KR102190397B1 - Lithium-sulfur battery cathode active material including the multi-armed nanoparticle, and lithium-sulfur battery including the cathode active material, and mechanical synthesis method for multi-armed nanoparticle - Google Patents

Abstract

A multi-armed nanoparticle sulfur composite can increase the degree of integration between nanoparticles. Therefore, the multi-armed nanoparticle sulfur composite can be effectively applied to a positive electrode of a lithium-sulfur battery.

Description

BACKGROUND ART A positive active material of a lithium-sulfur battery containing a sulfur complex of multi-branched nanoparticles, a lithium-sulfur battery containing the same, and a physical synthesis method of a multi-branched nanoparticle TECHNICAL FIELD [LITHIUM-SULFUR BATTERY AND LITHIUM-SULFUR BATTERY INCLUDING THE CATHODE ACTIVE MATERIAL, AND MECHANICAL SYNTHESIS METHOD FOR MULTI-ARMED NANOPARTICLE}

The present disclosure provides a positive electrode active material for a lithium-sulfur battery including a sulfur composite of multi-branched nanoparticles, a lithium-sulfur battery including the same, and a method for physical synthesis of multi-branched nanoparticles, and a lithium-containing sulfur composite of multi-branched nanoparticles. It relates to a positive electrode active material of a sulfur battery and a lithium-sulfur battery including the same.

Recently, small-sized and lightweight electronic products, electronic devices, communication devices, etc. are rapidly progressing, and as the necessity of electric vehicles emerges greatly in relation to environmental issues, the demand for performance improvement of secondary batteries used as power sources of these products is also increasing. It is a reality. Among them, lithium secondary batteries are receiving considerable attention as high-performance batteries because of their high energy density and high standard electrode potential.

In particular, a lithium-sulfur (Li-S) battery is a secondary battery that uses a sulfur-based material having an S-S bond (Sulfur-Sulfur bond) as a positive electrode active material and lithium metal as a negative electrode active material. Sulfur, the main material of the positive electrode active material, has the advantage of being very rich in resources, non-toxic, and low weight per atom. In addition, the theoretical discharge capacity of the lithium-sulfur battery is 1675mAh/g, and the theoretical energy density is 2,600Wh/kg. : 480Wh/kg, Li-MnO2 battery: 1,000Wh/kg, Na-S battery: 800Wh/kg), so it is the most promising battery among the batteries being developed so far.

During the discharge reaction of a lithium-sulfur battery, an oxidation reaction of lithium occurs at the anode, and a reduction reaction of sulfur occurs at the cathode. Sulfur before discharge has a cyclic S8 structure, and the oxidation number of S decreases as the SS bond is broken during the reduction reaction (discharge), and the oxidation number of S increases as the SS bond is re-formed during the oxidation reaction (charging). Electric energy is stored and generated using a reduction reaction. During this reaction, sulfur is converted from cyclic S ₈ to linear lithium polysulfide (Li ₂ Sx, x = 8, 6, 4, 2) by a reduction reaction, and eventually this lithium polysulfide is completely reduced. Then, lithium sulfide (Li2S) is finally produced. Unlike lithium ion batteries, the discharge behavior of lithium-sulfur batteries by the process of reduction to each lithium polysulfide is characterized by stepwise discharge voltages.

However, in the case of such a lithium-sulfur battery, problems such as low electrical conductivity of sulfur, elution and volume expansion of lithium polysulfide during charging and discharging, and low coulomb efficiency and rapid capacity reduction due to charging and discharging must be solved. In addition, most of the studies have overlooked tap density, another important measurement criterion for practical device applications where space is limited. In general, a large inner cavity and a larger interparticle space of the positive electrode material cause a low tap density, which is an important parameter affecting the volume energy density of the electrode. In addition, the low tap density results in a thicker electrode and a longer electron path due to the low volumetric capacity. From a practical point of view, a high sulfur content and a small amount of host material composed of dense and regular nanostructures is an effective way to achieve a sulfur anode having a high area/volume capacity. Therefore, there is a need to develop a positive electrode active material capable of increasing the tap density.

An object of the present disclosure is to provide a positive electrode active material capable of increasing tap density.

The present disclosure is to provide a lithium-sulfur battery including a positive electrode composed of a positive electrode active material capable of increasing the tap density.

