WO2016167591A1 - Negative electrode active material and method for preparing same - Google Patents

Abstract

The present invention relates to a negative electrode active material and a method for preparing the same and, more specifically, to a negative electrode active material obtained by modifying a surface of artificial graphite with nitrogen atoms, the nitrogen atoms having a content of 5-10 wt% on the basis of all atoms existing in the outermost shell of the negative electrode active material. The negative electrode active material according to the present invention, which is obtained by modifying a surface of artificial graphite with nitrogen, has improved dispersibility in an aqueous system, so the affinity between a binder and the negative electrode active material is improved, thereby increasing the binding strength of an electrode. In addition, only a particular quantity of the surface of the artificial graphite is modified with nitrogen, and thus the battery capacity is maintained high. Furthermore, in the method for preparing a negative electrode active material according to the present invention, only 10-20 wt% of an oxygen-containing functional group is linked to the artificial graphite through a mild oxidation process, so the nitrogen is easily attached to the artificial graphite to allow the artificial graphite to exhibit hydrophilicity, and the artificial graphite becomes an electrical conductor due to the maintenance of original crystallinity thereof, leading to excellent battery efficiency.

Description

Anode active material and method for preparing same

Cross Citation with Related Application (s)

This application claims the benefit of priority based on Korean Patent Application No. 10-2015-0053330 of April 15, 2015 and Korean Patent Application No. 10-2016-0045345 of April 14, 2016, All content disclosed in the literature is included as part of this specification.

Technical Field

The present invention relates to a negative electrode active material and a method of manufacturing the same.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and the most actively researched fields are power generation and storage using electrochemical reactions.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing. Recently, as the development and demand for portable devices such as portable computers, portable telephones, cameras, and the like, the demand for secondary batteries is rapidly increasing, and these secondary batteries exhibit high energy density and operating potential, and have a cycle life. Many studies have been conducted on this long, low self-discharge rate lithium battery and are commercially available and widely used.

In addition, as interest in environmental issues grows, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuel, such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

Conventionally, a typical lithium secondary battery uses graphite as a negative electrode active material, and charging and discharging are performed while repeating a process in which lithium ions of a positive electrode are inserted into and detached from a negative electrode. The theoretical capacity of the battery is different depending on the type of the electrode active material, but as the cycle progresses, the charge and discharge capacity is generally lowered.

This phenomenon is most likely due to the separation between the electrode active material or between the electrode active material and the current collector due to the volume change of the electrode generated as the charge and discharge of the battery progresses, the active material does not perform its function.

Meanwhile, as a related art related to an aqueous binder and a negative electrode active material, electronic devices using a method for manufacturing a negative electrode for a nonaqueous secondary battery, a nonaqueous secondary battery, a negative electrode for a nonaqueous secondary battery, and a nonaqueous secondary battery have been proposed. Specifically, in the negative electrode for a nonaqueous secondary battery comprising graphite, carbon black, and an aqueous binder, the carbon black includes particles having an aspect ratio of 1.0 or more and 5.0 or less and a maximum diameter of 0.05 μm or more and 10 μm or less. And a negative electrode for a nonaqueous secondary battery, characterized in that the electrode density of the negative electrode is 1.50 g / cm 3 to 1.80 g / cm 3, wherein graphite is disclosed that artificial graphite can be preferably used.

However, according to the above method, since artificial graphite manufactured by high temperature heat treatment and using hydrophobic characteristics is used, dispersibility in an aqueous system is inferior, so that the adhesion between the active material and the electrode is poor.

Accordingly, development of an electrode material capable of preventing structural separation of the electrode and thereby improving battery performance by preventing separation between electrode active materials or between electrode active materials and a current collector during manufacturing of the electrode is required.

Prior art literature

Republic of Korea Patent No. 10-1006121

The first technical problem to be solved of the present invention is to provide a negative electrode active material that can improve the dispersibility in the aqueous system by modifying the surface of the artificial graphite to hydrophilic.

The second technical problem to be solved of the present invention is to provide a method for producing the negative electrode active material.

The third technical problem to be solved of the present invention is to provide a secondary battery negative electrode including the negative electrode active material.

A third technical problem to be solved by the present invention is to provide a secondary battery including the negative electrode, a battery module and a battery pack including the same.

In order to solve the above problems, in one embodiment of the present invention

An anode active material comprising artificial graphite surface-modified with a nitrogen atom,

The nitrogen atom provides a negative electrode active material containing 5% by weight to 10% by weight based on the total weight of all the atoms present in the outermost portion of the negative electrode active material including artificial graphite.

In addition, in one embodiment of the present invention

Connecting the oxygen-containing functional group to the artificial graphite through a mild oxidation process (step 1); And

It provides a negative electrode active material of the present invention comprising the step (step 2) of reducing the artificial graphite connected to the oxygen-containing functional group of the step 1 in a nitrogen atmosphere (step 2).

