KR101729001B1 - Negative active material for lithium secondary battery, preparing method thereof and lithium secondary battery using the same - Google Patents

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same. More particularly, the present invention relates to a negative electrode active material for a lithium secondary battery, The present invention relates to a negative electrode active material for a lithium secondary battery, which can reduce a process cost by performing a sintering process and improve cell characteristics such as initial charge / discharge efficiency, a manufacturing method thereof, and a lithium secondary battery using the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative active material for a lithium secondary battery,

The present invention relates to a negative electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same. More particularly, the present invention relates to a negative electrode active material for a lithium secondary battery, The present invention relates to a negative electrode active material for a lithium secondary battery, which can reduce a process cost by performing a sintering process and improve cell characteristics such as initial charge / discharge efficiency, a manufacturing method thereof, and a lithium secondary battery using the same.

In recent years, lithium-ion secondary batteries have attracted attention as a power source for electric motors powered by electric motors, while the interest and demand for environmentally friendly green cars are increasing. In addition, they are also used as energy storage devices, and they are larger in size than the existing batteries for small-sized electronic devices, and their demand is increasing. In addition, in the field of small-sized electronic devices, utilization is gradually widening due to technological development of electronic devices such as smart phones, tablet PCs, and Ultrabooks.

Recently, the field of use of lithium secondary batteries has been widening, and as the size of the lithium ion secondary battery is becoming larger, it is required to have higher price competitiveness than conventional lithium secondary battery materials. Currently, most of the anode active materials for lithium secondary batteries are made of graphite active materials. As the size of the batteries becomes larger, the price competitiveness of batteries becomes an important factor for commercialization. Therefore, the negative active material has a tendency of increasing the share of natural graphite active materials having price competitiveness among black graphite systems.

Due to the price competitiveness of natural graphite, it has recently been applied to large-sized lithium secondary batteries for electric vehicles and energy storage. In order for the mid- to large-sized battery market to be activated, the cost competitiveness of the battery should be secured. The proportion of the raw materials in the cost structure of the battery is as high as about 50%. It can be said that it is directly related to securing price competitiveness. To overcome the price problem of such materials, in the field of natural graphite anode active material, in order to save manufacturing process cost, the temperature of the firing process is lowered, the heat treatment speed is increased to increase the production per unit time, To reduce manufacturing costs. However, the above methods have the effect of lowering the process cost. However, since the organic material coated on the surface of graphite is not sufficiently burned, the organic component remains and the charging / discharging rate is lowered and the initial efficiency is lowered. There is a problem that battery performance and manufacturing cost improvement are not compatible.

Conventionally, a method of modifying the surface of natural graphite using a petroleum-based or coal-based pitch and then firing is used. However, in this method, since the coating process of the pitch is a solid-phase process, the uniform coating itself is difficult. Even if the coating is uniform, the pitch is melted down during the firing process due to the thermoplastic property of the pitch, And thus it is difficult to ensure the uniformity of thickness of the carbon layer coated on the graphite surface. As a result, problems such as deterioration of the initial efficiency of the secondary battery, deterioration of the life span, and deterioration of the output performance are caused.

Korean Patent Laid-Open Publication No. 10-2014-0140323 (Patent Document 1) discloses a lithium secondary battery which is formed by coating with a low crystalline particulate material selected from petroleum pitch, coal pitch, Negative electrode active material. However, in this case, some irreversible reactions were suppressed, but the low-crystalline granular material coated on the surface of spherical graphite flowed down during the firing process, resulting in difficulty in uniform coating, and graphite peeling phenomenon easily occurred There is still a problem that the physical properties of the lithium secondary battery are lowered.

Korean Patent Laid-Open No. 10-2004-0096279 (Patent Document 2) discloses an anode active material for a lithium secondary battery, which is lithium-doped with graphite and has a metal oxide coated on its surface, Of the present invention. When metal particles having high conductivity are present on the surface of the negative electrode active material, the initial efficiency is lowered due to a side reaction with the electrolyte during charging and discharging. When the metal surface and the electrolyte interface are exposed, It can sufficiently be expected that the degradation reaction may lead to deterioration of battery life performance.

Accordingly, development of a negative electrode active material for a rechargeable lithium battery having excellent price competitiveness while maintaining the performance and stability of a lithium secondary battery is required. The present inventors have found that the surface of natural graphite can be modified, or natural graphite can be doped with metal, The present inventors have developed an anode active material manufacturing technology which is capable of reducing the manufacturing cost and has excellent capacity and efficiency.

