KR102365020B1 - Method for Preparing Composite Materials Using Slurries Containing Reduced Graphene Oxide - Google Patents

Abstract

In the present disclosure, a reduced graphene oxide is prepared by reducing graphene oxide whose surface has been modified to maintain good dispersibility even after reduction, and a metal oxide, for example (lithium)-nickel-, is prepared as a slurry containing the same. A method for manufacturing 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 cobalt-manganese-based oxide (NCM), and a cathode material for a lithium secondary battery using the same are described.

Description

Method for Preparing Composite Materials Using Slurries Containing Reduced Graphene Oxide

The present disclosure relates to a method for manufacturing a composite material using a slurry containing reduced graphene oxide (rGO). Specifically, the present disclosure prepares reduced graphene oxide by reducing graphene oxide whose surface has been modified to maintain good dispersibility even after being reduced, and a metal oxide, for example (lithium)- A method for manufacturing a composite material by coating or encapsulating the surface of a nickel-cobalt-manganese based oxide (NCM), particularly a metal oxide with low stability, such as nickel-rich NCM, and a lithium secondary battery with improved output and lifespan stability It is about cathode materials.

Lithium secondary batteries are used as power sources for small portable equipment because of their high energy density and light weight characteristics. there is.

Lithium ion secondary battery using lithium ion as a secondary battery has a tendency to expand from a small mobile device power source to a power source for mid- to large-sized electric vehicles (eg EV, HEV, etc.) and energy storage device (ESS). is in In particular, interest in electric vehicles, which are eco-friendly vehicles, is very high, and major automakers around the world are accelerating the development of related technologies by recognizing electric vehicles as the next-generation growth technology under the motto of eco-friendliness.

The development of high-capacity lithium-ion secondary batteries is essential to be used in medium and large-sized applications such as electric vehicles. In general, a cathode of a lithium ion secondary battery includes a cathode material, a conductor, a binder, and a current collector. In particular, a cathode material having excellent reversibility, low self-discharge rate, high capacity, and high energy density is required. . As such a cathode material, 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 in particular, 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 (HEV) and electric vehicles (EV) because of its high performance and good stability as a cathode material. Demand is also constantly increasing. In particular, as research for mass production of high-quality NCMs is being actively conducted, cathode materials in which nickel:manganese:cobalt are mixed in a 1:1 ratio are mainly used.

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

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

In this regard, methods for 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, nickel (Ni)-rich NCMs (especially LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) with a molar ratio of Ni:Co:Mn of 8:1:1) have poor thermal stability, and surface It can be difficult to apply a carbon coating because impurities such as residues of lithium synthesis from the solvent react with the solvent to leave an electrochemically inactive material on the surface or because an exchange between hydrogen ions and lithium ions occurs. Moreover, under high-temperature treatment conditions, not only graphene but also metal oxides are reduced. In order to avoid this, when heat treatment is performed at a relatively low temperature, additional problems such as not being able to sufficiently secure the sp ² structure of graphene are also induced. .

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

In one embodiment of the present disclosure, a composite material in which a carbon-containing coating layer having 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 for manufacturing the same.

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

According to the first aspect of the present disclosure,

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

b) reducing the surface-modified graphene oxide to form reduced graphene oxide (rGO);

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 the metal oxide particles to form a third dispersion; and

f) removing the liquid medium from the third dispersion;

There is provided a method for manufacturing a composite material comprising a.

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 or without an aromatic ring substituted. (Here, the substituent may be an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group, a carboxy 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 a second aspect of the present 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:

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 comprises 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 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 including 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 the epoxy group contained in the graphene oxide to bond with a carbon atom while ring-opening and to have a negative charge in an aqueous medium. Since the negative charge can be maintained even after the reduction treatment, it is possible to prepare a slurry that can solve the problem of difficult dispersibility of conventional reduced graphene oxide (rGO) by increasing the dispersion effect using electrical repulsion, and 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 a conductive three-dimensional carbon network layer can be formed on the surface of the metal oxide, it promotes the transport of electrons and ions and enhances the depolarization of the metal oxide, thereby providing stable electrochemical properties at a high rate. reaction can be achieved. Furthermore, by combining the reduced graphene oxide with conductive nanoparticles, the movement of lithium ions is hindered by incomplete coating resulting from the stacking and aggregation phenomena in which the reduced graphene oxide in the form of a sheet is overlapped again in the subsequent drying process. phenomenon can be suppressed. In addition, due to the three-dimensional carbon network layer, it is possible to suppress the electrical separation phenomenon due to micro-cracks in the metal oxide, the irreversible phase transition, and the side reaction with the electrolyte that may occur during the charge/discharge process.