In addition, an embodiment of the present invention provides a secondary battery negative electrode on which a negative electrode active material slurry including the negative electrode active material is coated on a negative electrode current collector.

In addition, an embodiment of the present invention provides a secondary battery including the negative electrode, the positive electrode and the nonaqueous electrolyte, and a battery module and a battery pack including the same.

According to the present invention, by including an anode active material made of artificial graphite surface-modified with a specific amount of nitrogen element, the dispersibility in the aqueous system is improved, the affinity with the binder is high, the adhesive strength is increased, and the battery capacity is high. A secondary battery negative electrode and a secondary battery including the same may implement the maintenance effect.

Furthermore, the negative electrode active material according to the present invention is connected to the surface of the artificial graphite by the oxygen-containing functional group only 10 to 20% by weight through a manufacturing method using a mild oxidation process, while exhibiting hydrophilicity, while maintaining the original crystallinity of the electrical conductor Therefore, it can exhibit excellent battery efficiency.

1 is a photograph of an artificial graphite doped with nitrogen element prepared in step 2 of Example 1 under a scanning electron microscope (SEM).

FIG. 2 is a graph showing the result of measuring binding energy of the artificial graphite doped with nitrogen element prepared in Example 1 according to Experimental Example 4 and the surface-treated artificial graphite of Comparative Example 1. FIG.

3 is an XRD graph measured by an X-ray diffraction analyzer for artificial graphite subjected to the mild oxidation process of Step 1 of Example 1 of the present invention.

4 is an XRD graph measured by an X-ray diffraction analyzer for artificial graphite doped with nitrogen element of step 2 of Example 1 of the present invention.

5 is an XRD graph of untreated surface graphite of Comparative Example 1. FIG.

Hereinafter, the present invention will be described in more detail to aid in understanding the present invention.

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

As used herein, the terms "comprise", "comprise" or "have" are intended to indicate that there is a feature, number, step, component, or combination thereof, that is, one or more other features, It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, components, or combinations thereof.

Since artificial graphite is manufactured through high temperature heat treatment, all oxygen-containing functional groups present on the surface of artificial graphite are removed to have hydrophobic characteristics. However, hydrophobic artificial graphite has various problems such as viscosity change and change over time in an aqueous system.

Accordingly, the present invention is intended to provide a more stable electrode by modifying the surface of the artificial graphite to hydrophilic, to exhibit excellent dispersibility in the aqueous system.

First, according to one embodiment of the present invention,

An anode active material comprising artificial graphite surface-modified with a nitrogen atom,

The nitrogen atom provides a negative electrode active material containing 5% by weight to 10% by weight based on the total weight of all atoms present in the outermost part of the negative electrode active material including artificial graphite.

As the negative electrode active material according to the present invention, flaky artificial graphite having a diameter of 5 to 20 μm may be used. The artificial graphite may be one or more selected from the group consisting of graphitized mesocarbon microbeads, graphitized mesophase pitch-based carbon fibers and graphitized coke, but the artificial graphite is not limited thereto.

Since the artificial graphite is manufactured through a high temperature heat treatment, all the oxygen-containing functional groups present on the surface of the artificial graphite are removed to have hydrophobic characteristics. Because of this, hydrophobic artificial graphite causes various problems such as viscosity change and change over time in an aqueous system.

In the present invention, in order to solve the problem that the surface of the artificial graphite is hydrophobic, it provides a negative electrode active material by modifying the surface of the artificial graphite with nitrogen.

In this case, the content of nitrogen may be 5% by weight to 10% by weight relative to the weight of all the atoms present in the outermost portion of the negative electrode active material including the artificial graphite. In this case, the degree (%) modified with nitrogen can be quantitatively analyzed through X-ray photoelectron spectroscopy (XPS). In addition, the outermost range of the negative electrode active material means a thickness within 100 nm from the surface of the negative electrode active material, that is, the surface of the negative electrode active material located farthest from the center in the core direction.

If the nitrogen element is modified to less than 5% by weight, the hydrophilicity is insufficient and the aqueous dispersibility is not improved, and the oxygen-containing functional groups are exposed a lot, which may cause electrochemical instability. In this case, the side reactivity with the electrolyte may increase, which may cause problems of initial efficiency, capacity reduction, and deterioration of battery life.

On the other hand, since the surface of artificial graphite has a low energy barrier, it is difficult to directly modify it with nitrogen. Thus, in the present invention, in order to partially modify the artificial graphite surface with nitrogen, a method of first modifying nitrogen using the oxygen-containing functional group may be used by lowering an energy barrier by connecting an oxygen-containing functional group to the surface of the artificial graphite.

That is, the surface of the artificial graphite is -CH ₂ or CH ₃ , one or more of the sites where the hydrogen is separated is replaced with an oxygen-containing functional group can be connected to the oxygen-containing functional group on the surface of the artificial graphite. Subsequently, the surface of the artificial graphite can be finally modified with a nitrogen atom by doping nitrogen to the artificial graphite to which the oxygen-containing functional group is connected to replace the oxygen-containing functional group itself or a part of the oxygen-containing functional group with a nitrogen atom.