An object of the present disclosure is to provide a method for physically synthesizing multi-geometric nanoparticles constituting a positive electrode active material capable of increasing tap density.

The positive electrode active material according to the embodiments includes a multi-branched nanoparticle/sulfur composite.

The lithium-sulfur battery according to the embodiments includes a positive electrode made of a multi-branched nanoparticle/sulfur composite.

The physical synthesis method for synthesizing the multi-geometric nanoparticles constituting the positive electrode active material capable of increasing the tap density according to the embodiments is to prepare a polymer template having an opal structure, and a nanoparticle precursor in which the nanoparticles are dispersed in water is used as the opal. The structure is injected into a polymer mold, and the spherical polymer mold in which the nanoparticle precursor is injected is sintered and removed to obtain an inverted opal structure, and the reversed opal structure is water-dispersed and then physically pulverized. To synthesize the particles.

Other specifics of aspects of the present invention are included in the detailed description below.

The morphology of the multi-geometric nanoparticles according to the embodiments has a high degree of integration between the nanoparticles. Therefore, when using these nanoparticles to form a positive electrode of a lithium-sulfur battery, a high tap density can be realized.

Therefore, since the electrode can be formed in the form of a thin film compared to other materials having the same tap density, excellent rate-limiting characteristics can be achieved.

The method for physically synthesizing the multi-geometric nanoparticles according to the embodiments is simpler than the conventional chemical synthesis method and can be applied in the same manner regardless of composition or material.

1 is a schematic diagram illustrating a method of physically synthesizing multi-geometric nanoparticles and a method of forming an anode by applying the same.
Figure 2a is an electron microscope image of TiO ₂ (IO TiO ₂ ) of an inverted opal structure.
2B is an electron microscope image of tetrapod TiO ₂ (T-TiO ₂ ).
3A and 3B are TEM (Transmission Electron Microscopy) images of tetrapod TiO ₂ (T-TiO ₂ ).
4A to 4C are images showing the result of EDS (Energy Dispersive Spectroscopy) mapping analysis of tetrapod TiO ₂ (T-TiO ₂ ).
5 is a graph showing XRD (X-ray diffraction) results of tetrapod TiO ₂ (T-TiO ₂ ).
6A and 6B are BET (Brunauer-Emmett-Teller) analysis results and BJH (Barrett-Joyner-) of TiO ₂ (IO TiO ₂ ) and tetrapod TiO ₂ (T-TiO ₂ ) having an inverted opal structure, respectively. Halend) These are graphs showing the analysis results.
6C and 6D are graphs showing BET analysis results and BJH analysis results of a sulfur complex (T-TiO ₂ /S) of tetrapod TiO ₂ (T-TiO ₂ ) and T-TiO ₂ , respectively.
7 is a graph showing a TGA (ThermoGravimetric Analysis) analysis of the T-TiO ₂ of sulfur composites _{(T-TiO 2 / S)} .
8 is a graph showing a result of proceeding the life characteristics of the positive evaluation consisting of a P25 / S cathode active material manufactured in comparison with the positive electrode made of a sulfur complex (T-TiO ₂ / S) the positive electrode active material of the T-TiO ₂ for example.
Figure 9 is a graph evaluating the rate controlling properties of the positive electrode composed of a P25 / S positive electrode active material produced in Comparative Example and an anode made of a sulfur complex (T-TiO ₂ / S) of the positive electrode active material T-TiO _2.

Hereinafter, embodiments will be described in detail so that those of ordinary skill in the art can easily implement them. The embodiments may be implemented in various different forms, and are not limited only to the specific embodiments described herein.

1 is a schematic diagram illustrating a method of physically synthesizing multi-geometric nanoparticles and a method of forming an anode by applying the same.

Multi-geometric nanoparticles can be synthesized by applying a method of physically pulverizing the inverted opal structure using a polymer mold having an opal structure.

The polymer mold may be a polymer opal mold. For example, the polymer mold may be a polystyrene opal template. The polystyrene opal mold can be formed by polymerizing styrene monomers by a dispersion polymerization method. After polymerization, centrifugation can be performed and dried to form polymer molds arranged in a face-centered cubic structure.

