KR20170002302A - An anode for lithium-ion secondary battery and secondary battery - Google Patents

Abstract

Provided is an anode for a lithium-ion secondary battery, in which destruction of a graphite material is suppressed, and orientation is appropriately controlled, thereby satisfying desired energy density and cycle characteristics in practice when the anode for a lithium-ion secondary battery is applied to various types of purposes related to resolution of an energy environment problem. The anode for a lithium-ion secondary battery includes an anode active material layer on a current collector. The anode active material layer includes an anode active material and a binder. The anode active material is made by mixing graphite particles, in which a ratio of particles having an elliptical aspect ratio of 0.5 or more is 60% or more and which has a particle density ranging from 1.90 to 2.16 g/cm^3, and low-crystalline carbon particles, in which a ratio of particles having an elliptical aspect ratio of 0.5 or less is 60% or more, at a mass ratio of 95:5-70:30. The anode for a lithium-ion secondary battery according to the present invention is suitable for a peak-strength ratio (I_110/I_004) of the anode active material layer or a pore surrounding length to satisfy a predetermined range.

Description

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

The present invention relates to a negative electrode and a secondary battery for a lithium ion secondary battery that are excellent in durability with a high capacity and can be used for various applications related to energy environment problems such as hybrid automobiles and solar power generation.

BACKGROUND ART [0002] In a modern society, which is maintained by electric energy, a rechargeable battery capable of recharging and discharging can be used nowadays. Particularly, the lithium ion secondary battery is relatively light in weight and low in environmental pollution while taking advantage of excellent characteristics such as a high operating potential, a large battery capacity, and a long cycle life. Therefore, a nickel- It is widely used instead of batteries.

In recent years, in order to cope with energy problems and environmental problems, a lithium ion secondary battery is mainly used as a hybrid electric vehicle in which an electric vehicle, a motor and a gasoline engine are combined, a plug- And is widely used as a large-sized battery of a hybrid electric vehicle. In addition to this, in combination with a generator whose output fluctuates, such as solar power generation or wind power generation, it is possible to control the absorption of the fluctuation or the output to be constant, or to control the fluctuation of the fluctuation or the peak shift on the demand side, Directional storage battery has been attracting attention and it is expected that the demand characteristics will gradually increase as the demand for various uses related to these energy environment problems increases.

As the negative electrode active material constituting the negative electrode of the lithium ion secondary battery, a carbon material including graphite, lithium titanate, silicon, tin and the like can be mentioned, but a carbon material is generally used from the viewpoints of safety and life span. Since graphite materials among carbon materials are excellent materials having a high energy density, they are currently used not only as power sources for portable electronic devices but also as power sources for hybrid electric vehicles and plug-in hybrid electric vehicles, lithium ion secondary batteries And research and development are underway.

In such a large-sized battery, it is demanded that the energy density is high, the deterioration of the discharge capacity is small with respect to repetition of charging and discharging, and the cycle characteristic is excellent. Particularly, in the above-mentioned hybrid electric vehicle and plug-in hybrid vehicle, rapid charging and rapid discharging performance are required, and a battery excellent in high input / output characteristics is required. However, in a battery using lithium ion, And it is expected to improve this.

Examples of the carbon material used as the negative electrode active material herein include amorphous carbon and graphite having high crystallinity. Graphite is generally used for applications requiring high energy density. Graphite is largely classified into natural graphite and artificial graphite. In general, natural graphite has a large specific surface area, has high reactivity with an electrolytic solution, and has high crystallinity. Therefore, natural graphite tends to be easily deformed by pressure and easily oriented. Thus, it is difficult to obtain high cycle characteristics as required for electric automobile batteries in natural graphite. Thus, attempts have been made to reduce the specific surface area by coating the surfaces of the particles with low-crystalline carbon to reduce the reactivity with the electrolytic solution. In addition, attempts have been made to reduce the orientation by sphering natural graphite. However, it is still insufficient to control the orientation, which is a practical difficulty when graphite is used for a negative electrode material for a lithium ion secondary battery.

On the other hand, artificial graphite is said to be excellent in cycle characteristics because it has lower reactivity with an electrolyte than natural graphite and has less orientation of particles. However, artificial graphite has various particle characteristics such as crystallinity, particle shape, and hardness of particles depending on its production method, and performance of artificial graphite can not be sufficiently drawn out unless an electrode design suitable for the particulate property is carried out.