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

1 is a diagram schematically illustrating a synthesis process and structure of a composite material encapsulated with 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 Examples;
3 is an XRD pattern (a), Raman spectrum (b), and XPS C 1s (c), Ni 2p (d), Co 2p (e), and Mn 2p ( f) is the spectrum;
Figure 4a is the Raman spectra of GO and AHNS-rGO;
Figure 4b is the XPS C 1s spectrum of GO and AHNS-rGO;
5 is a TGA plot (a), conductivity (b), adsorption-desorption isotherm (c), and pore size distribution (d) of BNCM, GNCM, and GCNCM;
6 is an SEM picture of BNCM (a,b), GNCM (c,d) and GCNCM (e,f), and a TEM picture of BNCM (g), GNCM (h) and GCNCM (i);
7 shows CV curves 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 is the equivalent circuit for EIS data fitting),
8 is a CV curve of BNCM (a), GNCM (b) and GCNCM (c) electrodes at scan rates ranging from 0.1 to 1 mV/s;
9 is a discharge profile of BNCM (a), GNCM (b) and GCNCM (c) electrodes at C-rates in the range of 0.1 to 10 C;
10 is a SEM picture of BNCM (a,b) and GCNCM (c,d) after 100 cycles, and a TEM picture of BNCM (e) and GCNCM (f) after 100 cycles (inset 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 of a three-dimensional structure.

The present invention can all be achieved by the following description. It is to be understood that the following description describes 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 regarding individual configurations may be properly 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 high specific surface area, thin film and conductivity. Typically, graphene oxide has insulating properties, and the sheet resistance exhibits 10 ¹² Ω/sq or more. However, when graphene oxide is reduced, the sheet resistance of rGO is significantly reduced and can be converted into a conductor or semiconductor.

"Carbon nanotube (CNT)" may mean a one-dimensional nanostructure in which a purely sp ² -bonded hexagonal carbon network is rolled (rolled) into a tubular or cylindrical structure having a nano size diameter. , single-walled carbon nanotubes (SWCNTs), depending on the number of bonds making up the walls or the number of graphite layers (single atom thick flat sheets of sp ² -bonded carbon atoms with a honeycomb crystal lattice structure). It is divided into double-walled carbon nanotubes (DWCNT) and multi-walled carbon nanotubes (MWCNT). In particular, SWCNTs have a diameter of about 1 nm, and can have a length of a significantly higher ratio (for example, 132,000,000 times) to the diameter, and DWCNTs have a diameter of about 5 to 10 nm, and larger nanotubes. It may contain one concentric SWCNT inside, and the MWCNT may contain two or more concentric CNTs inside the larger outer nanotube. CNTs can be prepared through a conventional arc process (arc discharge), laser ablation (ablation), CVD (Chemical Vapor Deposition), CCVD (Catalytic Chemical Vapor Deposition), such as methods.

"Encapsulation" may refer to a physicochemical or mechanical process for trapping an object within a particular material, wherein the core material may have a form 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.

Also, as used herein, "composite" or "composite material" may refer to a combination in which two or more individual components are mixed in different proportions to form a final material having properties different from those of the individual components.

It may be understood that the expressions “on” and “on” are used to refer to the concept of relative position. Accordingly, not only the case where other components or layers are directly present in the mentioned layers, but also other layers (intermediate layers) or components may be interposed or present therebetween. Similarly, the expressions “under”, “under” and “below” and “between” may also be understood as relative concepts with respect to location. Also, the expression “sequentially” may be understood as a concept of relative position.

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

Full disclosure

According to one embodiment of the present disclosure, reduced graphene oxide is converted into a liquid medium (for example, by giving a specific functional group (or functional group) through surface treatment during the production process of reduced graphene oxide (rGO)) A slurry well dispersed in absolute alcohol, specifically absolute ethanol) can be prepared. Using the reduced graphene oxide-containing slurry prepared as described above, it can be uniformly coated or encapsulated on a metal oxide, particularly a cathode active material such as nickel (Ni)-rich NCM having low stability.

It should be noted 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 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 changing metal oxides such as nickel-rich NCM with low thermal stability. In particular, when metal oxide particles such as NCM are brought into contact with the reduced graphene oxide slurry, the surfaces of the metal oxide and the reduced graphene oxide both show negative charges, so that the dispersion power of the particles in the slurry is improved by mutual electrical repulsion. A more uniform coating or encapsulation can be implemented.

According to one embodiment of the present disclosure, the reduced graphene oxide slurry further contains conductive nanoparticles, and these conductive nanoparticles inhibit the restacking of the graphene sheets to maintain a gap between the sheets, so that lithium ions are 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. can be converted into a network of structures, resulting in the formation of a carbon-containing network with higher conductivity (ie, the conductivity of the carbonaceous network can be further improved through the addition of conductive nanoparticles).

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

Hereinafter, a method for 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 hydroxyl group) as shown in the following Structural Formula 1, specifically single-layer graphite oxide (CO covalent bond introduced) By doing so, it may mean a two-dimensional material derived from graphene).

[Structural Formula 1]

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

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

According to an exemplary embodiment, the size of the graphene oxide may be, 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 an aqueous dispersion of concentrations. In order to prepare a uniform dispersion, a known grinding and/or homogenizing means may be used, for example, ultrasonic grinding, a homogenizer, or the like.