The oxygen-containing functional group may be used one or more selected from the group consisting of a hydroxyl group, an epoxy group, a carboxyl group and a lactol group, but is not limited thereto.

The oxygen-containing functional group may be linked in advance to a level equivalent to that of the modified nitrogen. Specifically, since the content of the modified nitrogen is about 5 to 10% by weight relative to the total weight of all the atoms present in the outermost portion of the negative electrode active material, the oxygen-containing functional group connected to the artificial graphite surface before the nitrogen modification is artificial About 10 to 20% by weight of the covalently bondable sites of the outermost carbon atoms of the graphite may be included in connection. In this case, the degree (substitution rate) of the oxygen-containing functional group connected to the surface of the artificial graphite can be quantitatively measured through an elemental analyzer.

If the oxygen-containing functional group is connected to less than 10% by weight, there is a problem that artificial graphite cannot show hydrophilicity properly because the amount of nitrogen to be connected in the subsequent process is small, and too much if connected to more than 20% by weight. Since the amount of oxidation occurs, it is difficult to reduce all of them in a subsequent process, and the graphite layer is separated because the (002) surface of the artificial graphite is large, so that the capacity is small or the structure is not rigid, which is insufficient for a battery.

The oxygen-containing functional group may be connected to the surface of the artificial graphite by a mild oxidation process, which has been described in detail in the method for preparing the negative electrode active material.

On the other hand, the oxygen-containing functional group may be bonded at about 5 to 15% by weight based on the covalently bonded site of the outermost carbon atom of the artificial graphite in the negative electrode active material after nitrogen modification.

This is an oxygen-containing functional group in which 25-50% by weight of the oxygen-containing functional group itself or a part of the oxygen-containing functional group itself or a portion of the oxygen-containing functional group is substituted with a nitrogen atom and not substituted with the remaining nitrogen atoms. It means the content of.

More specifically, in one embodiment of the present invention

An anode active material comprising artificial graphite surface-modified with a nitrogen atom,

Based on the total weight of the covalently bonded sites of the outermost carbon atom of the artificial graphite,

5 to 10 wt% may provide an oxygen-containing functional group in which a nitrogen atom or a portion is replaced by a nitrogen atom, 5 to 15 wt% of an oxygen-containing functional group, and 80 to 90 wt% of a negative electrode active material to which a hydrogen atom is bonded.

As described above, in the present invention, the surface of the artificial graphite used as the negative electrode active material has a low energy barrier, which makes it difficult to directly modify nitrogen. Therefore, in order to modify the surface of artificial graphite hydrophilically, a part of the surface of artificial graphite should be replaced with an oxygen-containing functional group to lower the energy barrier, and then nitrogen atoms should be connected using the oxygen-containing functional group.

Specifically, the surface of the artificial graphite is -CH ₂ or CH ₃ , one or more of the sites where the hydrogen is separated is replaced with an oxygen-containing functional group can be connected to the oxygen-containing functional group on the surface of the artificial graphite. By doping nitrogen to the artificial graphite to which the oxygen-containing functional group is connected, the oxygen-containing functional group itself or a part of the oxygen-containing functional group can be replaced with a nitrogen atom, and finally the outermost part of the artificial graphite can be modified with a nitrogen atom.

In this case, the content of the oxygen-containing functional group in which the nitrogen atom or part is replaced by a nitrogen atom may be 5 to 10% by weight relative to the covalently bonded site of the outermost carbon atom of artificial graphite. The oxygen-containing functional group in which part of which is substituted with a nitrogen atom may eventually exhibit hydrophilicity because the nitrogen atom is present at the outermost part of the negative electrode active material.

If the content of the nitrogen element or the nitrogen-substituted oxygen-containing functional group is less than 5% by weight, the hydrophilicity is insufficient, the aqueous dispersibility is not improved, and the oxygen-containing functional group is exposed a lot, which may cause electrochemical instability. On the other hand, when the content of the nitrogen element or nitrogen-substituted oxygen-containing functional group exceeds 10% by weight, side reaction with the electrolyte increases, which may cause problems of initial efficiency, capacity reduction, and deterioration of battery life.

In addition, since the content of the modified nitrogen is about 5 to 10% by weight relative to all the atoms present in the outermost portion of the negative electrode active material, the oxygen-containing functional group may also be connected to the equivalent level of 10 to 20% by weight. 25 to 50% by weight of the oxygen-containing functional group itself or a portion of the oxygen-containing functional group of the linked oxygen-containing functional group is replaced with nitrogen, the remaining of the oxygen-containing functional group that is not substituted with nitrogen on the surface of the negative electrode active material The content may be 5 to 15% by weight, relative to the covalently linkable site of the outermost carbon atom of the artificial graphite.