Korean Patent Publication No. 10-2014-0140323 (December, 2014) Korean Patent Publication No. 10-2004-0096279 (November 16, 2004)

SUMMARY OF THE INVENTION The present invention has been made in order to solve the problems of the prior art, and it is an object of the present invention to provide a graphite- Discharge efficiency, and the like, and a method for producing the negative electrode active material for a lithium secondary battery.

Another object of the present invention is to provide a lithium secondary battery comprising the above-described negative electrode active material for a lithium secondary battery.

According to an aspect of the present invention, there is provided an anode active material for an Fe-containing natural graphite based lithium secondary battery including a carbon composite in which an Fe-containing natural graphite based granular material coated with a thermosetting resin is heat- .

According to one embodiment of the present invention, the natural graphite-based granular material is not limited, but it may contain 0.05 to 0.8% by weight of Fe element, more preferably 0.1 to 0.5% by weight of Fe element, But is not limited to.

According to one embodiment of the present invention, the natural graphite-based granular material may satisfy the following formula (1).

0.4? Fe / M + Fe? 1.0 [Formula 1]

(Wherein Fe is the content (ppm) of Fe element contained in the natural graphite-based granular material, M is the content (ppm) of the metal element excluding iron contained in the natural graphite- Fe is the total amount (ppm) of the metal elements contained in the natural graphite-based granular material.)

According to an embodiment of the present invention, the carbon composite material includes a non-graphitizable carbon layer derived from a thermosetting resin on the surface of the granular material, and the average thickness of the non-graphitizable carbon layer is not limited, have.

According to an embodiment of the present invention, the thermosetting resin may include polyurethane and polyisocyanurate, and the thermosetting resin may include 3 to 35% by weight based on the entire negative electrode active material, It is not.

According to another aspect of the present invention, there is provided a lithium secondary battery comprising the anode active material for a Fe-containing natural graphite based lithium secondary battery.

According to an embodiment of the present invention, the irreversible capacity of the lithium secondary battery is 26 to 29 mAh / g, and the reversible capacity is 359 to 365 mAh / g.

According to another aspect of the present invention, there is provided a method of manufacturing an anode active material for an Fe-containing lithium secondary battery, comprising the steps of: heat-treating an Fe-containing natural graphite-based granular material coated with a thermosetting resin under an inert gas atmosphere; ≪ / RTI >

According to one embodiment of the present invention, the heat treatment temperature is not limited, but may be performed at 700 to 1300 ° C.

According to the negative electrode active material for a lithium secondary battery of the present invention, and the lithium secondary battery using the same, a non-graphitizable carbon layer is formed on the surface of a natural graphite based granular material containing an Fe element, Even when heat treatment is performed at a temperature, it is possible to achieve the same effect as that of calcination at a high temperature due to the catalytic effect of the Fe element, and there is an advantage that the process cost can be reduced.

In addition, by forming the non-graphitizable carbon layer on the surface of the natural graphite, the Fe element contained in the natural graphite is prevented from being exposed to the outside, whereby the side reaction with the electrolyte during the electrochemical reaction of the battery is suppressed, It is possible to improve battery characteristics such as charging / discharging speed, lifetime characteristics, initial efficiency improvement, and the like.

1 is an SEM photograph of natural graphite used in an anode active material for a lithium secondary battery according to an embodiment of the present invention.
2 is a SEM photograph of a section of a natural graphite doped with a metal used in a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.
3 is a SEM photograph of a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.

Hereinafter, preferred embodiments and evaluation test items of the negative electrode active material for a lithium secondary battery of the present invention and a method for producing the same will be described in detail. The present invention may be better understood by the following examples, which are for the purpose of illustrating the present invention and are not intended to limit the scope of protection defined by the appended claims.

In order to produce a negative electrode active material for a lithium secondary battery, a surface of a natural graphite-based granular material containing an Fe element is coated with a thermosetting resin and heat-treated in an inert gas atmosphere to form a non-graphitizable carbon layer, Fe element can act as a catalyst and can be carbonized by pyrolyzing at a lower temperature than in the conventional firing step. The Fe element present in natural graphite can improve the electric conductivity and improve the battery characteristics such as initial charge / discharge efficiency, and Fe It has been found that the desorption and elution of elements are effectively blocked by the non-graphitizable carbon layer, thereby completing the present invention.

The natural graphite based granular material containing the Fe element according to an embodiment of the present invention is not limited as long as it is well known in the art, but the carbon content is 80 to 98%, preferably 95% or more, At least one of natural crystalline graphite and natural crystalline graphite having a particle size of 1.0 to 50 占 퐉 may be selected.

According to one embodiment of the present invention, the natural graphite-based granular material is not limited, but may contain, for example, 0.05 to 0.8% by weight of Fe element. More preferably, the Fe element may be contained in an amount of 0.1 to 0.5% by weight, but is not limited thereto. When the Fe element is included in the above-mentioned range, the Fe element serves as a catalyst, so that a graphitizable carbon layer can be formed in the same manner as in the case of calcining at a temperature lower than an existing calcination temperature, And manufacturing costs and facility investment costs are reduced.

