KR20210012899A - Method for Preparing Composite Materials Using Slurries Containing Reduced Graphene Oxide - Google Patents

Abstract

In the present disclosure, described are: a method of preparing reduced graphene oxide by reducing graphene oxide with a modified surface in order to maintain good dispersibility even after reduction, and preparing a composite material with improved output and lifespan stability by coating or encapsulating the surface of a metal oxide with low thermal stability, such as lithium nickel cobalt manganese oxides (NCM), with slurry containing the reduced graphene oxide; and a positive electrode material for a lithium secondary battery using the same.

Description

Method for preparing composite material using slurry containing reduced graphene oxide {Method for Preparing Composite Materials Using Slurries Containing Reduced Graphene Oxide}

The present disclosure relates to a method of manufacturing a composite material using a slurry containing reduced graphene oxide (rGO). Specifically, the present disclosure provides a reduced graphene oxide by reducing the surface-modified graphene oxide in order to maintain good dispersibility even after reduction, and a metal oxide such as (lithium)- Nickel-cobalt-manganese-based oxide (NCM), in particular, a method of manufacturing a composite material by coating or encapsulating the surface of a low-stability metal oxide such as NCM rich in nickel, and a lithium secondary battery with improved output and life stability. It relates to a cathode material.

Lithium secondary batteries are used as a power source for small portable equipment because of their high energy density and lightweight characteristics, and have recently been in the spotlight as a power source for hybrid/electric vehicles (HEV/EV), as well as small home appliances and mobile electronic products. have.

Lithium-ion secondary batteries using lithium ions as secondary batteries are expanding from small mobile device power sources to mid-large-sized electric vehicles (eg, EV, HEV, etc.) and energy storage devices (ESS) power supplies. Is in. In particular, interest in electric vehicles, which is an eco-friendly vehicle, is increasing very much, and major automobile companies around the world are accelerating the development of related technologies by recognizing electric vehicles as the next-generation growth technology with the motto of eco-friendliness.

In order to be used in medium and large-sized applications such as electric vehicles, the development of high-capacity lithium-ion secondary batteries is essential. In general, a cathode of a lithium ion secondary battery includes a cathode material, a conductor, a binder, and a current collector, and in particular, a cathode material having excellent reversibility, low self-discharge rate, high capacity, and high energy density is required. . As such cathode materials, LiCoO ₂ (LCO), LiMn ₂ O ₄ (LMO), LiFePO ₄ (LFP), Li(Ni _x Mn _y Co _z )O ₂ (x+y+z=1)(NCM), LiNiCoAlO ₂ (NCA) and the like are known, and particularly, nickel-based layered oxides (NCM, NCA, etc.) are mainly used. Among them, lithium-nickel-cobalt-manganese oxide (NCM) is attracting attention as a cathode material for lithium secondary batteries such as hybrid vehicles (HEVs) and electric vehicles (EVs) due to its high performance and good stability as a cathode material. Demand is also constantly increasing. In particular, researches to mass-produce high-quality NCMs are actively being conducted, and the cathode material is mainly composed of nickel: manganese: cobalt in a 1:1 ratio.

The above-described cathode active material, NCM, exhibits different electrochemical properties depending on the composition of nickel, cobalt, and manganese constituting the same. For example, nickel contributes to capacity, cobalt to power, and manganese to contribute to structural stability. In particular, nickel contributes to the capacity expression of the active material based on various oxidation states, so for use in mid- to large-sized electric vehicles, the content of nickel is increased as much as possible and high-energy cathode active material (nickel (Ni)-rich NCM, Ni≥80 %) is advantageous. However, as described above, as the content of nickel increases, the ratio of cobalt and manganese contributing to the output and stability decreases, and thus the output or life stability of the battery may decrease.

In order to compensate for the above-described disadvantages, a method of surface coating or doping an active material with various materials has been proposed. A method of forming a double carbon (graphene) coating is known, and it can be easily complexed with various types of particles due to the layered two-dimensional structure of graphene while simultaneously improving output and life stability.

In this regard, methods of coating carbon (graphene) on the surface of active material particles generally require an aqueous solution process and high-temperature treatment in an inert gas environment. However, NCM rich in nickel (Ni) (especially LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) with a Ni:Co:Mn molar ratio of 8:1:1) not only has low thermal stability, but also surface It may be difficult to apply a carbon coating because impurities such as lithium synthesis residues react with a solvent to leave an electrochemically inert material on the surface or exchange between hydrogen ions and lithium ions. Moreover, under high-temperature treatment conditions, not only graphene but also metal oxides are reduced, and in order to avoid this, additional problems such as not being able to sufficiently secure the sp ² structure of graphene are also caused when heat treatment at a relatively low temperature to avoid this. .

Accordingly, there is a need for a method for effectively applying a carbonaceous coating to a metal oxide having low stability (especially thermal stability) and high sensitivity to moisture, such as nickel (Ni)-rich NCM.

In one embodiment of the present disclosure, it is intended to provide a composite material in which a carbon-containing coating layer of good properties (specifically, a carbon-containing network encapsulation layer having a three-dimensional structure) is formed on a metal oxide having low stability, and a method of manufacturing the same.

Another embodiment of the present disclosure is to provide a cathode material including the above-described composite material.

According to the first aspect of this disclosure,

a) Surface modification of graphene oxide using an amino group-containing compound that bonds with carbon atoms while ringing the epoxy group in the graphene oxide so that the surface of the graphene oxide has a negative (-) charge in the aqueous medium ;

b) forming reduced graphene oxide (rGO) by reducing the surface-modified graphene oxide;

c) preparing a first dispersion containing the reduced graphene oxide and conductive nanoparticles in a liquid medium;

d) concentrating the first dispersion to form a second dispersion in the form of a slurry;

e) combining the second dispersion and metal oxide particles to form a third dispersion; And

f) removing the liquid medium from the third dispersion;

A method of manufacturing a composite material comprising a is provided.

According to an exemplary embodiment, the amino group-containing compound may be at least one selected from compounds represented by the following general formulas 1 and 2, or a salt thereof:

[General Formula 1]

NH ₂ -R ₁ -SO ₃ H

[General Formula 2]

NH ₂ -R ₂ -PO ₃ H ₂

In the above formula, R ₁ and R ₂ are independently an alkylene group having 1 to 12 carbon atoms (specifically, 1 to 6 carbon atoms), an arylene group having 6 to 18 carbon atoms (specifically, 6 to 12 carbon atoms) with an aromatic ring substituted or unsubstituted (Here, the substituent may be an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group, a carboxyl group, or a halogen group).

According to an exemplary embodiment, the metal oxide may be a nickel-cobalt-manganese oxide (NCM).

According to an exemplary embodiment, the metal oxide may be represented by the following general formula 3:

[General Formula 3]

Li(Ni _x Co _y Mn _z )O ₂

In the above formula, x is 0.3 to 0.95, y is 0.025 to 0.35, and z is 0.025 to 0.35, and x+y+z=1.

According to the second aspect of this disclosure,

Metal oxide particles; And

An encapsulation layer formed on the metal oxide particles and including reduced graphene oxide and conductive nanoparticles;

As a composite material comprising a,

Based on the weight of the composite material, the content of the carbon component is in the range of 0.5 to 10% by weight,

The encapsulation layer has a thickness of 15 to 40 nm, and includes 30 to 70% by weight of reduced graphene oxide and 70 to 30% by weight of conductive nanoparticles, based on the weight of the encapsulation layer, and

The composite material is provided with a composite material having a specific surface area (BET) in the range of 1 to 50 m 2 /g and a pore volume in the range of 0.005 to 1 cm 3 /g.

According to a third aspect of the present disclosure, there is provided a cathode material for a secondary battery comprising the above-described composite material.

According to an embodiment of the present disclosure, the graphene oxide is surface-modified with an amino group-containing compound that allows ring-opening of the epoxy group contained in the graphene oxide and bonds to the carbon atom and exhibits a negative charge in an aqueous medium. Since the negative charge can be maintained even after the reduction treatment, a slurry that can solve the problem due to the difficult dispersibility of the conventional reduced graphene oxide (rGO) can be prepared by increasing the dispersion effect using the electric repulsive force. Specifically, it is possible to quickly form a uniform carbon-containing coating on a metal oxide such as nickel-rich NCM) under mild conditions. In particular, since it is possible to form a conductive three-dimensional carbon network layer on the surface of a metal oxide, it promotes the transport of electrons and ions, and strengthens the depolarization of metal oxides, resulting in a stable electrochemical process at a high rate. The reaction can be achieved. Moreover, by combining the reduced graphene oxide and the conductive nanoparticles, the movement of lithium ions is impeded by the incomplete coating resulting from the restacking and aggregation phenomenon in which the reduced graphene oxide in the form of a sheet is overlapped again in the subsequent drying process. It can suppress the phenomenon of becoming. In addition, due to the three-dimensional carbon network layer, electrical separation due to micro-cracks in metal oxides, irreversible phase transitions, and side reactions with electrolytes that may occur during charging and discharging can be suppressed.

As a result, according to a specific example of the present disclosure, a metal oxide to which a carbon-containing coating layer or an encapsulation layer is applied, in particular, an NCM-based metal oxide composite material is used as a cathode material for a secondary battery, specifically a lithium secondary battery, so that the output and lifetime It can improve stability, and can be effectively applied to electric vehicle batteries, energy storage devices, and the like.