For example, Patent Document 1 discloses a technique for providing a lithium ion secondary battery having excellent cycle characteristics by preventing deterioration of conductivity of an electrode, which is a problem when a graphite material having a small deformation and orientation is used by pressure have. However, in the graphite material of Patent Document 1, there is a problem that when the electrode is compressed, the electrode density can not be sufficiently increased, and the capacity density per volume is lowered. If the density of the electrode is intentionally increased at a high pressure, cracks or breakage occurs at the cross-sectional area in the graphite plane orientation at the time of consolidation, and electrochemical side reaction occurs in the newly formed graphite cross section. There was a problem that it was deteriorated.

Patent Document 2 discloses that by using composite graphite particles composed of a core material made of graphite having a specific interlayer distance and a surface layer having a low crystalline carbon having an R value of at least a specific value obtained from Raman scattering spectroscopy, Discloses an anode active material of a lithium ion secondary cell having excellent charge / discharge cycle characteristics in which crystalline orientation of graphite particles is suppressed at the time of consolidation. However, with respect to the composite graphite particles of Patent Document 2, since the low-crystalline carbon material is coated on the graphite surface to improve the compressive strength of the graphite particles, the particles are hardly deformed and the orientation of the graphite crystals is suppressed. However, It is insufficient only by the effect of reinforcing the particles. Therefore, when the electrode density is to be further improved, the coating pressure is broken by the press pressure, resulting in electrochemical side reaction in the newly formed graphite cross section, .

In Patent Document 3, it has been proposed to control both the porosity inside the negative electrode active material particle and the porosity outside the negative electrode active material in the electrode. By providing a predetermined amount of voids in the inside of the negative electrode active material particles, it is possible to use ions for the diffusion of the inside of the void, and the output characteristics are improved. In Patent Document 4, it has been proposed to use different negative electrode active materials to control the porosity of each of the negative electrode active material layers. Layered active material layer containing two kinds of negative electrode active materials different in rolling density and average particle size of the negative electrode active material on the electrode current collector, it is possible to improve the porosity of the electrode surface even after the rolling process, The chargeability and lifetime characteristics of the lithium secondary battery can be improved.

However, when a graphite material having a small degree of deformation or orientation due to the pressurization as described in Patent Document 3 is used, the electrode density can not be sufficiently increased when the electrode is compacted, and the capacity density per volume is lowered. There was a problem in that it could not be compatible. In addition, as in Patent Document 4, there is a problem that the productivity is significantly lowered by the method of forming the two-layer negative electrode active material layer.

WO2011-115247 WO2007-072858 Japanese Laid-Open Patent Publication No. 2014-53154 WO2014 / 116029

It is an object of the present invention to provide a lithium secondary battery anode which can be practically satisfactory in energy density and cycle characteristics required when it is applied to various applications related to energy environment problem resolution.

As a result of advanced studies, the inventors of the present invention have found that, by mixing graphite with specific low-crystallization carbon, orientation of graphite during consolidation is suppressed to prevent deformation and breakage, thereby increasing the cycle density while increasing the capacity density and charge- And have accomplished the present invention. Further, the present inventors have found that it is important not only to control the pore volume in the electrode but also to control the reaction surface area in the electrode, in order to improve the output characteristics. By mixing low-crystalline carbon containing a certain amount of particles of a specific shape other than graphite at a predetermined ratio into graphite containing a certain number of particles of a specific shape, it is possible to realize more efficient diffusion of lithium ions even at the same electrode density and porosity Thus, the inventors have found that not only the capacitance density and the input / output characteristics but also the productivity can be increased, thereby completing the present invention.

That is, the present invention is a negative electrode for a lithium ion secondary battery having a negative electrode active material layer on a current collector, wherein the negative electrode active material layer includes a negative electrode active material and a binder and the negative electrode active material has a ratio of particles having an? Graphite particles are mixed with low-crystalline carbon particles having a true specific gravity of 1.90 to 2.16 g / cm 3 and a ratio of particles having an aspect ratio of less than 0.5 of 60% or more at a mass ratio of 95: 5 to 70:30. It is a negative electrode for a lithium ion secondary battery.