If the content of Fe element is less than 0.05 wt% in the natural graphite-based granular material, the effect of adding Fe is insufficient and it is difficult to lower the firing temperature. When the content exceeds 0.8 wt%, excessive Fe element may be exposed or eluted . In this case, the electrolytic solution decomposition reaction may occur and the battery life and the battery capacity may decrease.

The natural graphite-based granular material may further include a metal (M) other than Fe. For example, a metal such as Zn, Ni, Mg, Cu, Cr, Co, Al, B, Ga, (Sn), titanium (Ti), and zirconium (Zr), but is not limited thereto.

According to one embodiment of the present invention, the natural graphite-based granular material may satisfy the following formula (1).

0.4? Fe / M + Fe? 1.0 [Formula 1]

(Wherein Fe is the content (ppm) of Fe element contained in the natural graphite-based granular material, M is the content (ppm) of the metal element excluding iron contained in the natural graphite- Fe is the total amount (ppm) of the metal elements contained in the natural graphite-based granular material.)

The carbon composite material according to an embodiment of the present invention may include a non-graphitizable carbon layer derived from a thermosetting resin on the surface of a natural graphite-based granular material containing an Fe element. The term " derived " as used in the present invention means that the thermosetting resin is formed by heat treatment and carbonization unless otherwise specified.

By forming the non-graphitizable carbon layer thinly and uniformly, exposure and elution of Fe and other metal elements contained in the natural graphite-based granular material can be effectively prevented. This prevents exposure of the metal elements contained in the negative electrode active material, thereby suppressing side reactions between the electrolyte and the metal element during the electrochemical reaction of the battery and preventing the elution of the metal elements into the electrolyte, thereby improving the charge / discharge rate, , Improvement of initial efficiency, and so on.

According to an embodiment of the present invention, the non-graphitizable carbon layer is not limited to an average thickness, but is preferably, for example, less than 40 nm, more preferably 3 to 30 nm, but is not limited thereto.

According to one embodiment of the present invention, the thermosetting resin is not limited as long as it is a polymer resin capable of forming a non-graphitizable carbon layer by heat treatment. The carbon composite material produced by carbonization through heat treatment of the particulate material coated with the thermosetting resin according to the present invention can be used as a negative electrode active material for a lithium secondary battery.

Generally, the graphite structure gradually develops as the heat treatment temperature increases, but non-graphitizable carbons mean carbon that does not develop into graphite structure even at temperatures of 2500 ° C or higher. Such non-graphitizable carbon has excellent mechanical properties and can prevent exposure and elution of metal elements contained in natural graphite. The thermosetting resin may include, for example, pitches such as coal-based pitch and petroleum-based pitch, phenol resin, furan resin, polyurethane and polyisocyanurate. More preferably polyurethane and polyisocyanurate, but is not limited thereto.

The thermosetting resin according to an embodiment of the present invention can form a polyurethane resin by curing the thermosetting resin composition containing the polyol composition and the isocyanate compound on the surface of the granular material.

The polyurethane resin according to an embodiment of the present invention may be formed by reaction of a polyol compound and an isocyanate compound contained in the polyol composition. The polyurethane resin may contain polyisocyanurate depending on the kind and content of the isocyanate compound.

The polyisocyanurate may be produced by an endothermic reaction of an isocyanate compound and may be catalyzed by a catalyst, particularly a basic catalyst, in the polyol composition.

According to one embodiment of the present invention, the polyol compound is a typical one used for producing a polyurethane resin and is not particularly limited. Specifically, the polyether polyol, the polyester polyol, the polytetramethylene ether glycol polyol, the polyarnstoff dispersion (PHD) polyol, the amine-modified polyol, the Mannich polyol and the mixture thereof One or two or more selected are preferable, and more preferably polyester polyol, amine-modified polyol, Manmich polyol or a mixture thereof is effective.

The molecular weight of the polyol compound is not limited, but is preferably 300 to 3,000 g / mol, and more preferably 400 to 1,500 g / mol. When the molecular weight of the polyol is less than 300 g / mol, the thermal stability of the polyurethane resin synthesized by the formation of the monol decreases, resulting in melting in the carbonization step. When the molecular weight of the polyol exceeds 3,000 g / mol , There is a fear that the amorphous carbon chain increases in the polyol structure and the thermal stability of the polyurethane resin is lowered.