1 is a view schematically showing a synthesis process and structure of a composite material encapsulated in a carbon network of a three-dimensional structure in a metal oxide (NCM811) according to an embodiment;
2 is a photograph for confirming the degree of dispersion of AHNS-rGO synthesized in Example;
3 is an XRD pattern (a), Raman spectrum (b), and XPS C 1s (c), Ni 2p (d), Co 2p (e), and Mn 2p for BNCM and GCNCM prepared in Examples ( f) is a spectrum;
4A is a Raman spectrum of GO and AHNS-rGO;
4B is the XPS C 1s spectrum of GO and AHNS-rGO;
5 is a TGA plot of BNCM, GNCM, and GCNCM (a), conductivity (b), adsorption-desorption isotherm (c), and pore size distribution (d);
6 is a SEM image of BNCM (a,b), GNCM (c,d) and GCNCM (e,f), and a TEM image of BNCM (g), GNCM (h) and GCNCM (i);
FIG. 7 is a CV curve for BNCM, GNCM, and GCNCM electrodes (a), initial charge-discharge profile at 0.1 C (b), rate performance (c), cycle stability (d), EIS after 1 cycle at 1 C. Spectrum (e), and EIS spectrum after 100 cycles at 1 C (inset drawing is an equivalent circuit for fitting EIS data),
Figure 8 is a CV curve of the BNCM (a), GNCM (b) and GCNCM (c) electrodes at a scanning rate ranging from 0.1 to 1 mV/s;
9 is a discharge profile of the BNCM (a), GNCM (b) and GCNCM (c) electrodes at a C-rate in the range of 0.1 to 10 C;
10 is a SEM image of BNCM (a,b) and GCNCM (c,d) after 100 cycles, and a TEM image of BNCM (e) and GCNCM (f) after 100 cycles (inset drawing is bulk from the surface FFT picture showing lattice transformation into phase);
11 is XPS Ni 2p (a,b), F 1s (c,d) and C 1s (e,f) spectra of BNCM and GCNCM electrodes after 100 cycles; And
12 is a diagram schematically showing the electrochemical effect of a carbon network encapsulation layer having a three-dimensional structure.

The present invention can all be achieved by the following description. The following description should be understood as describing preferred embodiments of the present invention, and the present invention is not necessarily limited thereto. In addition, the accompanying drawings are provided to aid understanding, and the present invention is not limited thereto, and details of individual configurations may be appropriately understood by the specific purpose of the related description to be described later.

"Reduced graphene oxide (rGO)" is a material having a structure similar to graphene and may exhibit a high specific surface area, thin film, and conductivity. Typically, graphene oxide has an insulating property, and the sheet resistance is 10 ¹² Ω/sq or more. However, when the graphene oxide is reduced, the sheet resistance of rGO is significantly reduced and can be converted into a conductor or a semiconductor.

"Carbon nanotubes (CNT)" can mean one-dimensional nanostructures in which a purely sp ² -bonded hexagonal carbon network is rolled (rolled up) into a tubular or cylindrical structure having a nano-sized diameter. , Single-walled carbon nanotubes (SWCNT) depending on the number of bonds forming the wall or the number of graphite layers (sp ² having a honeycomb crystal lattice structure-a flat sheet of single atom thickness of bonded carbon atoms). It is divided into double-walled carbon nanotubes (DWCNT) and multi-walled carbon nanotubes (MWCNT). In particular, SWCNTs have a diameter of approximately 1 nm, and may have a length of a remarkably high ratio (for example, 132,000,000 times) to the diameter, and DWCNTs have a diameter of approximately 5 to 10 nm, and a larger nanotube It may contain one concentric SWCNT inside, and the MWCNT may contain two or more concentric CNTs inside a larger outer nanotube. CNTs can be manufactured through a method such as an arc process (arc discharge), laser ablation, CVD (Chemical Vapor Deposition), CCVD (Catalytic Chemical Vapor Deposition).

“Encapsulation” may refer to a physicochemical or mechanical process for trapping an object in a specific material, wherein the core material may have a shape surrounded by a coating or cell. In the present specification, "encapsulation" and "coating" may be understood as mutually equivalent expressions in light of the purpose of the description.

In addition, in the present specification, "composite" or "composite material" may mean a combination in which two or more individual components are mixed in different proportions to form a final material having properties different from the individual components.

The expressions "on" and "on" may be understood as being used to refer to the concept of relative position. Therefore, not only the case in which another component or layer is directly present in the mentioned layer, but also another layer (intermediate layer) or component may be interposed or present therebetween. Similarly, the expressions “below”, “below” and “below” and the expressions “between” may also be understood as relative concepts of location. In addition, the expression "sequentially" may also be understood as a concept of a relative position.

In the present specification, when a certain component is "included", it means that other components and/or steps may be further included unless otherwise stated.

Overall disclosure

According to one embodiment of the present disclosure, by imparting a specific functional group (or functional group) through surface treatment during the manufacturing process of reduced graphene oxide (rGO), the reduced graphene oxide is a liquid medium (for example, Anhydrous alcohol, specifically anhydrous ethanol), it is possible to prepare a well dispersed slurry. The thus prepared reduced graphene oxide-containing slurry may be uniformly coated or encapsulated on a metal oxide, particularly a positive electrode active material such as nickel (Ni)-rich NCM having low stability.

Note that during the process of coating the metal oxide with the reduced graphene oxide-containing slurry, the surface of the metal oxide does not react with the liquid medium, and the liquid medium can be removed (or evaporated) over a short period of time at a relatively low temperature. It is possible to coat reduced graphene oxide with high conductivity on the surface of metal oxide particles without deteriorating metal oxides such as nickel-rich NCM with low thermal stability. Particularly, when metal oxide particles such as NCM are brought into contact with the reduced graphene oxide slurry, both the surfaces of the metal oxide and the reduced graphene oxide exhibit negative charges, and the dispersing power of the particles in the slurry is improved by electrical repulsion between them. It is possible to achieve a more uniform coating or encapsulation.

According to one embodiment of the present disclosure, the reduced graphene oxide slurry further contains conductive nanoparticles, and the conductive nanoparticles suppress the restacking of the graphene sheet to maintain the gap between the sheets so that lithium ions are It can be moved easily. That is, in the two-dimensional sheet structure of reduced graphene functionalized with a specific component as described above, by inserting conductive nanoparticles between the functionalized reduced graphene sheets, lithium ions can move with a high degree of freedom. It can be converted into a network of structures, resulting in the formation of a carbon-containing network with a higher conductivity (ie, the conductivity of the carbonaceous network can be further improved through the addition of conductive nanoparticles).

This can significantly improve rate capability and cycle stability when compared to a metal oxide without introducing a carbonaceous coating layer, particularly a metal oxide such as nickel-rich NCM.

Hereinafter, a method of manufacturing a composite material according to an embodiment will be described in detail.

A. Surface modification of graphene oxide (GO)

According to one embodiment, graphene oxide (GO) is graphene functionalized with an oxygen-containing functional group (eg, epoxy or hydroxy group) as shown in the following Structural Formula 1, specifically, a single layer graphite oxide (CO covalent bond is introduced) By doing so, it may mean a two-dimensional material derived from graphene).

[Structural Formula 1]

As shown in the above structural formula, in the case of graphene oxide, a carbonyl group and a carboxyl group exist at the edge of graphene to exhibit polar surface characteristics, and carbon in the graphene sheet is connected through an epoxy group. For example, it exhibits hydrophilicity due to the presence of a carboxyl group, and thus can be easily dispersed in water to form a colloidal solution. As such, the graphene oxide has a graphene-based lattice and various oxygen-containing functional groups, so that the functional groups on the surface of the graphene oxide can act as anchor sites for fixing various active species, and have adjustable electronic properties.

In this regard, graphene oxide may be prepared according to a method known in the art. As an example of such a method, a Brodie method, a Staudenmaier method, a Hummer method, and the like are known, and more typically, an improved Hummer method may be applied. In the case of the Hummer method, for example, using 3 weight equivalents of potassium permanganate (KMnO ₄ ) as an oxidizing agent and 0.5 weight equivalents of sodium nitrate, graphite (specifically in the form of a plate) in the presence of a strong acid (nitric acid or nitric acid/sulfuric acid). One weight equivalent can be converted to graphene oxide.

According to an exemplary embodiment, the size of the graphene oxide may range from, for example, about 100 nm to 10 μm, specifically about 200 nm to 5 μm, and more specifically about 400 nm to 1 μm.

In an exemplary embodiment, the graphene oxide prepared as described above is, for example, about 0.5 to 8 mg/ml, specifically about 0.5 to 5 mg/ml, more specifically about 1 to 3 mg/ml. It can be prepared as a concentration aqueous dispersion. In order to prepare a uniform dispersion, known pulverization and/or homogenization means such as ultrasonic pulverization, homogenizer, etc. may be used.

Even if the carboxyl group in the reduced graphene oxide (rGO) obtained by subsequent reduction of the thus prepared graphene oxide dispersion is reduced, the epoxy group in the graphene oxide is opened using a specific compound, that is, an amino group, to obtain good dispersibility. As a result of bonding with carbon atoms, surface modification is performed using an amino group-containing compound that causes the surface of graphene oxide to have a negative (-) charge in an aqueous medium. The ring-opening reaction of the epoxy group may be performed as shown in Scheme 1 illustrated below.