With respect to the graphene oxide dispersion prepared in this way, even if the carboxyl group in the reduced graphene oxide (rGO) obtained by subsequent reduction is reduced, a specific compound, that is, an amino group, is used to open the epoxy group in the graphene oxide to obtain good dispersibility. As a result of bonding with carbon atoms, surface modification is performed using an amino group-containing compound that allows 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 in Scheme 1 exemplified 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 formulas 1 and/or 2 (specifically, monomolecular compounds) or salts thereof (eg, alkali metals, alkalis 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 or without an aromatic ring substituted. (Here, the substituent may be an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group, a carboxy group, or a halogen group).

Representative examples of the above-described amino group-containing compound include aminomethanesulfonic acid, 2-aminoethanesulfonic acid, 3-amino-1-propanesulfonic 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 (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), and the like, One or two or more may be used in combination. According to a specific embodiment, 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 may be provided in the form of an aqueous solution, and may be combined with an aqueous dispersion of graphene oxide. In this case, 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, more specifically about 15 to 20% by weight, but this is for illustrative purposes only. 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, wherein the base to be added is ammonia (or ammonium hydroxide), sodium hydroxide, potassium hydroxide or It may be a combination thereof. At this time, the addition amount of the base is, based on the weight of the amino group-containing compound, for example, about 3 to 20% by weight, specifically about 4 to 15% by weight, more specifically about 5 to 10% by weight. 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 added in the graphene oxide dispersion is, based on the graphene oxide, for example, about 100 to 1000 weight. %, specifically from about 200 to 800% by weight, more specifically from about 300 to 500% by weight.

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

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

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

In this regard, a thermal reduction method is mentioned as a physical reduction. Illustratively, graphene oxide is subjected to instantaneous 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 be done.

Alternatively, a chemical reduction reaction using a reducing agent may be applied. As such a reducing agent, hydrazine, 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, etc. 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 and the like, then separated and dispersed again in an aqueous medium.

According to an exemplary embodiment, in the chemical reduction method using 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 wt%.

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

When the surface-modified graphene oxide is subjected to reduction treatment as described above, oxygen-containing groups such as carboxyl groups present in the graphene oxide are removed as shown in Scheme 2 below to recover sp ² bonds, whereas via an amino group Moieties of the bound surface modifying compound (eg, in the case of AHNS) remain.

[Scheme 2]

According to the above reaction formula, in the graphene oxide provided in step (a), in step (b), the amine group of AHNS attacks the epoxy group in the graphene oxide to open the ring by covalent interaction, and bond with a carbon atom ( step (c)). Thereafter, in step (d), as a reduction treatment using hydrazine hydrate (monohydrate) as a reducing agent, a significant portion of carboxyl groups and hydroxyl groups present in graphene oxide are removed, and SO ₃ H groups present in AHNS pass through amino groups. are combined These SO ₃ H groups are ionized in an aqueous medium to exhibit a negative charge (SO ₃ ⁻ ), thereby effectively suppressing the aggregation tendency of conventional reduced graphene oxide particles.

If the reduction treatment is performed as described above, the residual reactants in the dispersion may be removed, and a conventional post-treatment procedure (eg, water washing, physical or chemical separation, etc.) may be performed for separation/purification. Illustratively, during physical or chemical separation, filtering, centrifugation, dialysis, etc. may be applied, and specifically, 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, during the subsequent step, metal oxides such as NCM are vulnerable to moisture and deteriorate and the surface may be deactivated. To remove the residual moisture, the solid content of the reduced graphene oxide is redispersed in anhydrous alcohol (eg, absolute ethanol), methylpyrrolidone (N-methyl-2-pyrrolidone), dimethylformamide (Dimethylformamide), toluene, etc. This process may 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, it is dispersed in a liquid medium to prepare a dispersion (first dispersion). 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 (Dimethylformamide), toluene, etc. there is. According to an exemplary embodiment, the liquid medium may be absolute alcohol, and more specifically, absolute ethanol. At this time, absolute ethanol is a general-purpose 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 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 ml can be adjusted.

In addition, in one embodiment, conductive nanoparticles are included together with reduced graphene oxide in the first dispersion. These conductive nanoparticles can 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 when the composite material is applied as a cathode material by restacking the reduced graphene oxide in the form of a sheet in the subsequent drying process. In this regard, the conductive nanoparticles may be of a type that can be easily attached to the graphene sheet through interactions such as π-π bonds, van der Waals forces, and hydrophobic bonds. The conductive nanoparticles may be typically carbon nanotubes (CNTs), carbon black, conductive metal nanoparticles (nanowires), and the like, and one or two or more thereof may be used in combination.

- CNT

In the case of CNT, it may include SWCNT, DWCNT, MWCNT, or a combination thereof. However, as the ease of dispersion is affected by the CNT type, it may be advantageous to use MWCNTs. In addition, the shape of the CNTs 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 CNTs 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 (aspect ratio) may range from at least about 100, specifically from about 200 to 3000, and more specifically from about 400 to 1500. In addition, the specific surface area of the CNTs may be, for example, in the range of about 50 to 1315 m2/g, specifically about 100 to 1000 m2/g, and more specifically about 200 to 500 m2/g. In particular, when CNTs are used, the porosity of the reduced graphene oxide coating layer is improved, thereby providing the advantage of increasing the movement path of lithium ions.