When the nanoparticle precursor is injected into the polymer mold having an aligned structure and then the sintering process is performed to remove the polymer mold, a highly aligned inverted opal structure 10 is formed as shown in FIG. can do. In the case of nanoparticles TiO ₂ , a nanoarmo in which TiO ₂ nanoparticles are water-dispersed is injected into a polymer mold, followed by a sintering process for a predetermined period of time to remove the polymer mold to form a highly aligned and inverted opal. , IO) structure of TiO ₂ (IO TiO ₂ ) can be prepared. Sintering can be carried out at a temperature of around 500°C for about 2 hours.

The reversed opal structure may be dispersed in water and then physically pulverized to synthesize multi-armed nanoparticles 20. Physical grinding may be ultrasonication, and treatment may be within 1 hour.

When the connection between the vertices of the opal structure reversed by physical crushing is broken, multi-branched nanoparticles may be formed. When IO TiO ₂ is used, tetrapod TiO ₂ particles (T-TiO ₂ ) may be formed. The tetrapod TiO ₂ particles may have a size of about 1 μm.

If necessary, further centrifugation can be performed to uniformly form the size of the multi-branched nanoparticles. Since centrifugation is performed in the sense of fractionation, sieving may be performed to uniformly form the size of the multi-geometric nanoparticles.

In order to apply the prepared multi-geometric nanoparticles as a positive electrode active material for a lithium-sulfur battery, a sulfur compounding may be performed. Sulfur complexation is performed by dissolving sulfur in carbon sulfide, mixing multi-geometric nanoparticles, drying all of the sulfide carbon solvent, and then performing a molten sulfur impregnation method to obtain sulfur (30) of multi-geometric nanoparticles (20). The composite 40 can be synthesized.

The sulfur (30) composite 40 of the synthesized multi-geometric nanoparticles 20 may be formed as the positive electrode 50 of a lithium-sulfur battery using a conventional process. In the case of the anode 50 formed in this way, a high tap density may be exhibited due to the sulfur 30 composite 40 of the multi-branched nanoparticles 20. Therefore, if the density is increased, the volume can be reduced and the energy per volume can be increased. That is, energy per volume can be increased by forming a thin film compared to a conventional anode without increasing the amount of stored energy. Accordingly, it is possible to improve the life characteristics of the anode 50 and exhibit improved rate performance characteristics.

Hereinafter, preferred experimental examples are presented to aid the understanding of the present invention, but the following experimental examples are only illustrative of the present invention, and the scope of the present invention is not limited to the following examples.

Experimental example

Tetrapod TiO ₂ (T- TiO ₂ ) Physical synthesis of nanoparticles

First, a TiO ₂ structure (IO TiO ₂ ) having an inverted opal structure was prepared using a polystyrene opal template. For polystyrene opal mold, 60 mL of styrene monomer (Samchun, Acs reagent, 99.5%, KOR) was added to 600 mL of DI water, and the temperature was raised to 80° C. and purged with nitrogen for 30 minutes. Proceed. With the solution sufficiently stabilized at 80°C, 0.54g of 2,2'-Azobis(2-methylpropionitirle) (Junsei, Acs reagent, 98%, Japan) was added to the reactor as an initiator, and then at 80°C for 12 at 300RPM. Polymerization was carried out by stirring for hours.

Then, the produced polystyrene was centrifuged at 8000 RPM for 15 minutes, and then dried at room temperature for one day to prepare a spherical polymer mold having a face-centered cubic structure.

Then, after injecting a nanoamor having an average size of 15 nm in which TiO ₂ nanoparticles are water-dispersed, polystyrene particles are removed through a sintering process at 500°C for 2 hours, and TiO ₂ (IO TiO _{2) having an} inverted opal structure ) The structure was obtained.

After dispersing the inverted opal-structured TiO2 (IO TiO ₂ ) structure in water at a concentration of 5wt%, ultrasonication was performed for 1 hour.

2A shows an electron microscope (SEM) image of IO TiO ₂ . The size of the polystyrene particles used as a template was about 1.1 μm, the size of the IO TiO ₂ produced as shown in FIG. 2A was about 900 nm, and the size of the inner cavity was about 200 to 300 nm. .