If less than 5% by weight of oxygen-containing functional groups are present on the surface of the negative electrode active material, since excess nitrogen atoms are present on the surface of the negative electrode active material, the side reactivity with the electrolyte is increased, leading to an initial efficiency, capacity reduction, and lifetime of the battery. If a problem of deterioration may occur, and an oxygen-containing functional group of more than 15% by weight is present on the surface of the negative electrode active material, there is a problem that the negative electrode active material does not exhibit hydrophilicity because the nitrogen atoms are less doped on the surface, Due to the connection of oxygen-containing functional groups, the surface of the artificial graphite may be separated or the interplanar distance may be increased, resulting in inferior crystallinity.

On the other hand, as described above, the surface of the artificial graphite is present, the hydrogen atoms of the outermost because it is a -CH ₂ or CH _3, oxygen-containing functional group and a negative electrode active material nitrogen is connected. Accordingly, the negative electrode active material of the present invention may be 80 to 90% by weight of hydrogen atoms to the covalently bonded sites of the outermost carbon atoms of artificial graphite.

If less than 80% by weight of hydrogen atoms are bonded, excess oxygen-containing functional groups or nitrogen atoms are present on the surface of the negative electrode active material, resulting in increased side reactivity with the electrolyte, thereby degrading the initial efficiency, capacity, and lifespan of the battery. Problems in which the electrical properties of the active material may be degraded may occur, and when more than 90 wt% of hydrogen atoms are bonded to each other, since the nitrogen atoms are less doped on the surface, the negative electrode active material may not exhibit hydrophilicity.

In addition, the interplanar spacing (d ₀₀₂ ) of the carbon hexagonal net surface of the negative electrode active material according to the embodiment of the nitrogen-modified the present invention may be 0.3350 to 0.3400 nm. Since the negative electrode active material of the present invention is connected to the oxygen-containing functional group only on a portion of the surface of the artificial graphite, the surface of the artificial graphite is separated due to the connection of the excess oxygen-containing functional groups or the inter-plane distance increases to solve the problem of inferior crystallinity. In addition, the effect of exhibiting hydrophilicity can be realized while maintaining the original crystallinity of artificial graphite.

Artificial graphite in which the surface is separated by a conventional strong oxidation process, or graphite oxide inferior in crystallinity (graphite oxide) has an electrical insulator property. However, when the mild oxidation process such as the present invention is carried out, physical properties may be modified into an electric conductor.

In the case of a battery using artificial graphite or an anode active material containing graphite oxide that has not undergone a mild oxidation process, in order to cause an oxidation / reduction reaction, lithium ions move as much as electrons. Large, thus slowing down the reaction. Therefore, in order to smoothly drive the battery, an excessive amount of conductive material or the like must be added, so that the energy density of the battery is reduced, and the initial efficiency and the adhesive strength are also reduced.

In addition, in one embodiment of the present invention

Connecting the oxygen-containing functional group to the artificial graphite through a mild oxidation process (step 1); And

It provides a method for producing a negative electrode active material comprising a; doping the nitrogen by reducing the artificial graphite connected to the oxygen-containing functional group of step 1 in a nitrogen atmosphere (step 2).

Hereinafter, a method of manufacturing the negative electrode active material according to the present invention will be described in detail for each step.

In the method of manufacturing the negative electrode active material according to the present invention, (step 1) is a step of connecting a oxygen-containing functional group to the surface of the artificial graphite by performing a mild oxidation process before performing nitrogen doping to the artificial graphite.

The artificial graphite of (step 1) may be used at least one selected from the group consisting of graphitized mesocarbon microbeads, graphitized mesophase pitch-based carbon fiber and graphitized coke, but the artificial graphite is not limited thereto. . Since the surface of the artificial graphite has a low energy barrier and thus it is difficult to directly dope nitrogen, the surface of the artificial graphite may be doped with nitrogen after lowering the energy barrier by connecting an oxygen-containing functional group to the surface of the artificial graphite.

In this case, the mild oxidation of (step 1) means connecting the oxygen-containing functional group at a substitution rate of 10 to 20% by weight with respect to the covalently bonded site of the outermost carbon atom of artificial graphite. That is, the surface of the artificial graphite is -CH ₂ or CH ₃ , and the surface of the artificial graphite can be oxidized by connecting an oxygen-containing functional group to one or more of the hydrogen is separated.

If the oxygen-containing functional group of (step 1) is linked to the covalently bonded site of the outermost carbon atom of artificial graphite, artificial graphite is less because the amount of nitrogen connected in the subsequent process is less than 10% by weight. This hydrophilicity cannot be represented properly, and when connected at a substitution rate of more than 20% by weight, too much oxidation occurs, so that it is difficult to reduce all of them in a subsequent process, and the graphite layer becomes larger due to the (002) plane of artificial graphite. Because of the separation, the capacity is small, or the structure is not rigid, there is a problem that is insufficient for the battery.

At this time, the substitution rate indicating the extent to which the oxygen-containing functional group is connected to the surface of the artificial graphite can be measured quantitatively through an elemental analyzer.