According to one embodiment of the present invention, the isocyanate-based compound which reacts with the polyol compound is not particularly limited and is usually used for producing polyurethane resin. Specific examples thereof include hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (H12MDI), polyethylene polyphenyl isocyanate, toluene diisocyanate (TDI) -Diphenylmethane diisocyanate (2,2'-MDI), 2,4'-diphenylmethane diisocyanate (2,4'-MDI), 4,4'-diphenylmethane diisocyanate (4,4'- MDI, monomeric MDI), polymeric MDI, polymeric MDI, orthotoluidine diisocyanate (TODI), naphthalene diisocyanate (NDI), xylene diisocyanate (XDI), lysine diisocyanate (LDI) Methane triisocyanate (TPTI), and more preferably 4,4'-diphenylmethane diisocyanate (4,4'-MDI, polymeric diphenylmethane diisocyanate Isocyanate (polymeric MDI) Or polyethylene polyphenyl isocyanate are effective.

It is effective that the mixing ratio of the polyol compound and the isocyanate compound is 50 to 250 parts by weight of the isocyanate compound based on 100 parts by weight of the polyol compound. When the content of the isocyanate compound is less than 50 parts by weight, formation of isocyanurate bonds that increase thermal stability is not sufficient, and the resin is melted in a similar manner to graphitizable carbon during the carbonization process, In this case, there is a fear of exposure or elution of metal elements including Fe contained in the natural graphite-based granular material.

If the content of isocyanate exceeds 250 parts by weight, the isocyanurate bond is excessively generated, and the specific surface area after the carbonization process is increased. As a result, the water adsorption ratio is increased and the initial surface side reaction .

The polyol composition according to an embodiment of the present invention may further include a catalyst to effectively induce the reaction between the polyol compound and the isocyanate compound. The catalyst is not limited as long as it is a catalyst for synthesis of polyurethane which is well known in the art. For example, there may be mentioned pentamethyldiethylene triamine, dimethyl cyclohexyl amine, bis- (2-dimethylaminoethyl) ether, triethylenediamine one or two or more selected from (triethylene diamine) potassium octoate, tris (dimethylaminomethyl) phenol, potassium acetate or a mixture thereof may be used.

The content of the catalyst according to an embodiment of the present invention is preferably 0.1 to 5 parts by weight, more preferably 0.5 to 3 parts by weight, based on 100 parts by weight of the polyol compound, Can be properly formed. When the content of the catalyst is less than 0.1 part by weight, the reaction between the polyol compound and the isocyanate compound proceeds too slowly, resulting in a reduction in efficiency of producing the negative electrode active material. When the content of the catalyst is more than 5 parts by weight, As the surface of the granular material becomes uneven, the physical properties of the negative electrode active material deteriorate and the problem of exposure or elution of metal elements including Fe contained in the natural graphite-based granular material may occur have.

The polyol composition according to an embodiment of the present invention may further include an organic solvent and an additive for improving processability.

The organic solvent is not limited as long as it is an organic solvent well known in the art and includes, for example, aromatic hydrocarbons such as benzene, toluene, xylene and mesitylene, ketones such as methyl ethyl ketone, acetone, methyl amyl ketone, methyl isobutyl Ketones such as benzene, ketone and cyclohexanone, and alcohols such as ethanol, propanol, butanol, hexanol, cyclohexanol, ethylene glycol and glycerin.

According to one embodiment of the present invention, the solid content of the polyol composition is not limited, but may be 10 to 70% by weight, more preferably 10 to 60% by weight. If it is necessary to control the viscosity according to the type of the kneading process, the amount of the solvent may be adjusted to control the solid content of the polyol composition.

When the content of the solid content of the polyol composition is less than 10% by weight, the content of the organic solvent is relatively large, and unnecessary energy is consumed during drying and curing. When the solid content is more than 70% by weight, the viscosity of the thermosetting resin composition as a whole is too high to uniformly coat the surface of the granular material, and the viscosity of the thermosetting resin composition or the polyol composition is increased There may be difficulties.

According to an embodiment of the present invention, the thermosetting resin may be contained in an amount of 3 to 35% by weight based on the total weight of the negative active material. More preferably 10 to 25% by weight.

If the content of the thermosetting resin is less than 3% by weight, the effect of the coating is not remarkable and it is difficult to exhibit the performance improvement effect. There is a fear of exposure or elution of metal elements including Fe contained in the natural graphite- If the weight percentage is more than 10% by weight, the non-graphitizable carbon layer may be thickened, resulting in deterioration of the C-rate performance and necking of the carbon layers on the surface of the particles. Further, the surface of the granular material may be damaged due to an increase in the amount of crushing energy required for the crushing, and the initial charge-discharge efficiency may be deteriorated rapidly.

Next, a method for producing a negative electrode active material for a lithium secondary battery will be described in detail.