The prepared IO TiO ₂ was dispersed in water at a level of 5 wt% and then physically pulverized through an ultrasonication process to synthesize tetrapod TiO ₂ particles (T-TiO ₂ ).

2B is an electron microscope image of tetrapod TiO ₂ particles (T-TiO ₂ ). IO TiO ₂ via physical grinding It can be seen from the image of FIG. 2B that the connection between the vertices of the structure is cut off and that it is manufactured in the form of a multi-geometric (tetrapod).

The size of the tetrapod TiO ₂ particles (T-TiO ₂ ) was manufactured at a level of about 1 μm, and the morphology of the manufactured particles is a structure with a high degree of integration between particles, and a high tap density can be expected.

Thereafter, centrifugation was performed at 5000 RPM for 10 minutes, and the particles were assembled into monodispersed tetrapod TiO ₂ particles.

Tetrapod TiO ₂ Particle Sulfur complex (T- TiO ₂ /S)

In order to apply the synthesized tetrapod TiO ₂ particles (T-TiO ₂ ) as a positive electrode active material of a lithium-sulfur battery, a sulfur compounding was performed. In the sulfur complexation, 0.65 g of sulfur was dissolved in a carbon sulfide solvent, and then 0.35 g of tetrapod TiO ₂ particles (T-TiO ₂ ) were mixed. After mixing for 3 hours, a temperature of 40° C. was applied until all of the carbon sulfide solvent was dried. Thereafter, a sulfur complex (T-TiO ₂ /S) of tetrapod TiO ₂ particles was synthesized through melt diffusion at 155° C. for 6 hours using an autoclave.

Comparative example : Preparation of nanoparticles P25 sulfur composite P25/S particles

P25 was made into fine powder using agate induction. In the sulfur complexation, 0.65 g of sulfur was dissolved in a sulfide carbon solvent, and then 0.35 g of P25 powder was mixed. After mixing for 3 hours, a temperature of 40° C. was applied until all of the carbon sulfide solvent was dried. Thereafter, a P25 sulfur complex (P25/S) was synthesized through melt diffusion at 155° C. for 6 hours using an autoclave.

Multi-terrain Analysis of physical properties of nanoparticles

TEM (Transmission Electron Microscopy) analysis

TEM (transmission electron microscopy) analysis was performed to confirm the microstructure of the prepared tetrapod TiO ₂ (T-TiO ₂ ). The structure of the produced tetrapod can be confirmed in FIGS. 3A and 3B, and it can be confirmed that it was manufactured as a multi-branched structure. The fabricated tetrapod structure has a very low percolation threshold due to an isotropic, multi-geometric morphology, so it is known that it is highly integrated and can realize high electrical conductivity characteristics.

EDS(Energy Dispersive Spectroscopy) Mapping analysis

During the TEM analysis, a TEM image was taken, and EDS mapping analysis was performed on the TiO ₂ and sulfur elements of the tetrapod particles through the EDS analysis function at the corresponding location, and the results are shown in FIGS. 4A to 4C.

As can be seen in FIGS. 4A and 4C, it was confirmed from the EDS image that sulfur was present in the tetrapod TiO ₂ particles of the example, and uniformly distributed.

XRD (X-ray diffraction) analysis

After applying the tetrapod TiO ₂ particles, sulfur, and tetrapod TiO ₂ /S composite to the grid, while irradiating the X-ray beam, changing the angle of the beam to 2 θ/min and measuring the diffraction of the beam to determine the particle The sex was analyzed and the results are shown in FIG. 5.

As can be seen in FIG. 5, the confirmed tetrapod TiO ₂ peaks are confirmed at 25.4 ^o , 37.9 ^o , 48.1 ^o , 54.2 ^o , and 55.2 ^o , which are (105), (211), (204), (116), ( 220). It was confirmed that the confirmed tetrapod TiO ₂ peak was indexed into anatase TiO ₂ (JCPDS card no. 21-1272). The diffraction peaks representing sulfur can be found at 23.3 ^o , 26.15 ^o , 27.6 ^o , 29 ^o , 42.8 ^o , 48 ^o and 51.3 ^o . It can be seen that the tetrapod TiO ₂ /S complex appears as a mixture of the tetrapod TiO ₂ peak and the sulfur peak.