The lithium ion peak intensity of (110) of the graphite particles determined by XRD measurement of the negative electrode active material layer side of the secondary battery (I ₁₁₀₎ and the ratio (I ₁₁₀ / I ₀₀₄₎ between the peak intensity (I ₀₀₄₎ of face (004) Is preferably from 0.70 to 1.10, and the pore circumference per unit area observed in an electron microscope of the end face of the negative electrode active material layer is preferably 1.30 to 2.00 mu m / mu m < ^{2 &} gt ;. It is more preferable to satisfy both of these peak intensity ratios and the pore circumference field.

The negative electrode active material of the negative electrode for a lithium ion secondary battery preferably has a true specific gravity of 2.13 to 2.23 g / cm3, a tap density of 0.5 g / cm3 or more, and an average particle diameter D50 of 5 to 20 mu m.

The negative electrode for a lithium ion secondary battery preferably has a true specific gravity of 2.23 to 2.24 g / cm3 and an average particle size D50 of 5 to 20 占 퐉 in graphite particles in the negative electrode active material and the low-crystalline carbon particles in the negative electrode active material have an average particle size D50 of 5 To 20 mu m, and it is preferable that these graphite particles and low-crystalline carbon particles are used as essential components.

The low-crystalline carbon particles used in the negative electrode active material of the negative electrode for a lithium ion secondary battery are obtained by calcining cokes, coal or petroleum-based calcined cokes, or calcined cokes obtained by calcining coal or petroleum raw cokes at 900 to 1500 ° C, And at least one selected from coke treated at 1,500 ° C.

In addition, another aspect of the present invention is a lithium ion secondary battery using the negative electrode. In particular, a lithium ion secondary battery in which a negative electrode composed of a negative electrode active material layer formed by mixing the lithium ion secondary battery negative electrode active material and a binder is opposed to a positive electrode through a separator, wherein the negative electrode has an initial capacity N (mAh / Cm < 2 >) and the initial capacity P (mAh / cm2) of the positive electrode is 1.0 to 1.5.

Hereinafter, another preferred embodiment of the present invention will be described.

A negative electrode active material obtained by mixing graphite particles and low-crystalline carbon particles having a true specific gravity of 1.90 to 2.16 g / cm 3 in a mass ratio of 95: 5 to 70:30 and mixing the negative active material and the binder on the current collector, Wherein the negative electrode active material layer has a density of 1.10 to 1.40 g / cm3 after the compacting, and the peak intensity of the (110) plane of the graphite particle crystal in the negative electrode active material by XRD measurement I ₁₁₀₎ and (004) peak intensity of the face ₍₀₀₄ I) ratio (I ₁₁₀ / I ₀₀₄₎ the lithium ion secondary battery negative electrode, characterized in that between 0.70 ~ 1.10 in.

The negative electrode for a lithium ion secondary battery is characterized in that the low-crystalline carbon particles in the negative electrode active material have an average particle size D50 of 5 to 20 占 퐉 and a particle ratio of less than 0.50 (ratio of short axis per one strand to long axis per stride) Is preferably 70% or more, and the graphite particles in the negative electrode active material preferably have a true specific gravity of 2.23 to 2.24 g / cm3, an average particle size D50 of 5 to 20 占 퐉, and a particle ratio of not less than 0.50 of the void ratio per 70% .

Low-carbon graphite particles having a ratio of particles having a steric length per unit length ratio of 0.5 or more to graphite particles having a ratio of 70% or more to particles having an aspect ratio per unit length of less than 0.5 of 70% or more are mixed in a mass ratio of 95: 5 to 70:30 A negative electrode for a lithium ion secondary battery comprising a negative electrode active material and a negative electrode active material layer formed by mixing the negative electrode active material and a binder on the current collector, wherein the negative electrode active material layer has a density of 1.10 to 1.40 g / Lt; 3 > / cm < 3 >, and the periapical length per unit area observed in an electron microscope of the negative electrode cross section is 1.30 to 2.00 mu m / mu m < ² >.

It is preferable that the negative electrode for a lithium ion secondary battery has a true specific gravity of 1.90 to 2.16 g / cm 3 and an average particle size (D50) of 5 to 20 占 퐉. The true specific gravity of the graphite particles measured by a peak- Is in the range of 2.23 to 2.24 g / cm3, and the average particle diameter (D50) is preferably 5 to 20 占 퐉.