The method of manufacturing an anode active material for a lithium secondary battery according to an embodiment of the present invention may include a step of carbonizing the Fe-containing natural graphite-based granular material coated with a thermosetting resin by heat treatment in an inert gas atmosphere.

More specifically

a) mixing a Fe-containing natural graphite-based granular material with a thermosetting resin composition comprising a polyol composition and an isocyanate-based compound;

b) drying and curing a mixture of the thermosetting resin composition and the Fe-containing natural graphite-based granular material to prepare a granular material coated with a thermosetting resin;

c) subjecting the thermosetting resin-coated Fe-containing natural graphite-based granular material to heat treatment in an inert gas atmosphere to produce a carbon composite; And

and d) crushing and classifying the carbon composite material.

According to an embodiment of the present invention, in the step a), the mixing may be performed by simultaneously mixing the Fe-containing natural graphite-based granular material and the thermosetting resin composition, or by first mixing the polyol composition and the Fe- The compound may be added at a later time, but is not limited thereto.

The Fe-containing natural graphite-based granular material may be contained in natural graphite itself, or may be prepared through a pretreatment process such as doping with an Fe element or heat treatment through coating of a precursor containing an Fe element. The doping or pretreatment can be applied without limitation as long as it is a method well known in the art.

The natural graphite-based granular material may contain, for example, 0.05 to 0.8% by weight of Fe element, though not limited thereto. More preferably, the Fe element may be contained in an amount of 0.1 to 0.5% by weight, but is not limited thereto. When the Fe element is included in the above-mentioned range, even when the Fe element acts as a catalyst and is fired at a temperature lower than the conventional firing temperature, the fired graphitizable carbon layer can be formed as in the case of firing at a high temperature, , And the process cost can be reduced.

According to an embodiment of the present invention, the thermosetting resin composition may contain 3 to 35% by weight based on the total weight of the negative active material. When the total amount of the thermosetting resin composition is in the above range, A thin and uniform non-graphitizable carbon layer having a thickness of 3 to 30 nm can be formed.

In the step (a) of the present invention, the mixing method is not limited, but may be carried out at 10 占 폚 or less in a uniaxial kneader, a biaxial kneader or a batch type kneader. Preferably -20 to 10 ° C, delay the reaction so that the thermosetting resin composition is not cured in the kneading step, and as a result, the uniformity of kneading can be improved, which is effective.

The drying and curing in the step b) according to an embodiment of the present invention may be performed by heating the hot air at 60 to 150 ° C in an apparatus equipped with a chamber and an exhaust port, but the present invention is not limited thereto. For example, various heating devices such as radiant heat, direct heater heating, and microwave heating can be used.

The apparatus including the chamber and the exhaust port may be applied to any device known in the art without limitation, and examples thereof include a hot air dryer, an air dryer, a cake dryer, and a ring dryer.

The drying and curing may be hot air heating at 60 to 150 ° C, and the hot air speed may be 3 to 15 m / sec, but is not limited thereto.

The heat treatment in the step c) according to an embodiment of the present invention is not limited as long as it is a temperature range in which the carbon composite material can be formed. For example, the heat treatment may be performed at a temperature of 700 to 1300 캜.

In the conventional heat treatment, it is preferable to perform the first heat treatment at 500 to 1000 ° C and the second heat treatment at 900 to 1500 ° C to carbonize the thermosetting resin. However, by adopting the Fe-containing natural graphite based granular material, It can be seen that the same effect as that of secondary heat treatment at a high temperature is obtained even if the decomposition and desorption rate of organic elements such as H, O, N is increased during the heat treatment due to the Fe element contained therein as a catalyst, there was.

The heat treatment is performed in an inert gas atmosphere, and the inert gas is preferably helium, nitrogen, argon, or a mixed gas thereof.

According to an embodiment of the present invention, the heat treatment is preferably performed at 700 to 1,300 ° C, more preferably 800 to 1,000 ° C. When the heat treatment is carried out at a temperature lower than 700 ° C, the volatilization of the low-molecular-weight gases is less likely to remain in the material, thereby reducing the yield of the product and the thermoplastic resin on the surface of the granular material is not sufficiently carbonized, There is a possibility that the characteristics are deteriorated. In addition, when the heat treatment is performed at a temperature higher than 1,300 ° C., the supply of the heat is increased more than necessary to increase the manufacturing cost. If the thermal decomposition product of the tar gas discharged from the raw material is not properly treated due to the high temperature, have. As a result, the specific surface area is increased and the property of adsorbing moisture in the atmosphere is increased, so that lithium ions and moisture react with each other in the cell reaction to increase the irreversible capacity. In addition, in order to withstand the heat treatment temperature of 1,300 ° C. or higher, the material and composition of the electric furnace must be changed to a material having heat resistance, so that the manufacturing cost and the process cost may increase.