BET & BJH ( Brunauer , Emmett and Teller & Barrett - Joyner - Halenda ) analysis

The reverse opal structure of TiO ₂ (IO TiO ₂₎ and a physical grinding the tetra pot _{TiO 2 (T-TiO 2)} N 2 adsorption in the production through-desorption analysis (adsorption-desorption analysis) BET ( Brunauer-Emmett- for Teller) analysis was performed. The results are illustrated in Fig. 6A. The specific surface area of IO TiO ₂ is 51.8 m ² /g, and the tetrapod TiO ₂ (T-TiO ₂ ) has a specific surface area of 91.13 m ² /g, which is increased as the cut vertices are caught in the surface area after physical grinding. see.

In addition, after performing BJH (Barrett-Joyner-Halend) analysis, the results are shown in FIG. 6B. From FIG. 6B, it was confirmed that the tetrapod TiO ₂ (T-TiO ₂ ) had a 4 nm pore that was about 10 times that of IO TiO ₂ . Similarly, more than twice the pores could be observed in the mesopore area of 4 nm or less. This is due to the high degree of integration between the particles of the tetrapod TiO ₂ (T-TiO ₂ ) structure as IO TiO ₂ is physically pulverized, and the increase of the pores formed between the arms and the T-TiO ₂ sulfur complex (T -TiO ₂ /S) seems to be.

As a result of measuring BET after formation, it was confirmed that the surface area decreased to 6.25 m ² /g as shown in FIG. 5C. In addition, as shown in Fig. 5d, it can be seen that the 4 nm peak almost disappeared as can be seen by BJH analysis. As a result, it can be seen that since the tetrapod TiO ₂ is a high-density material, it promotes the diffusion of sulfur into the micropores through the connected arms and the scaffold of the arms. Through this, it can be expected that sulfur is sufficiently supported in the inner micropores, and as a result, the polysulfide intermediate is prevented from eluting into the electrolyte during charging and discharging.

Thermogravimetric Analysis ( Thermogravimetric analysis)

The results proceed to TGA (ThermoGravimetric Analysis) and analyzed for sulfur to determine the weight of TiO ₂ of the T-sulfur complex (T-TiO ₂ / S) is shown in Fig. The T-TiO ₂ of sulfur composites (T-TiO ₂ / S) it can be seen that the weight loss at about 300 ℃ 64%. This was confirmed that the sulfur is 64% complexed in the sulfur composite (T-TiO ₂ / S) of the T-TiO _2.

Evaluation of physical properties when applied to positive electrode of lithium-sulfur battery

T- TiO ₂ of Sulfur complex (T- TiO ₂ /S) to evaluate the life characteristics of the positive electrode

To demonstrate the superior life of the positive electrode formed by using the positive electrode active material consisting of sulfur composites (T-TiO ₂ / S) of the T-TiO ₂ it was confirmed evaluate the long-term service life properties. The results are illustrated in FIG. 8. Evaluation was performed on the T-TiO ₂ /S electrode under the current density condition of 1C, and the initial capacity was 860 mAh/g. The discharge capacity after each 200 cycles was 757 mAh/g (88% retention rate), and the average Coulomb efficiency was 99% and 98%, respectively.

On the other hand, evaluation of the life characteristics of P25/S produced in Comparative Example was evaluated under 1 C condition. It exhibited 454 mAh/g (44% retention) over 200 cycles. This difference in electrochemical performance is interpreted to be due to the effect that appears as a highly integrated structure of tetrapod TiO ₂ (T-TiO ₂ ) particles. Highly integrated, high tap density electrodes can provide a short electron transfer path. That is, by inducing close contact between tetrapod TiO ₂ (T-TiO ₂ ) and sulfur, active sulfur species can be effectively utilized during charging and discharging. At the same time, the anatase TiO ₂ used in the T-TiO ₂ sulfur composite (T-TiO ₂ /S) electrode prevents continuous elution as an electrolyte through a strong chemical bonding action with polysulfide, which is a polysulfide. Suppressing the shuttle effect of the electrode, it is possible to improve the electrochemical performance and stability of the electrode.