According to the present invention, it is possible to provide a lithium ion secondary battery in which charging and discharging efficiency is enhanced by suppressing breakdown of graphite particles by adding low-crystalline carbon particles specific to graphite particles. Further, at the same time, by controlling the orientation of the graphite particles, a lithium ion secondary battery having excellent cycle characteristics can be provided.

When graphite particles are used alone, deformation of the particles occurs during consolidation. When the low-crystalline carbon particles having a more elliptical (plate-like) particle shape are added to some extent, low-crystalline carbon particles It is presumed that the graphite particles are prevented from being deformed and thus the graphite particles are prevented from being broken and the orientation is appropriately controlled.

Further, according to the present invention, by using a negative electrode active material obtained by mixing a predetermined amount of low-crystalline carbon containing a certain amount or more of particles having a specific shape other than graphite in a graphite containing a certain number of particles of a specific shape, It is possible to suppress the reduction of the reaction area within the reaction zone. As a result, it is possible to provide a negative electrode and a lithium ion secondary battery for a lithium ion secondary battery which can realize more efficient diffusion of lithium ions as compared with a graphite electrode having the same electrode density and porosity, and excellent in high input / output characteristics. This lithium ion secondary battery is suitable for a wide range of applications such as a hybrid vehicle, a plug-in hybrid vehicle, and a power tool.

The negative electrode for a lithium ion secondary battery of the present invention is a negative electrode for a lithium ion secondary battery comprising a graphite particle containing 60% or more of grains having an aspect ratio of 0.5 or more and a low-crystalline carbon particle containing 60% or more grains having an aspect ratio As a negative electrode active material. As the graphite particles of the negative electrode active material, particles having an aspect ratio of 0.5 or more and an aspect ratio of 0.5 or more are more preferably 70% or more, and as the low-crystalline carbon particles, particles having an aspect ratio less than 0.5 of 70% or more are more preferable.

Here, the ratio of strand length to strand length means the short axis length per strand / long strand length per strand.

By combining such graphite particles and low-crystalline carbon particles, the orientation of the graphite particles in the negative-electrode active material layer before the compaction can be promoted and filled, and deformation / fracture during electrode compaction can be suppressed. On the other hand, when the ratio of the graphite particles to the graphite particles having an aspect ratio of 0.5 or more is excessively small, the graphite particles may have low resistance to deformation, and breakage may occur at the time of consolidation of the electrodes. When the proportion of the particles having an aspect ratio of less than 0.5 of the void fraction of the low-crystalline carbon particles is excessively small, the orientation property is not promoted in charging the graphite particles of the negative-electrode active material layer before the compaction, Excessive stress is applied and deformation / breakage occurs, which causes a side reaction when the battery is used, and a phenomenon such as a decrease in power performance due to a reduction in the reaction area due to the clogging of voids in the graphite particles.

In the present specification, the strand length ratio of the strand is calculated as the short axis length per strand / the length of the long strand per strand. Graphite particles or low-crystalline carbon particles, and when the ratio of the long axis to the oval phase is 0.5 or more, it is a spherical particle. On the other hand, if the ratio of the long axis to the oval phase is less than 0.5, (Plate shape). The surface roughness ratio of the strand can be analyzed by using an image analysis software (WinROOF: manufactured by Mitani Shoji Co., Ltd.) or the like for the particle image observed by the SEM method or the like. The particle image can be a layer or a molded body containing particles and a resin, and can be an image of particles appearing by polishing.

The number of particles a having a ratio of 0.5 or more and the number b of particles having a ratio of 0.5 or less of 0.5 or more are counted. Then, a / (a + b) or b / (a + b) is calculated and the value is expressed in%. It is understood that the ratio of the length of the ellipsoid is the ratio of the major axis of the ellipse having the same first and second moments and the minor axis perpendicular to the first and second moments.