[Scheme 1]

According to an exemplary embodiment, the compound for surface modification is at least one selected from compounds represented by the following general formula 1 and/or general formula 2 (specifically, a monomolecular compound) or a salt thereof (e.g., alkali metal, alkali Salts such as earth metals).

[General Formula 1]

NH ₂ -R ₁ -SO ₃ H

[General Formula 2]

NH ₂ -R ₂ -PO ₃ H ₂

In the above formula, R ₁ and R ₂ are independently an alkylene group having 1 to 12 carbon atoms (specifically, 1 to 6 carbon atoms), an arylene group having 6 to 18 carbon atoms (specifically, 6 to 12 carbon atoms) with an aromatic ring substituted or unsubstituted (Here, the substituent may be an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group, a carboxyl group, or a halogen group).

Representative examples of the above-described amino group-containing compounds are aminomethanesulfonic acid, 2-aminoethanesulfonic acid, and 3-amino-1-propanesulfonic acid. ), sulfanilic acid, 6-amino-4-hydroxy-2-naphthalenesulfonic acid, 4-amino-3-hydroxy-1-naphthalene Sulfonic acid (4-amino-3-hydroxy-1-naphthalenesulfonic acid), 4-amino-3-methylbenzenesulfonic acid, sodium 5-amino-9,10-dihydroanthracene -1-sulfonate (sodium 5-amino-9,10-dihydroanthracene-1-sulfonate), (4-aminophenyl) phosphonic acid ((4-aminophenyl)phosphonic acid), aminomethylphosphonic acid ((Aminomethyl)phosphonic acid ), 3-aminopropylphosphonic acid, 4-aminobutylphosphonic acid ((4-aminobutyl)phosphonic acid), L-2-amino-4-phosphonobutyric acid (L-2-amino-4 -phosphonobutyric acid), monosodium 4-amino-1-hydroxy-1-phosphonobutylphosphonate trihydrate (Monosodium (4-Amino-1-hydroxy-1-phosphonobutyl)phosphonate Trihydrate), etc. It can be used in combination of one or two or more. According to certain embodiments, the amino group-containing compound may be 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AHNS).

As an example, the amino group-containing compound for surface modification is provided in the form of an aqueous solution, and it may be combined with an aqueous dispersion of graphene oxide. At this time, the concentration of the aqueous solution of the amino group-containing compound may be, for example, about 5 to 30% by weight, specifically about 10 to 25% by weight, and more specifically about 15 to 20% by weight, but this is for illustrative purposes. Can be understood. In addition, the aqueous solution of the amino group-containing compound may be provided in the form of a basic (or alkaline) aqueous solution in order to increase the solubility of the amino group-containing compound, and the base added at this time is ammonia (or ammonium hydroxide), sodium hydroxide, potassium hydroxide, or It can be a combination of these. At this time, the amount of the base added is adjusted within the range of, for example, about 3 to 20% by weight, specifically about 4 to 15% by weight, more specifically about 5 to 10% by weight, based on the weight of the amino group-containing compound. It is possible.

According to an exemplary embodiment, when the amino group-containing compound is added to the graphene oxide dispersion, the amount of the amino group-containing compound in the graphene oxide dispersion is, based on the graphene oxide, for example, about 100 to 1000 weight. %, specifically about 200 to 800% by weight, more specifically about 300 to 500% by weight.

In this way, the amino group-containing compound may be added to contact the graphene oxide in the dispersion, and surface modification may be performed under elevated temperature conditions. Illustratively, the surface modification temperature may be, for example, at least about 60°C, specifically about 65 to 90°C, and more specifically about 70 to 80°C. Further, the surface modification (contact) time may range from, for example, about 10 to 48 hours, specifically about 12 to 30 hours, and more specifically about 18 to 24 hours. However, the above-described treatment conditions may be understood for exemplary purposes, and may be changed depending on the type of the amino group-containing compound and the concentration of the graphene oxide dispersion.

B. Reduction of surface-modified graphene oxide (GO)

According to one embodiment, oxygen-containing groups (carboxyl groups, etc.) present in the surface-modified graphene oxide are removed by performing a reduction treatment on the surface-modified graphene oxide. In this regard, a method of reducing graphene oxide known in the art, for example, a method such as physical reduction or chemical reduction, can be applied without particular limitation.

In this regard, a thermal reduction method may be mentioned as the physical reduction. Illustratively, graphene oxide is instantaneously subjected to high temperature heat treatment (eg, at least about 300°C, specifically about 400 to 1000°C, more specifically about 600 to 800°C) under an inert gas (nitrogen, neon, argon, etc.) atmosphere. ℃) can.

Alternatively, a chemical reduction reaction using a reducing agent can be applied. As such a reducing agent, hydrazine (hydrazine), hydrazine hydrate (hydrazine hydrate, N ₂ H ₄ .H ₂ O), sodium borohydrate (NaBH ₄ ), ammonia borane (NH ₃ BH ₃ ), ethylenediamine, thio Urea, hydroionic acid, L-ascorbic acid, urea, hydroquinone, benzyl amine, and the like may be used alone or in combination. In certain embodiments, hydrazine or a hydrate thereof may be used as the reducing agent.

For the reduction treatment, the surface-modified graphene oxide-containing dispersion may be used as it is, or the surface-modified graphene oxide may be purified through washing or the like, and then separated and dispersed in an aqueous medium.

According to an exemplary embodiment, in the chemical reduction method by a reducing agent, the amount of the reducing agent added is, based on graphene oxide, for example, about 200 to 2000% by weight, specifically about 500 to 1500% by weight, more specifically It may be in the range of about 800 to 1200% by weight.

According to an exemplary embodiment, the reduction treatment is performed at a temperature of, for example, at least about 40° C. (specifically about 60 to 100° C., more specifically about 80 to 95° C.), and about 10 to 30 hours (specifically about 12 To 24 hours, more specifically about 15 to 20 hours), but is not limited thereto.

When the surface-modified graphene oxide is reduced as described above, oxygen-containing groups such as carboxyl groups present in the graphene oxide are removed as in Scheme 2 below to restore sp ² bonds, whereas via an amino group The moiety of the bound surface-modifying compound (eg, in the case of AHNS) remains.

[Scheme 2]

According to the above reaction formula, in the graphene oxide provided in step (a), the amine group of AHNS attacks the epoxy group in the graphene oxide in step (b) to open the ring through covalent interaction, and bond with the carbon atom ( Step (c)). Thereafter, as a reduction treatment using hydrazine hydrate (monohydrate) as a reducing agent in step (d), a significant part of the carboxy group and hydroxyl group present in the graphene oxide are removed, and the SO ₃ H group present in the AHNS is passed through the amino group. Are combined. The SO ₃ H groups are ionized in the aqueous medium of the negative charge (SO ₃ ^-) it can be shown to inhibit the normal reduction yes tendency to flocculate pin oxide particles effectively.

When the reduction treatment is performed as described above, the remaining reactants in the dispersion may be removed, and conventional post-treatment procedures (eg, water washing, physical or chemical separation, etc.) may be performed for separation/purification. For example, in the case of physical or chemical separation, filtering, centrifugation, dialysis, etc. may be applied, and in detail, vacuum filtering may be performed.

According to an exemplary embodiment, optionally, a step of removing moisture remaining in the reduced graphene oxide (eg, the solid content of the reduced graphene oxide obtained by filtering) may be performed. This is because metal oxides such as NCM during the subsequent steps are susceptible to moisture and deteriorate to deactivate the surface. To remove such residual moisture, the solid content of the reduced graphene oxide is redispersed in anhydrous alcohol (eg, absolute ethanol), methylpyrrolidone, dimethylformamide, toluene, etc. The process of making can be repeated once or twice or more.

C. Preparation of Slurry of Reduced Graphene Oxide

According to one embodiment, in order to coat the reduced graphene oxide prepared by the above-described reduction treatment on the metal oxide particles, a dispersion (first dispersion) may be prepared by dispersing in a liquid medium. In this regard, the liquid medium for preparing the slurry may be at least one selected from alcohol (anhydrous alcohol), methylpyrrolidone (N-methyl-2-pyrrolidone), dimethylformamide, toluene, etc. have. According to an exemplary embodiment, the liquid medium may be anhydrous alcohol, more specifically anhydrous ethanol. At this time, anhydrous ethanol is a universal solvent that allows dissolution of hydrophilic and hydrophobic compounds, and has a low boiling point, so it may be advantageous for preparing a slurry for a subsequent carbonaceous coating.

According to an exemplary embodiment, the concentration of reduced graphene oxide in the first dispersion is, for example, about 1 to 20 mg/ml, specifically about 3 to 15 mg/ml, more specifically about 5 to 10 mg/ml. It can be adjusted in the ml range.

In addition, in one embodiment, conductive nanoparticles are included together with reduced graphene oxide in the first dispersion. These conductive nanoparticles may provide a function of converting the two-dimensional structure of the surface-modified reduced graphene oxide into a three-dimensional carbon-containing network structure by being inserted between the surface-modified reduced graphene oxide sheets. In addition, the conductive nanoparticles can prevent the movement of lithium ions from being hindered when the composite material is applied as a positive electrode material by restacking the reduced graphene oxide in the form of a sheet in a subsequent drying process. In this regard, the conductive nanoparticles can be easily attached to the graphene sheet through interactions such as π-π bonding, Van der Waals force, and hydrophobic bonding. The conductive nanoparticles may be typically carbon nanotubes (CNT), carbon black, conductive metal nanoparticles (nanowires), and the like, and one or two or more of them may be used in combination.