- Carbon Black

In general, a six-membered carbon ring is formed by incomplete combustion or thermal decomposition of hydrocarbons, and then carbon black crystallites having a hexagonal carbon network structure are formed through a polycyclic aromatic compound by a process such as dehydrogenation condensation. is referred to as Carbon black has a two-dimensional structure compared to conventional graphite having a three-dimensional structure. An exemplary atomic structure model of carbon black may be represented by Structural Formula 2 below.

[Structural Formula 2]

Such carbon black may be Furnace carbon black and/or Thermal carbon black, for example, in the form of particulates made of amorphous carbonaceous material, the size of which is, for example, about 10 to 100 nm, specifically about 30 to 80 nm, 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, for example, a material having a conductivity of 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 its It may be selected from the group consisting of combinations, specifically silver (Ag). Meanwhile, the size of the conductive metal nanoparticles may be, 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 above-described conductive nanoparticles, 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 wt%. However, when an excessively large amount is used, the capacity of the cell may be reduced due to the lowering of the specific gravity of the active material in the electrode, so it may be advantageous to properly use it within the above-described range.

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

Carbon coating on metal oxide

After preparing a high-concentration slurry-type dispersion (second dispersion), it may be combined with a metal oxide and coated.

According to an exemplary embodiment, in the form of particles, the metal oxide may be a primary particle or a secondary particle, and the size (diameter) thereof is, for example, about 1 to 50 μm, specifically As a result, 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, more specifically about 100 to 1000 nm. can

In a specific embodiment, 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

wherein 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 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 formulated in a ratio of about 8:1:1, this metal oxide is referred to as NCM811. is becoming

At this time, the surface of a metal oxide such as NCM may exhibit a negative charge because a hydroxyl group is generated on the surface in contact with the atmosphere. Therefore, since the reduced graphene oxide exhibiting a negative charge and an electrical repulsive or repulsive force are generated in the liquid medium, coating using the slurry can be performed in a state in which the particles in the slurry are uniformly dispersed.

In particular, aggregation of the conductive nanoparticles and the reduced graphene oxide added prior to the combination with the metal oxide may occur. However, as described above, as a result of the improvement in dispersing power by the surface modification of graphene oxide, the metal oxide particles and the conductive nanoparticles may be uniformly mixed.

According to an exemplary embodiment, a third dispersion is prepared by adding (combining) metal oxide particles to the second dispersion, and this combination or mixing process is performed under stirring conditions (eg, about 50 to 300 rpm, specifically about 100 to 200 rpm), for example, about 3 to 15 minutes, specifically about 5 to 10 minutes. Also, 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, based on the reduced graphene oxide, for example, about 1000 to 10000% by weight, specifically about 2000 to 8000% by weight, more specifically about 4000 to 6000% by weight It may be in the range .

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 40 to 80 ° C. (specifically about 50 to 70 ° C., more specifically about 55 to 65 ° C) 10 minutes) can be heated.

Optionally, for the purpose of substantially completely removing the residual liquid medium, it may be further treated by known drying means, for example in a vacuum oven. In this case, 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 is, for example, at least about 5 hours, specifically at least about 6 to 10 hours. can be These processing means and conditions can be understood as illustrative.

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 the metal oxide particles can 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 the aggregation of primary particles, and the composite material includes the above-mentioned carbon-containing 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 has, for example, 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 containing (ie, 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 conductive nanoparticles used, the amount of reduced graphene oxide used, the type and size of metal oxide, and the like.

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. An exemplary composition of the 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 about 70 to 30% by weight of nanoparticles (specifically about 60 to 40% by weight, more specifically about 55 to 45% by weight), which can be understood as illustrative.

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

In addition, the specific surface area (BET) of the composite material may be, for example, in the range of about 1 to 50 m2/g (specifically about 2 to 30 m2/g, more specifically about 4 to 15 m2/g), The volume may range, for example, from about 0.005 to 1 cm 3 /g (specifically from about 0.01 to 0.5 cm 3 /g, more specifically from about 0.02 to 0.1 cm 3 /g). In this regard, the specific surface area and pore volume of the coating layer not containing the conductive nanoparticles are relatively low, but in the case of the coating layer including the combination of reduced graphene oxide and conductive nanoparticles, the specific surface area and pores increased to a more significant level. 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, 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, in contrast 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 three-dimensional carbon-containing network layer described above is rate-limiting. Significantly improved results can be obtained in terms of properties and cycle stability. As an example, the rate-rate characteristic may increase, for example, by at least about 200%, specifically by at least about 300%, more specifically by at least about 400%, and the cycle stability may increase, for example, by at least about 20%, specifically By at least about 30%, more specifically, it can be improved by at least about 35%.