According to an embodiment of the present invention, in the step d), the carbon composite material may be shredded using a rotary-type grinder.

The step (d) is a step of dissolving the fused anode active material particles after firing to separate them into individual particles. Although the crushing process can be applied to any ordinary crusher, it is preferable to apply a weak crushing method to prevent the coating layer from being damaged as much as possible.

The pulverizer is rotated by a circular rotor rotated by a motor, and at least two pulverizing bars are mounted on the rotating rotor, and the sectional shape of the pulverizing bar can be processed by a pulverizer having a circular or polygonal shape. For example, at least one of a pin mill, a fine impact mill, a ball mill, a bead mill, an air flow type classifier equipped with a rotor, a dyno mill, a disc mill, a roll mill and a cyclone mill and a combination of two or more. More preferably, use of a pin mill or a fine impack mill is effective because it can prevent surface damage of the carbon composite material.

The irreversible capacity of the lithium secondary battery manufactured by the above-described method of the present invention is 26 to 29 mAh / g and the reversible capacity is 359 to 365 mAh / g. It is possible to provide a lithium secondary battery which is capable of producing a low-cost anode active material and has excellent price competitiveness.

Hereinafter, preferred embodiments of the negative active material for a lithium secondary battery of the present invention and a method of manufacturing the same will be described in detail.

<Evaluation test items>

1) Manufacturing method of measuring cell and evaluation of charge / discharge characteristics

 The measurement cell was a coin type half cell, which was made of a negative electrode active material and a binder in a ratio of 97: 3, and a lithium metal foil as a counter electrode. The electrolyte was mixed with an organic electrolyte at a ratio of 1: And impregnated with an electrolyte in which 1 M of LiPF 6 was dissolved to prepare a 2016 type coin cell.

2) Evaluation of charge / discharge characteristics

The lithium ion was inserted into the carbon electrode by a constant current method at a rate of 0.1 C at a rate of 0.1 C and the lithium ion insertion was proceeded from 0.01 V by a constant current method. When the current reached 0.01 C, the lithium ion insertion was terminated. The discharge was performed at a constant rate of 0.1 C at a constant voltage of 1.5 V by means of a constant current method to release lithium ions from the carbon electrode. The ratio of the amount of desorption to the amount of inserting was the initial efficiency, and the amount of desorption was converted into the capacity per weight to calculate the reversible capacity.

3) Hydrogen / carbon (H / C) atomic ratio

A sample of the anode active material for a lithium secondary battery according to an embodiment of the present invention was subjected to elemental analysis by using a CHN analyzer and hydrogen / carbon (H / C) atomic ratio was measured using a hydrogen / carbon atom ratio .

[Example 1]

Production of negative electrode active material

20 kg / hr of spherical natural graphite having an Fe element content of 3000 ppm (0.3 wt%) having a D50 of 18 μm, 1.3 kg / hr of a polyol (SSP-104HC, Aekyung Oil Company) and acetone of 3.1 kg / kg / hr. Then, isocyanate (JG55K, manufactured by Kumho Mitsui Chemical Co., Ltd.) was fed into the kneading tank at a mixing rate of 2.5 kg / hr while the stirring was in progress. The temperature of the cooling jacket of the kneading machine was set at -15 ° C. (※ At this time, the actual temperature of the dough inside the stirrer was measured at about 7 ° C.) The continuous drying conveyor belt type hot air dryer was used. Lt; 0 &gt; C, followed by drying and curing reaction for 15 minutes to obtain a spherical graphite carbon composite coated with a thermosetting resin. The resulting carbon composite material was heat-treated at 900 캜 in a nitrogen atmosphere to prepare a negative electrode active material having a non-graphitizable carbon layer having a thickness of about 15 nm. The properties of the prepared negative electrode active material were measured and are shown in Table 2 below. The amount of polyol and isocyanate is determined by setting the carbonization yield of graphitizable carbon to 30%.

 [Examples 2 to 5]

As shown in the following Table 1, negative electrode active materials were prepared in the same manner as in Example 1, except that natural graphite having different Fe contents was used or natural graphite having different Fe / M + Fe value was used. The properties of the prepared negative electrode active material were measured and are shown in Table 2 below. The carbonization yield of the flammable carbon was set in the same manner as in Example 1.