-CNT

In the case of CNT, SWCNT, DWCNT, MWCNT, or a combination thereof may be included. However, as the ease of dispersion is affected by the CNT type, it may be advantageous to use MWCNT. In addition, the shape of the CNT may be a strand type, a rod type, etc., and specifically may be a rod type. In an exemplary embodiment, the average diameter of the CNT is, for example, about 3 to 20 nm (specifically About 5 to 15 nm, more specifically about 6 to 13 nm), and an average length thereof, for example, in the range of about 1 to 100 μm (specifically about 2 to 50 μm, more specifically about 2.5 to 20 μm) However, it is not necessarily limited thereto. In this regard, the ratio of length to diameter (surface ratio) may range from at least about 100, specifically about 200 to 3000, and more specifically about 400 to 1500. In addition, the specific surface area of the CNT may be, for example, about 50 to 1315 m 2 /g, specifically about 100 to 1000 m 2 /g, more specifically about 200 to 500 m 2 /g. In particular, in the case of using CNT, it provides an advantage of improving the porosity of the reduced graphene oxide coating layer to increase the movement path of lithium ions.

-Carbon black

In general, a carbon six-membered ring is formed by incomplete combustion or thermal decomposition of hydrocarbons, and then through a polycyclic aromatic compound through a process such as dehydrogenation and condensation, carbon black crystals having a hexagonal carbon atom structure are formed. Referred to as. Carbon black has a two-dimensional order compared to a conventional graphite having a three-dimensional structure. An exemplary atomic structure model of carbon black can be represented by the following structural formula 2.

[Structural Formula 2]

Such carbon black may be Furnace carbon black and/or thermal carbon black, for example, in the form of fine particles composed of amorphous carbonaceous, and its size is, for example, about 10 to 100 nm, specifically about 30 to 80 nm, or more. Specifically, it may be in the range of about 40 to 60 nm.

-Conductive metal nanoparticles

The conductive metal nanoparticles may be made of a material having a conductivity of, for example, at least about 500 S/cm, specifically about 1000 to 15000 S/cm, and more specifically about 5000 to 10000 S/cm. According to an exemplary embodiment, the material of the conductive metal nanoparticles is copper (Cu), silver (Ag), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), rhodium (Rh), and It may be selected from the group consisting of combinations, and specifically may be silver (Ag). Meanwhile, the size of the conductive metal nanoparticles may range from, for example, about 1 to 100 nm, specifically about 5 to 50 nm, and more specifically about 10 to 30 nm.

According to an exemplary embodiment, the conductive nanoparticles described above, based on the weight of the reduced graphene oxide (or surface-modified reduced graphene oxide), about 10 to 300% by weight, specifically about 50 to 200% by weight, more Specifically, it may be added in an amount of about 80 to 150% by weight. However, when an excessively large amount is used, since the capacity of the cell may be reduced due to a decrease in the specific gravity of the active material in the electrode, it may be advantageous to properly use it within the above-described range.

According to an exemplary embodiment, a known mixing and/or homogenization means such as ultrasonic grinding and/or a homogenizer may be used so that the reduced graphene oxide and the conductive nanoparticles can be uniformly mixed in the dispersion.

The first dispersion can then be concentrated by partially removing the liquid medium to form a slurry or paste-like dispersion (second dispersion) having a higher solids content. To this end, the first dispersion is, for example, about 0.2 to 2 hours (specifically about 0.3 to 1.5) at a temperature of about 45 to 80 °C (specifically about 50 to 70 °C, more specifically about 55 to 65 °C). It can be prepared in the form of a slurry by performing the concentration step in a manner of heating over time, more specifically about 0.5 to 1 hour). At this time, the concentration of the reduced graphene oxide in the second dispersion may be, for example, about 20 to 100 mg/ml, specifically about 25 to 80 mg/ml, and more specifically about 30 to 50 mg/ml. .

Carbon coating on metal oxide

After preparing a dispersion (second dispersion) in the form of a slurry having a high concentration, a step of coating it by combining it with a metal oxide may be performed.

According to an exemplary embodiment, the metal oxide is in the form of particles, and may be a primary particle or a secondary particle, and the size (diameter) thereof is, for example, about 1 to 50 μm, specifically It may be in the range of about 3 to 30 μm, more specifically about 5 to 20 μm. However, when the metal oxide particles are in the form of secondary particles, the size of the primary particles constituting them is, for example, about 20 to 1500 nm, specifically about 50 to 1200 nm, and more specifically about 100 to 1000 nm. I can.

In certain embodiments, the metal oxide may be represented by the following general formula 3 as an NCM-based metal oxide:

[General Formula 3]

Li(Ni _x Co _y Mn _z )O ₂

In the above formula, x is 0.3 to 0.95 (specifically 0.5 to 0.9, more specifically 0.75 to 0.85), y is 0.025 to 0.35 (specifically 0.07 to 0.3, more specifically 0.1 to 0.2), and z is 0.025 to 0.35 (specifically As 0.07 to 0.3, more specifically 0.1 to 0.2), and x+y+z=1.

In certain embodiments, the metal oxide may be a nickel-rich metal oxide in which nickel (Ni): cobalt (Co): manganese (Mn) is blended in a ratio of about 8:1:1, such metal oxide is referred to as NCM811. Has become.

In this case, since a hydroxy group is generated on the surface of a metal oxide such as NCM in contact with the atmosphere, the surface may exhibit a negative (-) charge. Accordingly, since the reduced graphene oxide exhibiting a negative charge and electric repulsion or repulsion force are generated in the liquid medium, coating using the slurry can be performed while particles in the slurry are uniformly dispersed.

In particular, a phenomenon in which conductive nanoparticles added prior to combination with metal oxide and reduced graphene oxide aggregate may occur. However, as described above, as a result of improving the dispersibility by the surface modification of the graphene oxide, the metal oxide particles and the conductive nanoparticles may be evenly mixed.

According to an exemplary embodiment, a third dispersion is prepared by adding (combining) metal oxide particles to the second dispersion, and such a combination or mixing process is performed under agitation conditions (e.g., about 50 to 300) due to the characteristics of the slurry. rpm, specifically about 100 to 200 rpm), for example about 3 to 15 minutes, specifically about 5 to 10 minutes. In addition, the contact temperature for coating may be, for example, about 10 to 40°C, specifically about 15 to 35°C, and more specifically about 20 to 30°C.

At this time, the amount of the metal oxide added may range from, for example, about 1000 to 10000% by weight, specifically about 2000 to 8000% by weight, more specifically about 4000 to 6000% by weight, based on the reduced graphene oxide. .

It should be noted that the carbon coating process can perform an effective coating process in a short time under relatively mild conditions.

After performing the above-described coating process, a step of removing the liquid medium contained in the third dispersion may be performed to obtain a solid composite material. To this end, for example, about 3 to 15 minutes (specifically about 4 to 12 minutes, more specifically about 5 to about 5 to about 15 minutes) at about 40 to 80 ℃ (specifically about 50 to 70 ℃, more specifically about 55 to 65 ℃) 10 minutes).

Optionally, further treatment can be carried out in known drying means, for example vacuum ovens, for the purpose of substantially completely removing the residual liquid medium. Here, an exemplary temperature may be, for example, in the range of about 30 to 50°C (specifically, about 35 to 45°C), and the treatment time may be, for example, at least about 5 hours, specifically in the range of at least about 6 to 10 hours. Can be Such treatment means and conditions can be understood for illustrative purposes.

Through the above-described procedure, a composite material in which a carbonaceous (a combination of rGO and conductive nanoparticles) coating layer or an encapsulation layer is formed on metal oxide particles may be prepared.

In an exemplary embodiment, the metal oxide particles in the composite material may be in the form of primary particles or secondary particles formed by collecting the primary particles, and the composite material may be the aforementioned carbon-containing material. The coating layer (or encapsulation layer) may be in the form of surrounding or covering the primary particles or secondary particles.

In this regard, the carbonaceous coating layer that does not contain conductive nanoparticles, for example, has a thickness in the range of about 3 to 10 nm (specifically about 5 to 10 nm), whereas the combination of reduced graphene oxide and conductive nanoparticles The thickness of the carbonaceous coating layer including the is about 15 to 40 nm (specifically, about 20 to 30 nm), and the conductive nanoparticles act as a spacer between the graphene layers, thereby providing a wider path through which lithium ions can move. Can (i.e., a three-dimensional carbon-containing network structure).

According to an exemplary embodiment, based on the weight of the composite material, the content of the carbon component is, for example, in the range of about 0.5 to 10% by weight, specifically about 1 to 8% by weight, more specifically about 2 to 5% by weight. It can be changed according to the type and amount of used conductive nanoparticles, the amount of reduced graphene oxide, and the type and size of metal oxide.