Uses of Composite Materials

According to another embodiment of the present disclosure, the composite material in which the carbonaceous coating layer is formed on the metal oxide may be applied as a cathode material such as a cathode active material of a secondary battery, specifically, a lithium secondary battery. That is, it may be used as a material for storing and/or releasing lithium ions in the positive electrode of a lithium secondary battery. In particular, as the electrical conductivity increases as the electrical conductivity increases due to the role of the conductive nanoparticles in the carbonaceous coating as a spacer, the mobility of lithium ions is secured, thereby exhibiting excellent rate characteristics, and suppressing the occurrence of surface side reactions and metal oxide cracks to improve lifespan stability. can be improved As a result, it can be effectively applied to a battery for an electric vehicle, an energy storage device, and the like.

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

Hereinafter, preferred examples are presented to help the 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

The equipment used in this example is as follows.

- Characterization

XRD and Raman spectra were measured using MiniFlex 600 (Rigaku) and NRS-3100 (JASCO), respectively. 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 samples was determined by analysis using TGA (SDT Q600, TA Instruments).

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

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

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

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

- Measurement of electrochemical performance

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

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

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

The cell was 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. The rate-rate characteristics were measured by applying the same charging and discharging rates 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 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 material

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

First, the graphene oxide (Angstron Meterials; particle size: < 1㎛) was uniformly dispersed in deionized water using ultrasonic grinding (ultrasonic output: 100 W) and a homogenizer (Missang Science Co.) to obtain 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 aqueous AHNS 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 slowly added to 100 ml of 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 the fin oxide. At this time, the surface treatment was performed over 24 hours at 70 ℃.

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

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

Then, 100% by weight of CNT (US Research Nanometerials; particle average diameter: 15 nm) was added to the dispersion compared to the surface-modified reduced graphene oxide, and it was evenly dispersed by ultrasonic grinding and a homogenizer over 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.

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

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

Also, for comparative purposes, NCM811 without carbonaceous coating (ie, bare NCM) will be referred to as BNCM.

analyze

- Evaluation of dispersibility of AHNS-rGO

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 figure, the graphene gel in the form of aggregates appeared in rGO synthesized without surface modification did not appear at all, and sedimentation was also not observed. From this, it was confirmed that the reduced graphene oxide was well dispersed by the (-) 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 Examples, XRD, Raman spectroscopy, and XPS measurements were performed for BNCM and GCNCM, and the results were 3 is shown.

Figure 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 cation (Li ⁺ and Ni ₂ ⁺ ) mixing, where the I ₀₀₃ / I ₁₀₄ values for BNCM and GCNCM are 1.32 and 1.32 and I 104 respectively for BNCM and GCNCM. It was 1.41. These results suggest that the layered structure of NCM was not affected during the carbon encapsulation (or coating) process.

Figure 3b shows the Raman spectra of BNCM and GCNCM. Referring to the figure, the 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 associated with in-plane vibrations of sp ² carbon atoms. Also, the 2D band is related to the stacking order of graphite. As such, the 2D bands for carbon-encapsulated NCM support the existence of stacked graphene sheets.

4A and 4B respectively show Raman spectra and XPS analysis results for graphene oxide (GO) used in Examples and graphene oxide (AHNS-rGO) that was surface-modified with AHNS and then subjected to reduction treatment.

Referring to Fig. 4a, I _D / _IG 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, many new small graphite domains, which are smaller than the sp ² domains in GO, are generated. As the number of small sp ² domains increases, the disorder and the number of edge faces increase, resulting in an increase in I _D .

Meanwhile, the results of 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 XPS C 1s of BNCM and GCNCM. In the case of BNCM, the peak with a center of 289.5 eV may correspond to CO ₃ ^2- of Li ₂ CO ₃ , and the other peak at higher binding energy may be attributed to contaminants present on the NCM surface. For GCNCM, 5 peaks were observed due to the deconvolution of C s1 (median values were 284.5 eV, 285.5 eV, 286.5 eV, 288.1 eV, and 289.0 eV), which were CO ₃ ^2- , CC /C=C, CN, C=O and OC=O. These peaks are from AHNS-rGO and CNT present on the NCM.

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

Referring to Fig. 3c, the position and area ratio of the C 1s peak of GCNCM was consistent with that of AHNS-rGO, indicating that the surface of NCM was functionalized with AHNS-rGO.

Figure 3d shows XPS Ni 2p of BNCM and GCNCM. Referring to the figure, 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 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-mentioned XRD, Raman, and XPS measurement results support that the structure and chemical state of NCM are maintained even after the three-dimensional carbon-containing coating is applied.

- TGA analysis

A TGA test was performed to determine the content of carbon components in the carbon-coated NCM. The results are shown in Figure 5a. At this time, the sample was heated from 30°C to 750°C in an air atmosphere at a temperature increase rate of 10°C/min. Referring to the figure, BNCM, GNCM and GCNCM retained 99.3%, 97.2%, and 95.4% of the total weight, respectively. The difference in weight retention between BNCM and carbon-coated NCM refers to the carbon content in the carbon-coated NCM (which is 2.1% and 3.9% for GNCM and GCNCM, respectively). The measured carbon content closely matched the mass percentage of 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 conductivity improvement of NCM compared to AHNS-rGO. In particular, it is worth noting that the measured conductivities of the AHNS-rGO and CNT films were about 69 S/m and 173 S/m, respectively. GCNCM showed about 46 times higher conductivity than BNCM. It is considered that excellent rate-limiting properties can be achieved because the conductive carbon-containing layer formed on the NCM rapidly transfers electrons to the NCM surface.