 The mild oxidation process of (step 1) may be performed by heat treating artificial graphite in an air atmosphere at 500 to 600 ° C. for 1 hour to 1 hour and / or immersing artificial graphite in an acid solution for 4 to 6 hours and then drying it. Can be.

In this case, the acid solution is a nitric acid solution of 50 to 70 ℃ concentration of 30 to 50% by weight, the drying is preferably carried out in a vacuum of 250 to 300 ℃.

The oxygen-containing functional group can adjust its content by limited heat treatment time or immersion time of the acid solution. That is, in the method of the present invention, the oxygen-containing functional group content can be controlled within 10 to 20% by weight only after heat treatment or immersion under the above-described time conditions.

If the heat treatment time or the immersion time in the acid solution is less than the time, the oxygen-containing functional group content is reduced, and thus the nitrogen element content is lowered so that the aqueous dispersibility is not improved, and the time is exceeded. Since the oxygen-containing functional group content is increased, and thus the nitrogen element content is increased, the side reaction with the electrolyte is increased, which may cause problems of initial efficiency, capacity reduction, and deterioration of battery life.

The oxygen-containing functional group of (step 1) may be at least one selected from the group consisting of a hydroxyl group, an epoxy group, a carboxyl group, and a lactol group, but the oxygen-containing functional group is not limited thereto.

In addition, in the method of manufacturing the negative electrode active material according to the present invention, (step 2) is a step of doping nitrogen by reducing artificial graphite to which the oxygen-containing functional group of (step 1) is connected in a nitrogen atmosphere.

(Step 2) is a step of replacing the oxygen-containing functional group itself or a part of the oxygen-containing functional group connected to artificial graphite by (Step 1) with a nitrogen element. Even though only the oxygen-containing functional group is attached to the surface of the artificial graphite, the artificial graphite may exhibit hydrophilicity, but the oxygen-containing functional group alone does not satisfy the electrochemical characteristics required by the secondary battery. Therefore, in order to overcome this, by doping the surface of the artificial graphite with a nitrogen element in which two non-covalent electron pairs exist through (step 2), it is possible to produce artificial graphite having not only excellent electrochemical properties but also a hydrophilic surface.

The (step 2) can be carried out in a gas atmosphere in which hydrogen gas is mixed with one gas selected from the group consisting of hydrazine (N ₂ H ₂ ), ammonia (NH ₃ ) and mixtures thereof. Specifically, step (2) is carried out by reducing the hydrazine or ammonia gas and hydrogen gas in a nitrogen gas atmosphere in which 3: 3 is mixed, whereby the oxygen-containing functional group itself or part of the oxygen-containing functional group connected to the surface of the artificial graphite is nitrogen. It can be replaced by an element.

At this time, the (step 2) may be performed under a temperature condition of 800 to 1000 ℃. Specifically, in the method of the present invention, the temperature is raised to a temperature of 800 to 1000 ° C. over about 5 hours, and then reacted for about 2 hours, followed by lowering the temperature for 12 hours.

The amount of nitrogen doping can be controlled by the limited gas composition and the temperature range and the temperature increase time and the reaction time.

If the temperature range, the temperature and the reaction time are less than the range, there may occur a problem that the unreacted residual oxygen-containing functional group remains when the reaction is performed at a temperature below, for example, 800 ° C. or less than 2 hours. have. In addition, when the reaction is carried out at a temperature of more than 1000 ℃ or more than 2 hours, the amount of nitrogen reaction increases, so that the side reactivity with the electrolyte increases, which leads to a decrease in initial efficiency, capacity reduction, and lifetime characteristics of the battery. The problem may be that the doped portion is damaged.

In addition, another embodiment of the present invention provides a negative electrode on which a negative electrode active material slurry including the negative electrode active material is coated on a negative electrode current collector.

Specifically, the negative electrode may be prepared by applying a negative electrode active material slurry including the negative electrode active material of the present invention on a negative electrode current collector, followed by drying and rolling.

The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, calcined carbon; Or aluminum, stainless steel, etc. which surface-treated with carbon, nickel, titanium, silver, etc. can be used.

The negative electrode active material slurry may further include a binder as a component to assist in the bonding between the active material and the conductive material and the current collector. The binder is not particularly limited, and for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetra One selected from the group consisting of fluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber, and mixtures thereof can be used.

In some cases, the negative electrode active material slurry may further include a conductive material. The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

Since the negative electrode active material according to the present invention is hydrophilic by doping with nitrogen, it may exhibit higher dispersibility in an aqueous system for preparing an electrode using an aqueous solvent such as water and alcohol. In addition, an increase in the affinity between the active material and the aqueous binder improves the adhesive strength of the electrode, thereby providing a secondary battery having excellent electrical conductivity and stable for long-term use. Furthermore, since secondary graphite is modified to be hydrophilic by using nitrogen element, it is electrochemically stable, and the secondary battery including the secondary battery is excellent because the capacity of artificial graphite can be maintained by maintaining the interfacial distance of artificial graphite by mild mythification process. Can exhibit characteristics.