In addition, the composition of the carbon-containing composite coating layer (encapsulation layer) on the metal oxide particles may depend on the amount of individual components used during the manufacturing process. The composition of an exemplary carbon-containing composite coating layer is, based on the weight of the coating layer, about 30 to 70% by weight of reduced graphene oxide (specifically about 40 to 60% by weight, more specifically about 45 to 55% by weight), and conductivity. Nanoparticles may contain about 70 to 30% by weight (specifically, about 60 to 40% by weight, more specifically about 55 to 45% by weight), which may be understood as an exemplary purpose.

Meanwhile, according to an exemplary embodiment, the composite material exhibits porosity, and at this time, the pore size may be, for example, about 5 to 40 nm, specifically about 10 to 30 nm, and more specifically about 15 to 20 nm. .

In addition, the specific surface area (BET) of the composite material may be in the range of, for example, about 1 to 50 m2/g (specifically about 2 to 30 m2/g, more specifically about 4 to 15 m2/g), and The volume may range, for example, from about 0.005 to 1 cm 3 /g (specifically about 0.01 to 0.5 cm 3 /g, more specifically about 0.02 to 0.1 cm 3 /g). In this regard, the specific surface area and pore volume of the coating layer that does not contain the conductive nanoparticles is relatively low, but in the case of the coating layer containing the combination of the reduced graphene oxide and the conductive nanoparticles, the specific surface area and pores have increased significantly. It is noteworthy that it represents the volume.

In addition, the composite material may exhibit good conductivity, for example, at least about 0.1 S/m, specifically about 0.3 to 10 S/m, and more specifically about 0.5 to 5 S/m. In particular, the conductivity of the carbon network can be further improved by the addition of conductive nanoparticles.

In addition, compared to a metal oxide without a carbon-containing coating layer (for example, in the case of bare NMC811), the metal oxide coated (or encapsulated) with the above-described three-dimensional carbon-containing network layer is It can show remarkably improved results in terms of properties and cycle stability. As an example, the rate controlling property can be increased by, for example, at least about 200%, specifically at least about 300%, more specifically at least about 400%, and also the cycle stability is, for example, at least about 20%, specifically By at least about 30%, more specifically by at least about 35%.

Use of composite materials

According to another embodiment of the present disclosure, a composite material in which a carbonaceous coating layer is formed on a metal oxide may be applied as a secondary battery, specifically, a cathode material such as a cathode active material of a lithium secondary battery. That is, it can be used as a material for storing and/or releasing lithium ions from the positive electrode of a lithium secondary battery. In particular, the role as a spacer of the conductive nanoparticles in the carbonaceous coating, as the electrical conductivity increases, the mobility of lithium ions is also secured to show excellent rate characteristics, and also, it suppresses the occurrence of surface side reactions and cracks of metal oxides, thereby improving life stability. It can be improved. As a result, it can be effectively applied to batteries for electric vehicles and energy storage devices.

According to an exemplary embodiment, when the composite material is used as a cathode of a lithium secondary battery, even after performing 100 cycles at 1 C, at least about 80% of the initial capacity, specifically about 85 to 95% Can hold capacity.

Hereinafter, preferred embodiments are presented to aid in understanding of the present invention, but the following examples are provided for easier understanding of the present invention, and the present invention is not limited thereto.

Example

Equipment used in this example is as follows.

-Specialization

Each of the XRD and Raman spectra was measured using MiniFlex 600 (Rigaku) and NRS-3100 (JASCO). At this time, the XRD pattern was obtained using Cu Kα (λ = 0.15406 nm) irradiation (2θ range: 10 to 80°, scanning speed: 5° min ^-1 ).

XPS (K-Alpha, Thermo Fisher Scientific) analysis was performed to confirm the chemical composition and valence state of the sample. XPS spectra were recorded using Al Kα (hυ = 1486.7 eV) micro-focusing and monochromatic source).

The carbon content in the sample was determined by analysis using TGA (SDT Q600, TA Instruments).

N ₂ physical adsorption was performed using Micromeritics 3Flex, through which the surface area and porosity of the sample were measured. The N ₂ isotherm was collected after pretreatment at 100° C. for 24 hours. The specific surface area and pore size distribution were measured by the BET method and the BJH method, respectively.

The morphological characteristics and microstructure of the prepared particles were observed using FE-SEM (Verios G4 UC, FEI) and TEM (JEM 2100F, JEOL).

The conductivity of the sample was evaluated using a single-structure four-point probe method equipped with a resistance measurement system (Keithley, Tektronix). Powder-type samples were pelleted into 1 mm thick samples and measured 10 times at 5 different points on the pellet.

In order to evaluate the post-cycling properties, the coin-cell was disassembled, and then the cathode was taken out and washed three times with dimethyl carbonate, followed by drying at 60° C. under vacuum conditions overnight. The dried electrodes were characterized by SEM, TEM, and XPS. In the case of TEM analysis, particles collected from the electrode in a discharge state were cut into a thin film having a thickness of 70 nm by FIB (focused ion beam; Scios, FEI), and loaded onto a Mo grid.

-Electrochemical performance measurement

In order to manufacture the working electrode, the active material was mixed with Super P and PVdF binder in a weight ratio of 90:5:5 in NMP. The obtained slurry was coated on a current collector (Al foil) using a doctor blade, and dried overnight under vacuum and 110° C. temperature conditions. The dried electrode was punched into a 14 mm diameter disk.

The average loading amount of the active material was about 4.0 mg/cm 2. Lithium foil and polypropylene membrane (Celgard 2400) were used as negative electrodes and separators, respectively. As the electrolyte, a solution (1.0 M) in which LiPF ₆ was dissolved in a solvent in which ethylene carbonate: ethylmethyl carbonate: dimethyl carbonate was mixed in a volume ratio of 1:1 was used.

The CR2032 coin-shaped cell was assembled in an Ar-filled glove box and the cell was aged under open circuit over 24 hours. An electrostatic charge-discharge test was performed under 25° C. and 2.8 to 4.3 V conditions (1 C = 200 mA g ⁻¹ ) using a WBCS3000S electrochemical workstation (WonA-tech).

The cells were charged using a constant current/constant voltage (CC/CV) method, and a constant voltage step was provided until the current reached 10% of the corresponding CC step current. Rate-rate characteristics were measured by applying the same charging rate and discharging rate, while gradually increasing from 0.1 C to 10 C. After performing two charge-discharge cycles (0.1 C), the cell was subjected to 100 charge-discharge cycles at 1 C to evaluate cycle stability.

CV was performed in the same voltage range using ZIVE SP 1 (WonA-tech).

Electrochemical impedance spectroscopy (EIS) was performed after charging up to 4.3 V using ZIVE SP 1 in the frequency range from 100 kHz to 10 mHz. All measured potentials are described as values for Li/Li ⁺ .

-Manufacture of rGO/CNT/NCM composite materials

The rGO/CNT/NCM composite material was prepared according to the procedure shown in FIG. 1.

First, the graphene oxide (Angstron Meterials; particle size: <1 μm) was uniformly dispersed in deionized water using ultrasonic grinding (ultrasonic output: 100 W) and a homogenizer (Misung Science Corporation) to achieve 2 mg/ml. A dispersion (dispersion) was prepared. Then, 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AHNS; TCI) was dissolved in deionized water to prepare an AHNS aqueous solution (concentration: 20% by weight). Subsequently, an alkaline AHNS solution prepared by adding 6 mg of ammonia solution (Sigma-Aldrich) to 5 ml of AHNS aqueous solution was gradually added to 100 ml of the previously prepared GO aqueous dispersion to modify the surface of graphene oxide. It was added to the graphene oxide-containing dispersion in an amount of 500% by weight based on pin oxide. At this time, the surface treatment was performed at 70°C over 24 hours.

Then, 2 ml of hydrazine monohydrate (Sigma-Aldrich) as a reducing agent (amount of 1000% by weight based on graphene oxide) was added to the surface-treated dispersion, and the surface-modified graphene at 100°C over 20 hours The oxide was reduced, washed with deionized water, and vacuum filtered using cellulose acetate paper to sufficiently remove the residue.

The surface-modified reduced graphene oxide from which the residue was removed was redispersed in 100 ml of absolute ethanol (Daejeonghwa Geum), and vacuum filtered with absolute ethanol using PTFE paper to remove residual moisture. After removing moisture, the reduced graphene oxide surface-modified to a concentration of 5 mg/ml was redispersed in absolute ethanol. At this time, ultrasonic grinding and homogenizer were used so that the graphene sheet was peeled and uniformly dispersed.

Then, 100% by weight of CNT (US Research nanometerials; particle average diameter: 15 nm) relative to the surface-modified reduced graphene oxide was added to the dispersion, followed by ultrasonic grinding and homogenizer for 1 hour. In order to minimize the solvent in the dispersion, it was heated at 70° C. for 30 minutes, and as a result, a high concentration slurry of 30 mg/ml was prepared.

To the slurry was added 4800 wt% (100 wt% CNT) of NCM (LG Chemistry, NCM811, particle size 15 µm) compared to the surface-modified reduced graphene oxide, and evenly mixed for 5 minutes at 150 rpm at 25°C. Thereafter, the residual ethanol was removed by heating at 60° C. for 5 minutes, and subsequently dried in a vacuum oven maintained at 60° C. for 6 hours to completely remove the ethanol to prepare an AHNS-rGO/CNT/NCM composite material. . Hereinafter, this will be referred to as "GCNCM".