- Surface characterization

The texture properties of BNCM, GNCM and GCNCM were evaluated through N ₂ physisorption experiments, and the results are shown in FIG. 5c . The figure shows the adsorption-desorption isotherms of BNCM, GNCM and GCNCM and the corresponding BET surface areas, which were evaluated as 0.37 m2/g, 0.72 m2/g, and 5.07 m2/g, respectively. The specific surface area of GNCM did not change much compared to BNCM, because the AHNS-rGO sheets were densely restacked on the NCM during the encapsulation process. On the other hand, the specific surface area of GCNCM was significantly increased, because CNTs inhibited the restacking of AHNS-rGO sheets and induced the formation of a porous network between the sheets.

Figure 5d shows the pore size distribution of BNCM, GNCM and GCNCM. GNCM (encapsulated only in AHNS-rGO) showed no significant changes 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 abundant micropores (<2 nm) and mesopores (20-50 nm), which was more than 5.5 times larger than that of the two-dimensional AHNS-rGO layer (0.0048 cm3/g). Pore volume (0.0262 cm 3 /g) is shown. In addition, it had a smaller pore size of 17.2 nm due to the development of micropores and mesopores.

- Morphological characterization

Morphological characteristics and uniformity of the carbon-containing coating layer formed on the NCM were analyzed using SEM and TEM. 6a to 6f are low and high magnification SEM photographs of BNCM, GNCM, and GCNCM.

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 low magnification and high magnification SEM pictures of GCNCM. According to the figure, the AHNS-rGO layer completely covers the entire NCM particle. 6g and 6h are cross-sectional TEM images of BNCM and GNCM particles. In the case of GNCM, as shown in Fig. 6h, the surface of the NCM is completely covered with a carbon layer of densely stacked AHNS-rGO sheets, 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 sheets can be hindered by the narrow and long diffusion path in the carbon layer.

6e and 6f are SEM pictures of GCNCM. As can be seen from Fig. 6f, CNTs were well mixed with AHNS-rGO and uniformly covered the NCMs. Compared with GNCM, GCNCM showed a rougher surface.

6i is a cross-sectional TEM image of GCNCM particles. According to the figure, unlike the carbon layer of the two-dimensional structure of the GNCM (Fig. 6h), the carbon layer of the GCNCM exhibited a hierarchical three-dimensional porous structure. This is because CNTs were inserted into the AHNS-rGO sheet to inhibit stacking, thereby having multiple wrinkles and forming a non-densely packed AHNS-rGO sheet. As such, the addition of CNTs 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 that of GNCM (7 nm) due to the formation of a three-dimensional network structure, with CNTs intercalated between the AHNS-rGO layers (Fig. 6h and Fig. 6i). .

The above-mentioned SEM and TEM analysis support that the carbon layer is uniformly coated on the surface of the NCM, and the carbon layer with a three-dimensional structure is formed by the insertion of CNTs into the AHNS-rGO 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 NCMs.

Figure 7a shows the CV curves of the BNCM, GNCM and GCNCM electrodes in the 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. Comparing with the CV curve of the BNCM electrode ( ΔV _BNCM = 0.155 V), the positive and negative peaks of the GNCM electrode each broadened and shifted to a larger, lower potential ( ΔV _GNCM = 0.214 V), which was of polarity. means increase. Despite the higher conductivity of GNCM than that of BNCM, the GNCM electrode showed higher polarity than that of the BNCM electrode, which inhibited diffusion of lithium ions from the bulk electrolyte to the NCM surface (densely packed AHNS on the NCM surface). -caused by the rGO sheet). On the other hand, the GCNCM electrode showed a narrower redox peak width and interval ( ΔV _GCNCM = 0.102 V) between the positive and negative peaks 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, which can promote the transport of electrons and lithium ions. As such, the GCNCM electrode is expected to achieve excellent rate-limiting performance.

Meanwhile, FIGS. 8A to 8C are CV curves of BNCM, GNCM and GCNCM electrodes (scan 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 increased, the shift of the redox peak also increased. Therefore, for electrodes with high polarity, this peak shift may limit some redox reactions at high scan rates. In Fig. 8b, compared with the BNCM electrode (Fig. 8a), the anode peak and the cathode peak of the GNCM electrode, respectively, are wider and shifted to a higher potential and a lower potential, which is because it has a higher electrode polarity than the BNCM. am. In addition, the redox peak current of GNCM slightly increased with increasing scanning rate, 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 gap between the positive and negative peaks was smaller than that of other types, indicating that the polarity of the electrode rapidly decreased due to rapid charge transport by the highly conductive three-dimensional carbon layer formed on the surface of the NCM. it means. However, when only AHNS-rGO is coated, a two-dimensional dense AHNS-rGO layer is formed on the surface of the NCM, which improves electron conduction but prevents lithium ion diffusion and increases electrode polarity.