The graphite particles used in the negative electrode active material of the negative electrode for a lithium ion secondary battery of the present invention are preferably graphite particles having a high specific gravity of 2.23 g / cm3 to 2.24 g / cm3. The true specific gravity indicates the development of the crystal structure of the carbon material, and generally, as the crystal structure develops, the capacitance density per weight improves. Therefore, it is preferably 2.23 g / cm 3 or more. Artificial graphite and natural graphite can be mentioned as the graphite giving the true specific gravity, but natural graphite is more preferable because of low cost and easiness of electrode production.

Herein, the true specific gravity is measured by a pycnometer method with respect to the particles after the treatment when the graphite particles are subjected to spheroidizing treatment, surface treatment, etc., and if there is a void in which the liquid can not penetrate, It is included in volume. The true specific gravity referred to in the present specification is understood to be a value measured under the conditions described in the examples, unless otherwise specified.

The graphite preferably has a small amount of impurities from the viewpoint of suppressing side reactions, and is subjected to various purification treatments if necessary. Examples of the natural graphite include impression graphite, scaly graphite, and soil graphite. Of these natural graphite, the graphite in the ground is generally small in particle size, and has low purity. On the other hand, flaky graphite or impression graphite has advantages such as high graphitization degree and low impurity amount, and therefore can be preferably used in the present invention.

Examples of the artificial graphite include graphite pitch, coal heavy oil, atmospheric residue, petroleum heavy oil, aromatic hydrocarbon, nitrogen-containing cyclic compound, sulfur-containing cyclic compound, polyphenylene, polyvinyl alcohol Organic materials such as polyacrylonitrile, polyvinyl butyral, natural polymers, polyphenylene sulfide, polyphenylene oxide, furfuryl alcohol resin, phenol-formaldehyde resin and imide resin are baked and heated at a temperature of 2500 ° C. to 3200 ° C. Lt; 0 > C. On the other hand, when firing, a silicon-containing compound, a boron-containing compound, or the like may be used as a graphitization catalyst, or phosphorus or phosphorus compounds may be added.

The graphite to be used in the active material of the negative electrode for a lithium ion secondary battery of the present invention can be appropriately selected from flaky, fibrous, amorphous particles and the like, but is preferably spherical. Since graphite particles are generally flat, they have a high specific surface area and are difficult to charge at high temperatures. In addition, there has been a problem that lithium ion adsorption / desorption occurs only on the edge surface. For this reason, spheroidizing treatment is carried out for the purpose of reducing the specific surface area and obtaining an isotropic crystal structure. By carrying out this spheroidizing treatment, it is possible to control the stiffness ratio of the graphite particles. However, there is an advantage that a slightly elliptical shape than a true spherical shape is easy to be oriented at the time of consolidation.

The spheroidizing treatment may be a mechanical treatment or a method of performing granulation using a pitch or the like. However, the shape of the graphite particles after the spheroidizing treatment is not particularly limited, / Long axis length per strand) 0.50 or more is preferably 70% or more. If the ratio of the long axis to the tangential phase is less than 0.50, the anisotropy of the crystal structure in the graphite particles becomes large, and the in-plane orientation becomes too large when the negative active material layer is formed by consolidation. As a result, the expansion and contraction in the planar direction during storage and release of lithium ions increases, and the cycle characteristics remarkably deteriorate.

The graphite particles preferably have a BET specific surface area of 3.0 to 8.0 m2 / g. This BET specific surface area is determined by the shape of the graphite particles and the properties of the surface coat layer. If the BET specific surface area is too small, the charging / discharging rate of the lithium ion is delayed. If the BET specific surface area is excessively large, the tap density is not increased and the electrode density is not sufficiently increased. Since the BET specific surface area affects the rate of surface reaction when lithium ions enter and exit the carbon structure, it is preferable to control the BET specific surface area to an appropriate value.

As the negative electrode active material of the negative electrode for a lithium ion secondary battery of the present invention, the low-crystalline carbon particles to be blended in the graphite particles preferably have a ratio of particles having an aspect ratio less than 0.5 of the steric phase of less than 0.5, but have a true specific gravity of 1.90 to 2.16 g / cm < 3 >. If the true specific gravity of the low-crystalline carbon particles is too low, a side reaction may occur at the time of charging / discharging when applied to a lithium ion secondary battery, which may lower the efficiency, and if too high, the discharge capacity may decrease. If the true specific gravity is too low, the crystal growth of the carbon particles proceeds, so that the graphite crystals laminated inside the particles are liable to slip between the layers due to the external pressure, so that the deformability of the internal particles at the time of compaction is lowered, Destruction). In the present invention, it is possible to suppress the closure of the internal pores of the graphite particles during consolidation by adding the low-crystalline carbon particles which are less likely to be broken than graphite in the deformable graphite particles, and to suppress the breakage of the graphite particles during consolidation . Therefore, if the deformability of the low-crystalline carbon particles added to the graphite particles is low, the particles of the negative-electrode active material may be broken during consolidation, resulting in a decrease in charge-discharge efficiency.