On the other hand, except that CNT was not used, the same process as the manufacturing method of GCNCM was repeated to prepare a composite material coated with surface-modified reduced graphene oxide (AHNS-rGO) on NCM811. Will be referred to as ",

In addition, for comparison purposes, NCM811 (ie, bare NCM) without carbonaceous coating will be referred to as BNCM.

analysis

-Evaluation of dispersibility of AHNS-rGO

In order to evaluate the dispersibility of AHNS-rGO, 0.2 g of AHNS-rGO was dispersed in 200 ml of absolute ethanol, and the results are shown in FIG. 2.

Referring to the drawing, graphene gel in the form of aggregates appeared in rGO synthesized without surface modification was not observed at all, and a phenomenon of sedimentation was also not observed. From this, it was confirmed that the reduced graphene oxide was well dispersed by the negative charge of the SO ₃ ^- group of AHNS.

-In order to analyze the structure and chemical state of the carbon layer and nickel (Ni)-rich NCM (NCM811) prepared in the examples, XRD, Raman spectroscopy, and XPS measurements were performed for BNCM and GCNCM, and the results were It is shown in Figure 3.

3A shows the XRD patterns of bare NCM (BNCM) and three-dimensional carbon-coated NCM (GCNCM). According to the figure, the peak positions of BNCM and GCNCM were the same. The intensity ratio ( I ₀₀₃ / I ₁₀₄ ) of the (003) and (004) peaks indicates the degree of mixing of cations (Li ⁺ and Ni ₂ ⁺ ), where the values of I ₀₀₃ / I ₁₀₄ for BNCM and GCNCM are 1.32 and It was 1.41. These results suggest that the layered structure of NCM was not affected during the carbon encapsulation (or coating) process.

3B shows Raman spectra of BNCM and GCNCM. Referring to the drawing, peaks at 483 cm ^-1 and 594 cm ^-1 may correspond to the E _g and A _1g Raman-active modes (R3m space group) of NCM811. The peaks at 1347 cm ^-1 , 1590 cm ^-1 , 2690 cm ^-1 , and 2894 cm ^-1 may correspond to the D, G, 2D, and D+G bands of carbon, respectively. The D band is caused by the disorder-induced phonon mode vibration of the sp ³ carbon atom, indicating the presence of structural defects. The G band is related to the in-plane vibration of the sp ² carbon atom. In addition, the 2D band is related to the stacking order of graphite. As such, the 2D band for the carbon-encapsulated NCM supports the presence of the stacked graphene sheets.

Each of FIGS. 4A and 4B shows Raman spectra and XPS analysis results for each of graphene oxide (GO) and graphene oxide (AHNS-rGO) subjected to reduction treatment after surface modification with AHNS used in Examples.

Referring to FIG. 4A, I _D / I _G increased as GO was converted to rGO, indicating that the average size of the sp ² domain decreased as GO was reduced. This result is because, as the sp ² structure is regenerated during the reduction process, a number of new small graphite domains, which are smaller than the sp ² domain in GO, are generated. As the number of small sp ² domains increases, the degree of disorder and the number of edge faces increase, resulting in an increase in I _D.

Meanwhile, the results of performing XPS measurement to confirm the chemical composition and valence state of the transition metals of BNCM and GCNCM are shown in FIGS. 3C to 3F. Figure 3c shows the XPS C 1s of BNCM and GCNCM. In the case of BNCM, a peak with a center of 289.5 eV may correspond to CO ₃ ^2- of Li ₂ CO ₃ , and other peaks at higher binding energies may be considered to be due to contaminants present on the NCM surface. In the case of GCNCM, five peaks were observed due to the deconvolution of C s1 (median values are 284.5 eV, 285.5 eV, 286.5 eV, 288.1 eV, and 289.0 eV), which are CO ₃ ^2- , CC, respectively. Corresponds to /C=C, CN, C=O and OC=O. This peak is due to the AHNS-rGO and CNTs present on the NCM.

Figure 4b shows the XPS C 1s of GO and AHNS-rGO. As GO was converted to AHNS-rGO, the intensity of each of the C-O and C-C/C=C peaks decreased and increased, and a new C-N peak was observed.

Referring to FIG. 3C, the location and area ratio of the C 1s peak of GCNCM was consistent with that of AHNS-rGO, which means that the surface of NCM was functionalized with AHNS-rGO.

3D shows the XPS Ni 2p of BNCM and GCNCM. Referring to the drawing, the Ni 2p _3/2 peak can be deconvolved into the Ni ³⁺ and Ni ²⁺ peaks, wherein the binding energies were 856.1 eV and 855.0 eV, respectively. As the fraction of Ni ³⁺ increased, the electrochemical activity of NCM increased, while the degree of cation mixing of Ni ²⁺ and Li ⁺ decreased. In addition, the Ni ³⁺ /Ni ²⁺ ratios of BNCM and GCNCM were 3.14 and 3.18, respectively, such that similar values between the two samples indicate that the three-dimensional carbon-containing coating process does not change the chemical state of the NCM.

The above-described XRD, Raman, and XPS measurement results support that the structure and chemical state of the NCM are maintained even after the three-dimensional carbon-containing coating is applied.

-TGA analysis

A TGA test was performed to measure the content of the carbon component in the carbon-coated NCM. The results are shown in Fig. 5A. At this time, the sample was heated from 30° C. to 750° C. at a temperature increase rate of 10° C./min in an air atmosphere. Referring to the figure, BNCM, GNCM and GCNCM maintained 99.3%, 97.2%, and 95.4% of the total weight, respectively. The difference in the degree of weight retention between BNCM and carbon-coated NCM means the carbon content (2.1% and 3.9% for GNCM and GCNCM, respectively) in the carbon-coated NCM. The measured carbon content almost coincided with the mass percentage of the carbon components (AHNS-rGO and CNT) in the carbon-coated NCM.

-Conductivity measurement

According to FIG. 5B, the conductivity of BNCM, GNCM, and GCNCM was 0.015 S/m, 0.031 S/m, and 0.684 S/m, respectively. It can be seen that CNT contributes more to the improvement of the conductivity of NCM compared to AHNS-rGO. In particular, it is worth noting that the measured conductivity of each of the AHNS-rGO and CNT films is about 69 S/m and 173 S/m. GCNCM showed about 46 times higher conductivity than BNCM. It is judged that the conductive carbon-containing layer formed on the NCM can quickly transfer electrons to the NCM surface, thereby achieving excellent rate-limiting properties.

-Surface characteristic analysis

Test characteristics of BNCM, GNCM, and GCNCM were evaluated through an N ₂ physical adsorption experiment, and the results are shown in FIG. 5C. The figure shows the adsorption-desorption isotherms of BNCM, GNCM, and GCNCM and corresponding BET surface areas, respectively, and were evaluated as 0.37 m 2 /g, 0.72 m 2 /g, and 5.07 m 2 /g. The specific surface area of GNCM did not change much compared to that of BNCM, because the AHNS-rGO sheet was densely restacked on the NCM during the encapsulation process. On the other hand, the specific surface area of GCNCM was remarkably increased because CNT inhibited the restacking of the AHNS-rGO sheet and induces the formation of a porous network between the sheets.

5D shows the pore size distribution of BNCM, GNCM and GCNCM. GNCM (encapsulated only with AHNS-rGO) showed no particular change in pore volume (0.0048 cm 3 /g ¹ ) and average pore size (30.9 nm) compared to BNCM (0.0023 cm 3 /g and 27.8 nm). In the case of GCNCM, the three-dimensional carbon network had rich micropores (<2 nm) and mesopores (20 to 50 nm), which was 5.5 times larger than that of the two-dimensional AHNS-rGO layer (0.0048 cm 3 /g). The pore volume (0.0262 cm 3 /g) was shown. In addition, it had a smaller pore size of 17.2 nm due to the development of micropores and mesopores.

-Analysis of morphological characteristics

The morphological characteristics and uniformity of the carbon-containing coating layer formed on the NCM were analyzed using SEM and TEM. 6A to 6F are SEM photographs of BNCM, GNCM and GCNCM at low and high magnifications.

As shown in FIGS. 6A and 6B, BNCM is composed of hundreds of nanometer-level NCM primary particles, and forms secondary particles having a diameter of 10 to 15 μm.

6C and 6D are SEM images of low and high magnification of GCNCM. According to the figure, the AHNS-rGO layer completely covers the entire NCM particles. 6G and 6H are cross-sectional TEM photographs of BNCM and GNCM particles. In the case of GNCM, as shown in Fig. 6H, the surface of the NCM is completely covered with the carbon layer of the densely stacked AHNS-rGO sheet, and the thickness of the carbon layer is about 7 nm. The diffusion of lithium ions through the carbon layer of the densely stacked AHNS-rGO sheet can be hindered by the narrow and long diffusion path within the carbon layer.

6E and 6F are SEM photographs of GCNCM. As can be seen from Fig. 6f, CNTs are well mixed with AHNS-rGO and uniformly coat NCM. In contrast to GNCM, GCNCM showed a rougher surface.