[Comparative Example 1]

10 kg of spherical natural graphite having a D50 of 18 mu m and an Fe element content of 3000 ppm (0.3 wt%), 65% of a carbonization yield and a mean particle size of about 2 [mu] m were impregnated in an impeller type stirrer 0.65 kg of the system pitch were simultaneously introduced and mixed at 150 rpm for 30 minutes to prepare a graphite pitch complex having evenly distributed pitch fine powder. The prepared composite was subjected to a primary heat treatment at 700 ° C in a nitrogen atmosphere and a secondary heat treatment at 1250 ° C to produce a negative electrode active material having a graphitizable carbon layer. The pitch, which is a precursor of the biodegradable carbon, was set at a carbonization yield of 64%.

[Comparative Example 2]

As shown in the following Table 1, a negative electrode active material was prepared in the same manner as in Example 1, except that spherical natural graphite having a D50 of 18 mu m and an Fe element content of 1 ppm (0.0001 wt%) was used. The properties of the prepared negative electrode active material were measured and are shown in Table 2 below. The carbonization yield of the flammable carbon was set in the same manner as in Example 1.

[Comparative Example 3-4]

As shown in the following Table 1, negative electrode active materials were prepared in the same manner as in Example 1, except that natural graphite having different Fe contents was used or natural graphite having different Fe / M + Fe value was used. The properties of the prepared negative electrode active material were measured and are shown in Table 2 below. The carbonization yield of the flammable carbon was set in the same manner as in Example 1.

 [Table 1]

[Experimental Example 1] Preparation of secondary battery

(a) Electrode Fabrication

 1.5 parts by weight of SBR (Styrene Butadiene Rubber) and 1.5 parts by weight of CMC (Carboxyl Methyl Cellulose) were added to 97 parts by weight of the negative electrode active material prepared above, and distilled water was added thereto. The mixture was uniformly stirred in a sludge form and uniformly coated on a copper foil. The coating was uniformly coated at 110 μm using a doctor blade, dried in an oven at 60 ° C. for 30 minutes, and pressed at a pressure of 0.6 MPa. A foil-shaped electrode was punched out in a circle of 1 cm 2 in width and dried in a 120 ° C vacuum oven for 12 hours.

(b) Preparation of test cell

 The negative electrode active material prepared in the above Examples and Comparative Examples was used for the negative electrode of the water-based electrolyte secondary battery, and the charging (lithium insertion) capacity and the discharging (lithium talli) capacity of the negative electrode active material were not influenced by the performance of the counter electrode, The lithium secondary battery was constructed by using lithium metal as a counter electrode to evaluate the characteristics of the secondary battery.

A lithium secondary battery was assembled in a glove box of 2016 size (20 mm in diameter, 16 mm in thickness) under an argon atmosphere, and 1 mm thick metallic lithium was squeezed on the bottom of a can of canned battery cell, And the cathode was made to face the lithium. The electrolyte used was prepared by adding 1.2M LiPF6 salt to a solvent prepared by mixing EC (Ethylene Carbonate), DMC (Dimethyl Carbonate) and EMC (Ethyl Methyl Carbonate) at a volume ratio of 1: 1: The battery was put into a doll battery, and the can cover was contacted and pressed to assemble the lithium secondary battery.

(c) Battery capacity measurement

Characterization of the assembled lithium secondary battery was carried out by charging and discharging at 25 DEG C by a constant current-constant voltage method (CCCV) using a TOSCAT-3100 charging and discharging test apparatus manufactured by TOYO SYSTEM. 'Charging' is a reaction in which lithium is inserted into a cathode, and a 'coin-type battery' is a reaction in which a voltage of a coin-type battery is lowered. . Also, the constant current-constant voltage condition is such that the charging is performed at a constant current density (0.1 C standard) until the voltage of the coin-type battery becomes 0.05 V, and then the constant current And the charging is continued. The value obtained by dividing the amount of electricity supplied at this time by the weight of the negative electrode active material of the electrode was defined as a charging capacity per unit weight (mAh / g) of the negative electrode active material. After completion of charging, the battery was stopped for 10 minutes and discharged. The discharge was carried out at a constant current until the voltage of the coin-type battery reached 1.5 V, and the discharge capacity (mAh / g) per unit weight of the negative electrode active material was obtained by dividing the discharged electricity by the weight of the negative electrode active material Respectively. The reversible capacity was defined as the discharge capacity. The irreversible capacity was calculated by subtracting the discharge capacity from the charge capacity, and the efficiency was calculated as the discharge capacity (%) in relation to the charge capacity. The characteristics of the basic coin-type battery are shown by averaging three or more characteristic values of the same battery.

(d) Measurement of high rate charge / discharge characteristic

The high rate charge / discharge characteristics of the assembled lithium secondary battery were analyzed at 25 DEG C by the constant current-constant voltage method (CCCV) in the same manner as in (c). The high rate charge / discharge characteristics are expressed by the capacity (mAh / g) measured by charging / discharging at the current density by changing the current density during charging / discharging and increasing the constant current density supplied / discharged cycle by cycle.