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

7B shows the initial charge-discharge profile (0.1 C, 2.8 to 4.3 V potential range) of the electrode. The initial charge-discharge efficiencies of BNCM, GNCM and GCNCM were 89.7%, 90.6%, and 92.3%, respectively. Compared with BNCM, GCNCM showed higher initial charge/discharge efficiency, which is attributed to the conductive three-dimensional AHNS-rGO layer formed on the NCM.

9A to 9D show the discharge profiles of the electrodes according to various C. The specific discharge capacities of the BNCM, GNCM and GCNCM electrodes at 0.1 C are 200 mAh/g, 196 mAh/g, and 195 mAh, respectively. /g. The reason why the specific discharge capacities of GNCM and GCNCM were somewhat lower than those of BNCM was 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 with the BNCM electrode, the IR drop increased rapidly as C increased. In addition, the GNCM electrode exhibited lower lithiation potential and discharge capacity than the BNCM electrode, due to increased polarization derived from inhibition of lithium ion diffusion. On the other hand, the GCNCM electrode showed a lower IR drop and a higher reduction potential compared to the other electrodes at the same C ( FIGS. 9a and 9c ). This is because the conductivity was increased by coating with the highly conductive AHNS-rGO/CNT composite layer. Despite the high conductivity of AHNS-rGO, the densely stacked two-dimensional structure prevented diffusion of lithium ions from the bulk phase electrolyte, resulting in low rate-rate properties. 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 the NCM, but also reduced the 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) that was about 5 times higher than that of the BNCM.

Referring to FIG. 7d , the GCNCM electrode showed good cycle performance (capacity retention of 88% after 100 cycles) (65% and 49% for BNCM and GNCM electrodes, 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 sheets inhibit lithium ion diffusion from the bulk phase electrolyte by the bottleneck effect, causing polarization of the electrode, thereby causing an irreversible phase shift and side reaction with the electrolyte. Unlike stacked two-dimensional carbon layers, AHNS-rGO/CNT composites, which provided carbon networks with conductive three-dimensional structures, could provide abundant electron/ion hydrogen channels to improve conductivity and lithium ion diffusivity.

- Cycling Characteristics 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 is the equivalent circuit model of the Nyquist plot). The resistance values of the electrodes calculated from the Nyquist plot are shown in Table 1 below.

7e and 7f, the GNCM electrode showed high R _sf (38.7 Ω) and R _ct (29.1 Ω) values compared to other electrodes, which is due to inhibition of lithium ion diffusion and reaction with 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. 7f shows Nyquist plots of BNCM, GNCM and GCNCM electrodes after 100 cycles. In this regard, R _ct can be seen as the main factor for the decrease in the capacity of the electrode, and in the case of GCNCM, the side reaction is 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, showed the smallest R _ct value even after 100 cycles compared to the other electrodes.

Meanwhile, FIGS. 10A to 10F show SEM and TEM images of BNCM and GCNCM electrodes after 100 cycles, respectively. Referring to FIGS. 10A and 10B , the lithiation/delithiation process produced micro-cracks in the NCM particles, because a volume change occurred during the process. When the micro-cracks are formed, the primary particles are physically separated to cut the electrical path and increase the resistance. 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 a side reaction, thereby promoting surface degradation and reducing 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 cracks occur, 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 cracks can be suppressed.

10e and 10f are cross-sectional TEM pictures of BNCM and GCNCM electrodes after 100 cycles. The changes in the 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 images from the surface to the bulk phase, three regions separated by red and yellow lines for both BNCM and GCNCM samples can be identified. Inside the red line, the layered structure is converted to the rock-salt phase as the outermost surface layer with some disorder. In the region between the red and yellow lines, the layered structure is disordered but weakly maintained. The yellow interior is the 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.

Structural changes from stratified to disordered and rock-salts result from cationic mixing and are supported by FFT photographs (insets in FIGS. 10E and 10F ). Each FFT picture was taken of three regions for BNCM and GNCM. This result is because the carbon layer not only improved the conductivity, but also protected the NCM surface by minimizing the contact between the NCM and the electrolyte.

After cycling 100 times, XPS analysis was performed to identify the components of the electrode surface layer, and XPS Ni 2p spectra of BNCM and GCNCM, respectively, 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 the NCM and the electrolyte.

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 three-dimensional 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, resulting in a large decrease in capacity. 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 binder (PVdF) of the electrode. The peak area of Li ₂ CO ₃ /ROCO ₂ Li at 289.5 eV for BNCM was significantly larger than that of GCNCM, meaning that the three-dimensional carbon layer suppressed electrolyte decomposition on the electrode surface, thereby preventing the thickening of the CEI layer. . By suppressing the side reaction, electrons and lithium ions can easily pass through the surface layer during the redox reaction, thereby suppressing the decrease in capacity.