6I is a cross-sectional TEM image of GCNCM particles. According to the above drawing, unlike the carbon layer of the two-dimensional structure of GNCM (FIG. 6H), the carbon layer of GCNCM exhibits a hierarchical three-dimensional porous structure. This is because CNTs are inserted into the AHNS-rGO sheet to suppress stacking, thereby resulting in a number of wrinkles, forming a non-densely packed AHNS-rGO sheet. As such, the addition of CNT induces small wrinkles in the AHNS-rGO layer and induces the formation of micropores and mesopores. The carbon layer thickness (30 nm) of GCNCM was larger than the carbon layer thickness (7 nm) of GNCM due to the formation of a three-dimensional network structure, and at this time, the CNT is inserted between the AHNS-rGO layers (FIGS. 6h and 6i). .

The above-described SEM and TEM analysis supports that the carbon layer is uniformly coated on the surface of the NCM, and the CNT is inserted into the AHNS-rGO layer, thereby forming a three-dimensional carbon layer.

-Electrochemical performance analysis

CV and electrostatic charge-discharge (GCD) measurements were performed to evaluate the effect of three-dimensional carbon coating on the electrochemical performance of NCM.

7A shows CV curves of BNCM, GNCM, and GCNCM electrodes in a potential band of 2.8 to 4.3 V. According to the figure, all electrodes exhibited three pairs of redox peaks. In this case, the potential interval ΔV of the redox peak may indicate the electrochemical reversibility and the degree of polarization of the electrode. Compared to the CV curve of the BNCM electrode ( ΔV _BNCM = 0.155 V), the positive and negative peaks of the GNCM electrode were respectively widened and shifted to a larger and lower potential ( Δ V _GNCM = 0.214 V), which was polar Means increase. Although the conductivity of GNCM is higher than that of BNCM, the GNCM electrode showed higher polarity than the BNCM electrode, which inhibits the diffusion of lithium ions from the bulk electrolyte to the NCM surface (AHNS densely packed on the NCM surface). -rGO sheet). On the other hand, the GCNCM electrode exhibited a narrower redox peak width and spacing ( ΔV _GCNCM = 0.102 V) between the anode peak and the cathode peak compared to other samples, indicating the lowest polarity. This is due to the conductive porous three-dimensional carbon network layer formed on the surface of the NCM, and can promote the transport of electrons and lithium ions. As such, the GCNCM electrode is expected to achieve excellent rate performance.

Meanwhile, FIGS. 8A to 8C are CV curves of the BNCM, GNCM, and GCNCM electrodes (scanning speed: 0.1 to 1 mV/s, voltage range: 2.8 to 4.3 V). Referring to the figure, as the scanning speed increases, the potential interval (ΔV) of the redox peak becomes larger, indicating that the redox reaction is controlled by diffusion. In addition, as the polarity of the electrode increases, the shift of the redox peak also increases. Thus, for electrodes with high polarity, this peak shift can limit some redox reactions at high scan rates. In FIG. 8B, compared with the BNCM electrode (FIG. 8A), each of the anode peak and cathode peak of the GNCM electrode was wider and shifted to a higher potential and a lower potential, because it has a higher electrode polarity than BNCM. to be. In addition, the redox peak current of GNCM slightly increased as the scanning speed increased, because the redox reaction was hampered by the densely stacked AHNS-rGO sheets on the NCM surface. On the other hand, in the case of the GCNCM electrode, the interval between the anode peak and the cathode peak was smaller than that of other types, indicating that the polarity of the electrode was rapidly reduced due to rapid charge transport by the highly conductive 3D carbon layer formed on the NCM surface. it means. However, in the case of coating only with AHNS-rGO, a two-dimensional dense layer of AHNS-rGO is formed on the surface of the NCM, so that although electron conduction is improved, the electrode polarity is increased by interfering with lithium ion diffusion.

In order to compare the rate-limiting characteristics of the samples, a charge-discharge test was performed in the CC/CV mode.

Figure 7b shows the initial charge-discharge profile (0.1 C, 2.8 to 4.3 V potential range) of the electrode, the initial charge and discharge efficiencies of each of BNCM, GNCM and GCNCM were 89.7%, 90.6%, and 92.3%. In contrast to BNCM, GCNCM exhibited higher initial charge/discharge efficiency, which is due to the conductive three-dimensional AHNS-rGO layer formed on the NCM.

9A to 9D show discharge profiles of electrodes according to various C, and specific discharge capacities of BNCM, GNCM, and GCNCM electrodes at 0.1 C are 200 mAh/g, 196 mAh/g, and 195 mAh, respectively. /g. The reason that the specific discharge capacity of GNCM and GCNCM is somewhat lower than that of BNCM is due to the carbon content, and did not contribute to the capacity in the potential range of 2.8 to 4.3 V. In the case of the GNCM electrode (FIG. 9B), compared to the BNCM electrode, the IR drop rapidly increased as C increased. In addition, the GNCM electrode exhibited lower lithiation potential and discharge capacity than the BNCM electrode, because polarization induced from inhibition of lithium ion diffusion increased. On the other hand, the GCNCM electrode exhibited lower IR drop and higher reduction potential compared to other electrodes at the same C (Figs. 9A and 9C). This is because the conductivity was increased by coating with a high conductivity AHNS-rGO/CNT composite layer. In spite of the high conductivity of AHNS-rGO, the densely stacked two-dimensional structure prevented the diffusion of lithium ions from the bulk phase electrolyte, resulting in low rate-rate characteristics. On the other hand, the three-dimensional porous carbon network layer having high conductivity exhibited excellent rate-limiting properties because it not only increased the conductivity of NCM, but also reduced polarity by providing a diffusion channel, and promoted the diffusion of lithium ions. As such, the GCNCM electrode exhibited a specific discharge capacity (10 C) about 5 times higher than that of BNCM.

Referring to FIG. 7D, the GCNCM electrode exhibited excellent cycle performance (88% capacity retention after 100 cycles) (BNCM and GNCM electrodes 65% and 49%, respectively). In particular, the GNCM electrode showed the lowest cycle performance, which can be attributed to the stacking of AHNS-rGO. The stacked AHNS-rGO sheet inhibits the diffusion of lithium ions from the bulk phase electrolyte by the bottleneck effect and causes polarization of the electrode, thereby causing irreversible phase shift and side reactions with the electrolyte. Unlike the stacked two-dimensional carbon layer, the AHNS-rGO/CNT composite, which provides a carbon network having a conductive three-dimensional structure, provides an abundant electron/ion hydrogen channel to improve conductivity and lithium ion diffusivity.

-Cycling characteristic analysis

EIS was performed to further analyze the cause of the improvement of the electrochemical performance of the GCNCM electrode. 7E and 7F show Nyquist plots of BNCM, GNCM and GCNCM electrodes after 1 cycle and 100 cycles (inset figure is an equivalent circuit model of the Nyquist plot). The resistance values of the electrodes calculated from the Nyquist plot are shown in Table 1 below.

According to FIGS. 7E and 7F, the GNCM electrode exhibited higher R _sf (38.7 Ω) and R _ct (29.1 Ω) values than other electrodes, due to inhibition of lithium ion diffusion and reaction with the electrolyte. On the other hand, it can be seen that GCNCM forms a thin anode-electrolyte interface (CEI) by promoting lithium ion diffusion and suppressing side reactions through a three-dimensional network structure. In the case of Figure 7f, after 100 cycles, a Nyquist plot of the BNCM, GNCM and GCNCM electrodes is shown. In this regard, R _ct can be seen as a major factor in the decrease in the capacity of the electrode. In the case of GCNCM, side reactions are limited by the surface protective layer, and the reversible electrochemical reaction is maintained by the increased conductivity. Therefore, the R _ct of the GCNCM electrode did not increase significantly compared to the BNCM electrode, and in particular, the R _ct value was the smallest even after 100 cycles compared to the other electrodes.

Meanwhile, FIGS. 10A to 10F show SEM and TEM images of the BNCM and GCNCM electrodes after 100 cycles, respectively. Referring to FIGS. 10A and 10B, the lithiation/delithiation process generated micro-cracks in the NCM particles, because a volume change occurs during the process. When micro-cracks are formed, the primary particles are physically separated, the electrical path is cut off, and resistance is increased. In addition, the electrolyte penetrates into the inner space of the NCM generated by the micro-cracks to generate an inert phase similar to NiO by side reactions, thereby promoting surface deterioration and causing a decrease in the capacity of the NCM. On the other hand, in the case of GCNCM, the separation of primary particles during the cycling process is suppressed (FIGS. 10C and 10D). In addition, even if a crack occurs, an electron transport path is provided between the separated particles due to the three-dimensional carbon network structure, and penetration of the electrolyte through the crack can be suppressed.

10E and 10F are cross-sectional TEM photographs of BNCM and GCNCM electrodes after 100 cycles. The changes in crystal structure caused by cationic disorder were analyzed by observing the surface of the primary particles in the two samples. As a result of observing the TEM image from the surface to the bulk phase, three areas separated by red and yellow lines for both BNCM and GCNCM samples can be confirmed. Inside the red line, the layered structure is the outermost surface layer with some degree of disorder and is converted into the rock-salt phase. In the area between the red line and the yellow line, the layered structure becomes disordered but remains weak. The yellow interior is an area where the layered structure is well maintained. Compared to the thickness of the disordered region of BNCM (<30 nm), the thickness of GNCM (<10 nm) was much thinner. This means that the phase change propagated from the surface is suppressed.

The structural changes from stratified to disordered and rock-salt originate from cation mixing and are supported by FFT photographs (insets in FIGS. 10E and 10F). Each FFT picture was taken of three areas for BNCM and GNCM. This result is because the carbon layer not only improves the conductivity, but also protects the NCM surface by minimizing the contact between the NCM and the electrolyte.