The initial charge / discharge capacities and efficiencies of the negative electrode active materials prepared in the above Examples and Comparative Examples were measured and shown in Table 2 below.

[Table 2]

FIG. 1 is a SEM photograph of a natural graphite used in an anode active material for a lithium secondary battery according to an embodiment of the present invention, and FIG. 2 is a SEM photograph of a section of a natural graphite doped with a metal. 3 is an SEM photograph of a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention.

As shown in Fig. 2, it was confirmed that Fe and other metals were observed between the structures of the layered natural graphite.

The natural graphite is coated with a thermosetting resin and fired to form a uniform non-graphitizable carbon layer, thereby preventing exposure or elution of the metal particles, thereby improving durability and initial charge / discharge efficiency.

As shown in Table 2, the anode active material prepared according to Examples 1 to 5 exhibited excellent battery capacity and initial charge / discharge efficiency despite the heat treatment at a lower temperature than the conventional firing process temperature.

On the other hand, in Comparative Example 1, since the coating efficiency of the pitch was mixed with the powder state, the irreversible capacity was increased and the initial charge / discharge efficiency was lower than that of the thermosetting resin coated with the liquid phase process. It was found that the H / C ratio was increased due to the absence of Fe element in the natural graphite, and the initial charge / discharge efficiency was lowered due to the increase of the initial irreversible reaction.

In addition, in Comparative Example 3, it was found that the excessive charge of Fe element was contained in the natural graphite, the battery capacity was low due to the high metal content as a whole, and the initial charging / discharging efficiency was lowered due to the metal particles being exposed to the electrolyte solution. , It is difficult to expect an effective catalytic effect due to reduction of the Fe action area due to intermetallic bonding when the Fe / M + Fe ratio is too small even if the content of Fe element is appropriate in the natural graphite, Could know.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. It will be clear to those who have knowledge of.

Claims (10)

Based natural graphite-based granular material coated with a thermosetting resin obtained by coating a thermosetting resin composition comprising a polyol composition and an isocyanate-based compound on the surface of an Fe-containing natural graphite-based granular material and curing the resultant mixture is heat-treated in an inert gas atmosphere to form carbon &Lt; / RTI &gt;
Wherein the natural graphite-based granular material contains Fe element in an amount of 0.05 to 0.8 wt%, and the carbon composite material has a non-graphitizable carbon layer formed by carbonizing an average thickness of 3 to 30 nm on the surface of the granular material Negative electrode active material for rechargeable lithium secondary battery. The method according to claim 1,
Wherein the natural graphite based granular material contains 0.1 to 0.5% by weight of an Fe element as an anode active material for an Fe-containing natural graphite based lithium secondary battery. The method according to claim 1,
Wherein the natural graphite-based granular material satisfies the following formula (1).
0.4? Fe / M + Fe? 1.0 [Formula 1]
(Wherein Fe is the content (ppm) of Fe element contained in the natural graphite-based granular material, M is the content (ppm) of the metal element excluding iron contained in the natural graphite- Fe is the total amount (ppm) of the metal elements contained in the natural graphite-based granular material.) delete The method according to claim 1,
Wherein the thermosetting resin comprises a polyurethane and a polyisocyanurate. The method according to claim 1,
Wherein the thermosetting resin comprises 3 to 35% by weight based on the total weight of the negative active material, and the thermosetting resin is a negative active material for an Fe-containing natural graphite based lithium secondary battery. A lithium secondary battery comprising a negative electrode active material for a Fe-containing natural graphite based lithium secondary battery according to any one of claims 1 to 3, 5 and 6. 8. The method of claim 7,
Wherein the irreversible capacity of the lithium secondary battery is 26 to 29 mAh / g and the reversible capacity is 359 to 365 mAh / g. Based natural graphite-based granular material coated with a thermosetting resin obtained by coating a thermosetting resin composition comprising a polyol composition and an isocyanate-based compound on the surface of a Fe-containing natural graphite-based granular material and curing the resultant mixture is heat-treated in an inert gas atmosphere to form carbon &Lt; / RTI &gt;
Wherein the natural graphite based granular material contains 0.05 to 0.8% by weight of an Fe element, and the thermosetting resin composition contains 3 to 35% by weight based on the total of the negative electrode active material, and has an average thickness of 3 to 30 nm Wherein the non-graphitizable carbon layer is formed on the surface of the non-graphitizable carbon layer. 10. The method of claim 9,
Wherein the annealing temperature is 700 to 1,300 DEG C. 5. A method for producing an anode active material for a Fe-containing natural graphite based lithium secondary battery,

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