Meanwhile, in the embodiment, the function of the carbon network layer having a conductive three-dimensional structure to improve the electrochemical stability of the NCM can be described as shown in FIG. 12 .

Referring to the drawings, the carbon network layer of the conductive three-dimensional structure (i) maintains electrical connection between primary particles when micro-cracks occur due to volume change during repeated cycles, (ii) ) reduce the irreversible phase transition on the NCM surface by minimizing the contact between the NCM and the electrolyte and inhibiting the electrolyte from penetrating into the micro-cracks, and (iii) reducing the side reactions at the electrode/electrolyte interface by protecting the surface. 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-modifying the graphene oxide using an amino group-containing compound that bonds with carbon atoms while ring-opening the epoxy group in the graphene oxide so that the surface of the graphene oxide has a negative (-) charge in an aqueous medium ;
b) reducing the surface-modified graphene oxide to form a surface-modified reduced graphene oxide (rGO);
c) preparing a first dispersion containing the surface-modified 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 the metal oxide particles to form a third dispersion; and
f) removing the liquid medium from the third dispersion;
As a method of manufacturing a composite material for a cathode material of a secondary battery comprising a,
The composite material includes an encapsulation layer containing surface-modified reduced graphene oxide and conductive nanoparticles on metal oxide particles, and the conductive nanoparticles are surface-modified by being inserted between sheets of surface-modified reduced graphene oxide. A method for manufacturing a composite material in which the two-dimensional structure of reduced graphene oxide is converted into a three-dimensional carbon-containing network structure. The method according to 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 an alkylene group having 1 to 12 carbon atoms, an arylene group having 6 to 18 carbon atoms in which an aromatic ring is substituted or unsubstituted (wherein the substituent is an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group , a carboxy 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. According to claim 2, wherein the amino group-containing compound is aminomethanesulfonic acid (Aminomethanesulfonic acid), 2-aminoethanesulfonic acid (2-aminoethanesulfonic acid), 3-amino-1-propanesulfonic acid (3-amino-1-propanesulfonic acid) propanesulfonic acid), sulfanilic acid, 6-amino-4-hydroxy-2-naphthalenesulfonic 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 (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) A method of manufacturing a composite material, characterized in that at least one selected from the group. The method according to claim 5, wherein the amino group-containing compound is 6-amino-4-hydroxy-2-naphthalenesulfonic acid. The method of claim 1, wherein step b) is performed by thermal reduction or a chemical reduction reaction using a reducing agent. 8. The method of claim 7, wherein the thermal reduction comprises thermally treating the graphene oxide under an inert gas atmosphere, and
The chemical reduction reaction is a reducing agent, hydrazine (hydrazine), hydrazine hydrate (N ₂ H ₄ .H ₂ O), sodium borohydrate (sodium borohydrate, NaBH ₄ ), ammonia borane (NH ₃ BH ₃ ), ethylenediamine, thiourea, hydroionic acid, L-ascorbic acid, urea, hydroquinone, and benzyl amine. A method for manufacturing a composite material. The method according to 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 according to 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 conductive nanoparticles are, based on the weight of the surface-modified reduced graphene oxide, a method for producing a composite material, characterized in that contained in an amount of 10 to 300% by weight. The composite material according to 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 (area ratio) of at least 100. manufacturing method. 11. The method of claim 10, wherein the conductive metal nanoparticles are 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, and its size is in the range of 1 to 100 nm, characterized in that the manufacturing method of the composite material. According to claim 1, wherein the concentration of the surface-modified reduced graphene oxide in the first dispersion is 1 to 20 mg / ml, and the concentration of the surface-modified reduced graphene oxide in the second dispersion is 20 to 100 A method for producing a composite material, characterized in that the range of mg/ml. The method according to claim 1, wherein the metal oxide is in the form of particles, the size of which ranges from 1 to 50 μm, and
The method for producing a composite material, characterized in that the metal oxide in the third dispersion is contained in an amount in the range of 1000 to 10000 wt% based on the surface-modified reduced graphene oxide. The composite material according to claim 1, wherein step e) comprises combining the second dispersion and the metal oxide particles over 3 to 15 minutes under stirring 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 surface-modified reduced graphene oxide and conductive nanoparticles;
As a composite material for a cathode material of a secondary battery comprising a,
The encapsulation layer has a three-dimensional carbon-containing network structure converted from the two-dimensional structure of the surface-modified reduced graphene oxide by inserting conductive nanoparticles between the sheets of surface-modified reduced graphene oxide,
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 contains 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,
the composite material exhibits 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, and
The surface-modified reduced graphene oxide is a composite material that is surface-modified with an amino group-containing compound, which 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 an alkylene group having 1 to 12 carbon atoms, an arylene group having 6 to 18 carbon atoms in which an aromatic ring is substituted or unsubstituted (wherein the substituent is an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a ketone group , a carboxy group, or a halogen group). The method of claim 16, wherein the metal oxide particles are nickel-cobalt-manganese oxide (NCM) material,
At this time, the metal oxide particles are a composite material, characterized in that the primary particles or secondary particles formed by the aggregation of the primary particles. 17. 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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