After cycling 100 times, XPS analysis was performed to determine the component of the electrode surface layer, and XPS Ni 2p spectra of each of BNCM and GCNCM are shown in FIGS. 11A and 11B. The Ni ²⁺ and Ni ³⁺ peaks result from the presence of NiF ₂ and LiNi _1-x M _y O ₂ respectively. As shown, the Ni ²⁺ /Ni ³⁺ ratio to GCNCM was significantly lower than that of BNCM, suggesting that the carbon layer effectively suppressed the side reaction between NCM and the electrolyte.

As shown in FIGS. 11C and 11D, the intensity of the LiF peak formed on the surface of GCNCM was much lower than that of BNCM, which means that the 3D carbon coating layer effectively suppressed the formation of LiF. In addition, since NiF ₂ and LiF are insulators, as the thickness of the CEI layer increases, the movement of electrons and lithium ions is further hindered, thereby increasing the width of capacity reduction. 6E and 6F show XPS C 1s spectra of BNCM and GCNCM, respectively. The deconvolved peak at 289.5 eV supports the presence of Li ₂ CO ₃ and ROCO ₂ Li on the electrode. The other peaks at 284.5 eV, 285.5 eV, 286.5 eV, 288.1 eV, 289 eV, and 290.3 eV correspond to CC/C=C, CH ₂ /CN, CO, C=O, OC=O, and CF _{2 respectively} This is generated from the conductive carbon materials (Super P, AHNS-rGO and CNT) and the binder (PVdF) of the electrode. The peak area of Li ₂ CO ₃ /ROCO ₂ Li at 289.5 eV for BNCM was remarkably larger than that of GCNCM, indicating that the 3D carbon layer inhibited electrolyte decomposition on the electrode surface to prevent thickening of the CEI layer. . By suppressing side reactions, electrons and lithium ions can easily pass through the surface layer during the redox reaction, thereby suppressing capacity reduction.

Meanwhile, the function of improving the electrochemical stability of the NCM by the carbon network layer having a conductive three-dimensional structure in the embodiment may be described as illustrated in FIG. 12.

Referring to the above drawing, the carbon network layer having a conductive three-dimensional structure is configured to maintain electrical connection between primary particles when (i) micro-cracks occur due to volume change during the repeated cycle process, and (ii ) By minimizing the contact between the NCM and the electrolyte and preventing the electrolyte from penetrating into the micro-cracks, it reduces irreversible phase transitions on the NCM surface, and (iii) prevents side reactions at the electrode/electrolyte interface by surface protection Can be suppressed.

Simple modifications or changes of the present invention can be easily used by those of ordinary skill in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (20)

a) Surface modification of graphene oxide using an amino group-containing compound that bonds with carbon atoms while ringing the epoxy group in the graphene oxide so that the surface of the graphene oxide has a negative (-) charge in the aqueous medium ;
b) forming reduced graphene oxide (rGO) by reducing the surface-modified graphene oxide;
c) preparing a first dispersion containing the reduced graphene oxide and conductive nanoparticles in a liquid medium;
d) concentrating the first dispersion to form a second dispersion in the form of a slurry;
e) combining the second dispersion and metal oxide particles to form a third dispersion; And
f) removing the liquid medium from the third dispersion;
Manufacturing method of a composite material comprising a. The method of claim 1, wherein the amino group-containing compound is at least one selected from compounds represented by the following general formulas 1 and 2, or a salt thereof:
[General Formula 1]
NH ₂ -R ₁ -SO ₃ H
[General Formula 2]
NH ₂ -R ₂ -PO ₃ H ₂
In the above formula, R ₁ and R ₂ are independently a C 1 to C 12 alkylene group, an aromatic ring substituted or unsubstituted C 6 to C 18 arylene group (wherein the substituent is a C 1 to C 3 alkyl group, a hydroxyl group, a ketone group , A carboxyl group, or a halogen group). The method of claim 1, wherein the metal oxide is a nickel-cobalt-manganese oxide (NCM). The method of claim 3, wherein the metal oxide is represented by the following general formula 3:
[General Formula 3]
Li(Ni _x Co _y Mn _z )O ₂
In the above formula, x is 0.3 to 0.95, y is 0.025 to 0.35, and z is 0.025 to 0.35, and x+y+z=1. The method of claim 2, wherein the amino group-containing compound is aminomethanesulfonic acid, 2-aminoethanesulfonic acid, and 3-amino-1-propanesulfonic acid. propanesulfonic acid), sulfanilic acid, 6-amino-4-hydroxy-2-naphthalenesulfonic acid, 4-amino-3-hydroxy-1 -Naphthalenesulfonic acid (4-amino-3-hydroxy-1-naphthalenesulfonic acid), 4-amino-3-methylbenzenesulfonic acid, sodium 5-amino-9,10-di Hydroanthracene-1-sulfonate (sodium 5-amino-9,10-dihydroanthracene-1-sulfonate), (4-aminophenyl) phosphonic acid ((4-aminophenyl)phosphonic acid), aminomethylphosphonic acid ((Aminomethyl) phosphonic acid), 3-aminopropylphosphonic acid, 4-aminobutylphosphonic acid ((4-aminobutyl)phosphonic acid), L-2-amino-4-phosphonobutyric acid (L-2-amino -4-phosphonobutyric acid), and monosodium 4-amino-1-hydroxy-1-phosphonobutylphosphonate trihydrate (Monosodium (4-Amino-1-hydroxy-1-phosphonobutyl) phosphonate Trihydrate) Method for producing a composite material, characterized in that at least one selected from the group. The method of claim 5, wherein the amino group-containing compound is 6-amino-4-hydroxy-2-naphthalenesulfonic acid. The method of claim 1, wherein the step b) is performed by thermal reduction or a chemical reduction reaction using a reducing agent. The method of claim 7, wherein the thermal reduction comprises the step of thermally treating graphene oxide under an inert gas atmosphere, and
The chemical reduction reaction is a reducing agent, hydrazine (hydrazine), hydrazine hydrate (hydrazine hydrate, N ₂ H ₄ .H ₂ O), sodium borohydrate (NaBH ₄ ), ammonia borane (NH ₃ BH ₃ ), Characterized in that it is performed using at least one selected from ethylenediamine, thiourea, hydroionic acid, L-ascorbic acid, urea, hydroquinone, and benzyl amine A method for producing a composite material. The method of claim 1, wherein the liquid medium used in step c) is at least one selected from the group consisting of alcohol, methylpyrrolidone, dimethylformamide, and toluene. The method of claim 1, wherein the conductive nanoparticles contained in the first dispersion in step c) are at least one selected from the group consisting of carbon nanotubes (CNT), carbon black, and conductive metal nanoparticles, and
The method of manufacturing a composite material, wherein the conductive nanoparticles are contained in an amount of 10 to 300% by weight, based on the weight of the reduced graphene oxide. The composite material of claim 10, wherein the carbon nanotubes have an average diameter of 3 to 20 nm, an average length of 1 to 100 μm, and a ratio of length to diameter (surface ratio) of at least 100. Manufacturing method. The method of claim 10, wherein the conductive metal nanoparticles are made of copper (Cu), silver (Ag), gold (Au), platinum (Pt), palladium (Pd), nickel (Ni), rhodium (Rh), and combinations thereof. It is selected from the group consisting of, the size of the method for producing a composite material, characterized in that in the range of 1 to 100 nm. The method of claim 1, wherein the concentration of the reduced graphene oxide in the first dispersion is 1 to 20 mg/ml, and the concentration of the reduced graphene oxide in the second dispersion is in the range of 20 to 100 mg/ml. Method for producing a composite material characterized in that. The method of claim 1, wherein the metal oxide is in the form of particles, the size of which ranges from 1 to 50 μm,
The method for producing a composite material, wherein the metal oxide in the third dispersion is contained in an amount ranging from 1000 to 10000% by weight, based on the reduced graphene oxide. The method of claim 1, wherein the step e) comprises the step of combining the second dispersion and the metal oxide particles over 3 to 15 minutes under agitation and temperature conditions of 10 to 40°C. Manufacturing method. Metal oxide particles; And
An encapsulation layer formed on the metal oxide particles and including reduced graphene oxide and conductive nanoparticles;
As a composite material comprising a,
Based on the weight of the composite material, the content of the carbon component is in the range of 0.5 to 10% by weight,
The encapsulation layer has a thickness of 15 to 40 nm, and includes 30 to 70% by weight of reduced graphene oxide and 70 to 30% by weight of conductive nanoparticles, based on the weight of the encapsulation layer, and
The composite material is a composite material having a specific surface area (BET) in the range of 1 to 50 m 2 /g and a pore volume in the range of 0.005 to 1 cm 3 /g. The method of claim 16, wherein the metal oxide particles are made of nickel-cobalt-manganese oxide (NCM),
At this time, the metal oxide particle is a composite material, characterized in that the form of primary particles or secondary particles formed by collecting the primary particles. The composite material of claim 16, wherein the composite material has a pore size in the range of 5 to 40 nm. 17. The composite material of claim 16, wherein the composite material exhibits a conductivity of at least 0.1 S/m. A cathode material for a secondary battery comprising the composite material according to any one of claims 16 to 19.

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