In another embodiment of the present invention,

It provides a secondary battery including the negative electrode, a positive electrode, a separator interposed between the positive electrode and the separator and a nonaqueous electrolyte.

The positive electrode may be prepared by applying a positive electrode active material slurry including a positive electrode active material on a positive electrode current collector, followed by drying and rolling.

Although the positive electrode active material is not particularly limited, specifically, a lithium transition metal oxide may be used. Examples of the lithium transition metal oxide include Li.Co-based composite oxides such as LiCoO ₂ , Li.Ni.Co.Mn-based composite oxides such as LiNi _x Co _y Mn _z O ₂ , and Li.sub.2 such as LiNiO ₂ . Ni-based composite oxide may be mentioned, such as LiMn ₂ O ₄ of the Li-Mn composite oxide such, may be mixed alone or a plurality of them.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include stainless steel, aluminum, nickel, titanium, calcined carbon; Or aluminum, stainless steel, etc. which surface-treated with carbon, nickel, titanium, silver, etc. can be used.

The positive electrode active material slurry may further include a binder. The binder is not particularly limited, and for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetra One selected from the group consisting of fluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluorine rubber, and mixtures thereof can be used.

In some cases, the positive electrode active material slurry may further include the same or different conductive material as used in the negative electrode active material slurry.

In the secondary battery of the present invention, the nonaqueous electrolyte may be composed of an electrolyte solution and a metal salt, and the nonaqueous organic solvent is used as the electrolyte solution.

As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1,2-dime Methoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxoron, acetonitrile, nitromethane, methyl formate, Methyl acetate, phosphate triester, trimethoxy methane, dioxoron derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate Aprotic organic solvents, such as ethyl propionate, can be used.

The metal salt may be a lithium salt, the lithium salt is a material that is good to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF _{3 SO 3, LiCF 3 CO 2,} LiAsF 6, LiSbF 6, LiAlCl 4, CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenyl Lithium borate, imide and the like can be used.

According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided. Since the battery module and the battery pack include the secondary battery exhibiting stable and excellent battery characteristics, a power tool, an electric vehicle (EV), a hybrid electric vehicle (HEV), and a plug-in are included. It can be used as a power source of any one or more of the electric vehicle, including a plug-in hybrid electric vehicle (PHEV), or a system for power storage.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

Example

Example 1 Preparation of Anode Containing Nitrogen Doped Artificial Graphite

Step 1: The oxygen-containing functional group was attached through a mild oxidation process in which flaky artificial graphite (product name: BTR S360) having a diameter in the range of 5-20 μm was heat-treated in an air atmosphere at 550 ° C. in a tube furnace.

Step 2: hydrazine (N ₂ H ₂ ) of 800 ℃ the artificial graphite attached to the oxygen-containing functional group Nitrogen element was doped to the surface of the artificial graphite through a process of reducing for 24 hours in a nitrogen gas atmosphere of 3: 7 gas and hydrogen gas mixed.

Step 3: The negative electrode active material slurry was prepared by mixing 96 g of the artificial graphite doped with nitrogen element, 1 g of CMC, an aqueous binder, 2 g of SBR, 1 g of acetylene black, a conductive material, and 220 g of water, a solvent. The negative electrode active material slurry was applied to a copper current collector, and then dried and rolled in a vacuum oven at 130 ° C. to prepare a negative electrode.

Comparative example  Artificial Graphite

Step 1: A negative electrode active material slurry was prepared by mixing 96 g of general artificial graphite without surface modification, 1 g of CMC as an aqueous binder, 2 g of SBR, 1 g of acetylene black as a conductive material and 220 g of water as a solvent. The negative electrode mixture was coated on a copper current collector, dried in a vacuum oven at 130 ° C., and rolled to prepare a negative electrode.

Experimental Example

Experimental Example  One.

After observing the artificial graphite doped with nitrogen element prepared in (Step 2) of Example 1 with a scanning electron microscope (SEM), the results are shown in FIG.

As shown in Figure 1, the artificial graphite can be confirmed that the flaky form having a diameter in the range of 5 to 20 ㎛, it can be seen that the shape is maintained stable without breaking.

Through this, it can be seen that artificial graphite can serve as a stable anode active material even after the mild oxidation process and the nitrogen doping process.

Experimental Example  2.

Oxygen content at the outermost surface of the artificial graphite subjected to the mild oxidation process in step 1 of Example 1 was measured by an elemental analyzer, and the results are shown in Table 1.

	Oxygen content
Artificial graphite subjected to the mild oxidation process of (Step 1) of Example 1	9 to 10 wt%

Looking at Table 1, it can be seen that the oxygen-containing functional group is connected at a substitution rate of 9 to 10% by weight with respect to the outermost surface of the artificial graphite.