T- TiO ₂ of Sulfur complex (T- TiO ₂ /S) of the positive electrode Rate Property evaluation

Increase the charge/discharge current density from 0.1C rate to 2C rate in the electrode made of the sulfur composite (T-TiO ₂ /S) of T-TiO ₂ synthesized in the experimental example and the electrode made of P25/S synthesized in the comparative example. Compared by doing. The results are illustrated in FIG. 8. It can be seen that the electrode made of T-TiO ₂ /S exhibited a much higher discharge capacity than the electrode made of P25/S, and was better under various current density conditions. Electrodes made of T-TiO ₂ /S showed discharge capacities at 0.1, 0.2, 0.5, 1 and 2 C of 1039, 926, 806, 698 and 601 mAh/g, respectively. On the other hand, the discharge capacities of the electrodes made of P25/S were 942, 809, 663, 551 and 451 mAh/g, respectively. Compared to the capacity at the 0.1C rate, the T-TiO ₂ /S electrode maintains 58% even at the 2C rate, while the P25/S electrode shows a low retention rate of 47%. High-speed charge carrier transport in the entire electrode is a decisive factor in battery performance, and a short transport distance for electrons and lithium ions can achieve excellent rate performance. T-TiO ₂ /S can be implemented as an electrode thinner than P25/S per weight due to high tap density as described above. Therefore, it can be seen that the fabricated electrode provides a short transport distance for electrons and lithium ions, and thus exhibits improved rate performance characteristics. Although various embodiments have been described above, the scope of the rights is not limited thereto. The implementation form can be implemented in various ways within the scope of the detailed description of the invention and the accompanying drawings, and it is natural that this also belongs to the scope of the rights.

Claims (16)

delete delete delete delete Prepare a polymer mold with an opal structure,
Injecting a nanoparticle precursor in which nanoparticles are water-dispersed into a polymer mold having the opal structure,
Sintering and removing the spherical polymer mold in which the nanoparticle precursor was injected to obtain an inverted opal structure,
A method of physically synthesizing multi-branched nanoparticles by dispersing the inverted opal structure in water and then physically pulverizing it to synthesize the multi-branched nanoparticle. The method of claim 5,
The physically pulverizing is a physical synthesis method of multi-branched nanoparticles pulverized by ultrasonic treatment. The method of claim 5,
The nanoparticles are TiO ₂ ,
The multi-branched nanoparticle is a method for physical synthesis of a multi-branched nanoparticle of TiO ₂ having a tetrapod structure. The method of claim 5,
The opal-structured polymer mold is a method for physically synthesizing multi-branched nanoparticles that are polystyrene opal molds. The method of claim 5,
After dispersing the inverted opal structure in water and pulverizing it physically to synthesize multi-branched nanoparticles,
A physical synthesis method of multi-branched nanoparticles in which the multi-branched nanoparticles are assembled into monodispersed multi-branched nanoparticles by additional centrifugation or sieving. A positive electrode active material for a lithium-sulfur battery comprising a multi-regional nanoparticle-sulfur composite formed by compounding the multi-regional nanoparticles synthesized according to any one of claims 5 to 9. A lithium-sulfur battery comprising a positive electrode made of the positive electrode active material of claim 10. Prepare a polystyrene opal mold,
TiO ₂ in the polystyrene opal mold Inject the precursor,
TiO _{2 having} an inverted opal structure by sintering the polystyrene opal mold To obtain a structure,
TiO ₂ of the reversed opal structure A physical synthesis method of multi-branched nanoparticles in which the structure is dispersed in water and then physically pulverized to synthesize tetrapod TiO ₂ particles. The method of claim 12,
The physically pulverizing is a physical synthesis method of multi-branched nanoparticles pulverized by ultrasonic treatment. The method of claim 12,
TiO ₂ of the reversed opal structure After dispersing the structure in water, it was physically pulverized to synthesize tetrapod TiO ₂ particles,
A physical synthesis method of multi-branched nanoparticles that is further subjected to centrifugation or sieving to be assembled into monodispersed tetrapod TiO ₂ particles. Of claim 12 to claim 14 to form a Tetra pot TiO ₂ particles synthesized by any one of the composite of sulfur wherein the tetra-pot TiO ₂ A cathode active material for a lithium-sulfur battery made of a sulfur composite. A lithium-sulfur battery comprising a positive electrode made of the positive electrode active material of claim 15.

Images