Experimental Example  3.

X-ray photoelectron spectroscopy (XPS) was used to measure the binding energy of the artificial graphite doped with the nitrogen element of (Step 2) of Example 1 and the surface-modified artificial graphite of Comparative Example 1 The results are shown in FIG.

Referring to FIG. 2, the artificial graphite of Comparative Example 1 exhibits an even distribution of intensity in the energy range of 392 eV to 408 eV, whereas the artificial energy doped with nitrogen element of Example 1 has a binding energy of 396 eV. You can see that the intensity starts increasing and then decreases again at 404 eV.

Through this, the oxygen-containing functional group is connected to a part of the surface of the artificial graphite through the mild oxidation process, it can be confirmed that some or all of the oxygen-containing functional group is replaced with nitrogen, so that a certain amount of nitrogen is doped to the surface of the artificial graphite.

Further, from the binding energy graph of FIG. 2, the nitrogen element doping concentration was calculated through the ratio of the peak area of carbon appearing between 280 and 292 eV and the peak area of nitrogen appearing between 396 eV and 404 eV. It was found that about 7.21 atom% of nitrogen was doped on the surface.

Experimental Example  4.

X-ray diffraction of the interfacial distance (d ₀₀₂ ) of the surface-modified artificial graphite of Comparative Example 1, the artificial graphite subjected to the mild oxidation process of Step 1 of Example 1, and the artificial graphite doped with the nitrogen element of Step 2 After the measurement using the analyzer, the results are shown in FIGS. 3 to 5 and Table 2 below.

	Plane distance (d 002 )
Comparative Example 1 (Surface Graphite without Surface Modification)	0.3372 nm
Step 1 of Example 1 (artificial graphite subjected to a mild oxidation process)	0.3385 nm
Step 2 of Example 1 (Artificial Graphite with Nitrogen Element Doping)	0.3377 nm

As shown in FIGS. 3 to 5 and Table 2, the interfacial distance of the artificial graphite of Comparative Example 1, which is not surface modified, was 0.3372 nm (see FIG. 5), and the artificial oxidation process of performing the mild oxidation process of (Step 1) in Example 1 was performed. The interplanar spacing of graphite was 0.3385 nm (see FIG. 3), and the interplanar spacing of artificial graphite further subjected to nitrogen element doping of (step 2) was 0.3377 nm (see FIG. 4).

That is, after performing the mild oxidation process, the interfacial distance of artificial graphite increased by 0.38% compared to before the mild oxidation process, and then decreased by 0.23% after doping nitrogen element. As such, it can be seen that the artificial graphite of FIG. 4 surface-modified according to the method of the present invention has an artificial graphite having an interplanar distance difference of about 0.1% compared to the artificial graphite of Comparative Example 1 of FIG. .

Therefore, it can be seen that the mild oxidation process and nitrogen element doping of the artificial graphite can be performed hydrophilic treatment without significant change in the structure of the artificial graphite.

Experimental Example  5.

In order to find out the dispersibility of the artificial graphite prepared in Example 1 and Comparative Example 1, the negative electrode active material slurry prepared in Step 3 of Example 1 and the negative electrode active material slurry prepared in Comparative Example 1 for 7 days at room temperature After storage, the supernatant was extracted and the weight remaining after evaporation of the solvent was measured and the results are shown in Table 3.

	Solid mass (g)
Example 1	1.3
Comparative Example 1	0.6

As shown in Table 3, in the case of the negative electrode active material slurry including the artificial graphite doped with the nitrogen element of Example 1, about 1.3 g of solid remained, whereas the surface-modified artificial graphite of Comparative Example 1 was included. About 0.6 g of solid remained for the negative electrode active material slurry. That is, it can be seen that the negative electrode active material slurry of Example 1 is about 2 times more solid than the negative electrode active material slurry of Comparative Example 1.

As a result, the artificial graphite which is not surface-modified as in Comparative Example 1 is inferior in dispersibility, so that the artificial graphite is not evenly dispersed in the solvent and sinks to the lower liquid, so that the weight of the solid remaining even when the supernatant is evaporated is small. Since, as in Example 1, nitrogen-doped artificial graphite has improved hydrophilicity and excellent dispersibility in an aqueous system, it is dispersed evenly in both the supernatant and the supernatant in the solvent, so that a large amount remains even after the supernatant is evaporated. It can be predicted.

Experimental Example  6.

The adhesive strength of the anodes prepared in Example 1 and Comparative Example 1 was measured. At this time, the adhesive force was measured by a 180 ^o peel test. The results are shown in Table 4 below.

	Adhesive force (gf / 15 mm)
Example 1	30
Comparative Example 1	13

As shown in Table 4, the adhesion of the negative electrode of Example 1 of the present invention is 30 gf / 15 mm, 2.3 times compared to the negative electrode (13 gf / 15 mm) of Comparative Example 1 using the surface-modified artificial graphite It can be seen that it has excellent adhesion.

 As a result, since the dispersibility of the negative electrode active material including the artificial graphite doped with nitrogen of the present invention is excellent, the secondary battery having excellent adhesive strength and high electrical conductivity and stable for long-term use is provided. It can be predicted that it can be done.

Claims (23)

1. An anode active material comprising artificial graphite surface-modified with a nitrogen atom,

   Wherein the nitrogen atom is a negative electrode active material containing 5% by weight to 10% by weight based on the total weight of all the atoms present in the outermost portion of the negative electrode active material including artificial graphite.
2. The method according to claim 1,

   The artificial graphite is a negative active material 5 to 15% by weight of the oxygen-containing functional group is bonded to the covalently bonded site of the outermost carbon atoms.
3. The method according to claim 2,

   The oxygen-containing functional group is a method for producing a negative electrode active material is one or more selected from the group consisting of a hydroxyl group, an epoxy group, a carboxyl group and a lactol group.
4. The method according to claim 1,

   Interfacial distance (d ₀₀₂ ) of the carbon hexagonal network surface of the negative electrode active material is 0.3350 nm to 0.3400 nm.
5. An anode active material comprising artificial graphite surface-modified with a nitrogen atom,

   Based on the total weight of the covalently bonded sites of the outermost carbon atom of the artificial graphite,

   5% to 10% by weight of a nitrogen atom or an oxygen containing functional group partially substituted with a nitrogen atom,

   5 wt% to 15 wt% of an oxygen containing functional group, and

   An anode active material having 80 to 90 wt% of hydrogen atoms bonded thereto.
6. Connecting the oxygen-containing functional group to the artificial graphite through a mild oxidation process (step 1); And

   Reducing the artificial graphite in which the oxygen-containing functional group of the step 1 is connected in a nitrogen atmosphere (doping step 2); The method of manufacturing a negative electrode active material of claim 1 comprising a.
7. The method according to claim 6,

   In the step 1, the artificial graphite is at least one selected from the group consisting of graphitized mesocarbon microbeads, graphitized mesophase pitch-based carbon fibers and graphitized coke coke.
8. The method according to claim 6,

   The mild oxidation process of step 1 is to be carried out by heat treatment in an air atmosphere of 500 ℃ to 600 ℃.
9. The method according to claim 8,

   The mild oxidation process is carried out for 1 hour to 1 hour and a half of the manufacturing method of the negative electrode active material.
10. The method according to claim 6,

    The mild oxidation process of step 1 is a method for producing a negative electrode active material is performed by immersing artificial graphite in an acid solution for 4 to 6 hours, and then drying.
11. The method according to claim 10,

    The acid solution is a method of producing a negative electrode active material using a nitric acid solution of 50 ℃ to 70 ℃ concentration of 30% by weight to 50% by weight.
12. The method according to claim 10,

    The drying is a method of producing a negative electrode active material is carried out in a vacuum state of 250 ℃ to 300 ℃.
13. The method according to claim 6,

    The oxygen-containing functional group of step 1 is a method for producing a negative electrode active material is at least one selected from the group consisting of a hydroxyl group, an epoxy group, a carboxyl group and a lactol group.
14. The method according to claim 6,

    The oxygen-containing functional group of step 1 is a method for producing a negative electrode active material is bonded at a substitution rate of 10 to 20% by weight relative to the covalently bonded sites of the outermost carbon atoms of the artificial graphite.
15. The method according to claim 6,

    The nitrogen doping of the step 2 is performed in a nitrogen gas atmosphere in which a mixture of hydrogen gas and one gas selected from the group consisting of hydrazine (N ₂ H ₂ ), ammonia (NH ₃ ) and mixtures thereof. Manufacturing method.
16. The method according to claim 15,

    The nitrogen doping of the step 2 is a method for producing a negative electrode active material is carried out in a nitrogen gas atmosphere of hydrazine or ammonia gas and hydrogen gas 3: 7 mixed.
17. The method according to claim 15,

    Nitrogen doping of the step 2 is a method of producing a negative electrode active material is carried out at a temperature of 800 ℃ to 1000 ℃.
18. A negative electrode for a secondary battery, wherein a negative electrode active material slurry containing the negative electrode active material of claim 1 is coated on a negative electrode current collector.
19. The method according to claim 18,

    The negative electrode active material slurry further comprises a binder negative electrode for a secondary battery.
20. The method according to claim 19,

    The binder is polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene- A negative electrode for a secondary battery, which is a single substance selected from the group consisting of propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), and fluorine rubber or a mixture of two or more thereof.
21. A secondary battery comprising a negative electrode, a positive electrode, a separator modified between the negative electrode and the positive electrode, and a nonaqueous electrolyte, wherein the negative electrode includes the secondary battery negative electrode of claim 18.
22. A battery module comprising the secondary battery of claim 21 as a unit cell.
23. A battery pack comprising the battery module of claim 22, and used as a power source for medium and large devices.

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