On the other hand, the reason why the charging / discharging efficiency of the negative electrode is lowered due to the destruction of the negative electrode active material particles is that some of the electric capacity consumed in the charging in the negative electrode is consumed in side reactions and competitive reactions on the surface of the active material newly generated by the destruction, These side reactions and competing reactions are primarily due to the decomposition of the electrolyte solution at the edge of the hexagonal planar laminate exposed to the surface of the graphite material, although lithium is not consumed in the occlusion reaction. In general, a number of short-circuited bonds, that is, valence electron bonds, do not saturate in the edge plane of a hexagonal plane-plane stack, and there are many local electrons present in the non-opposing bond. In addition to the original charging reaction that lithium intercalates between the layers of the hexagonal mesh plane at the interface of the surface of the negative electrode graphite material in the charging process, that is, the interface between the electrolyte and the graphite material, It is considered that the charge / discharge efficiency of the negative electrode is lowered due to the side reaction and the competitive reaction caused by the reduction and decomposition of the electrolytic solution.

As the low-crystalline carbon particles, coke, coal or petroleum calcined coke obtained by calcining coal or petroleum raw coke at 900 to 1500 ° C or coke obtained by further calcining the calcined coke at 900 to 1500 ° C is suitable . These are used alone or in combination. On the other hand, coal-based and petroleum-based ones include those obtained from a mixture of coal-based oil and petroleum-derived oil.

The method for obtaining the low-crystalline carbon particles suitable for the present invention will be described in detail below. First, using a coking plant such as a petroleum-based or coal-based heavy oil, for example, a delayed coker, A raw coke such as a coal system is obtained by conducting pyrolysis / polycondensation reaction at a temperature of about 700 ° C for about 24 hours.

The heavy oil used herein may be either petroleum heavy oil or coal-based heavy oil, but the coal-based heavy oil is rich in aromatics, less impurities such as sulfur, vanadium and iron, and less volatile, Do. Mixtures of coal-based and petroleum-based oils are also desirable. Therefore, coal-based coke obtained therefrom is preferable.

The obtained raw coke of coal or the like is ground to a predetermined size, for example, 5 to 20 占 퐉, if necessary. A grinder used industrially can be used for grinding.

Next, the raw coke such as the coal-based coal is calcined in a low-oxygen atmosphere at a maximum reaching temperature of 800 ° C to 1500 ° C to produce a calcined coke such as coal system. The calcination temperature is preferably in the range of 900 占 폚 to 1500 占 폚, more preferably 1000 占 폚 to 1400 占 폚. The calcination treatment removes moisture and volatile components from the raw coke and catalyses the growth of crystals by converting the remaining hydrocarbons as a polymer component into coke. As the heat treatment of raw coke of coal or the like, a lead hammer capable of mass heat treatment can be used, such as a shuttle, a tunnel, a rotary kiln, a roller hearth kiln or a microwave, but is not limited thereto. These heat treatment facilities may be either continuous type or batch type. Next, a mass of the obtained calcined calcined coke is pulverized to a predetermined size, for example, 5 to 15 탆, by using a grinder such as an attritor or the like which is industrially used in the same manner. It is also preferable that the pulverized coke powder is cut to a predetermined particle size, for example, by cutting the fine powder by classification or by removing the coarse powder with a sieve or the like. Crystalline growth of carbon progresses by calcining treatment, and anisotropy is obtained in the plane direction and the thickness direction. By crushing the anisotropic coke, a low-crystalline carbon particle having a particle ratio of less than 0.50 in the aspect ratio of an oval phase of 70% or more is obtained have.

The low-crystalline carbon particles used in the present invention may be the above-mentioned coal-based calcined coke, but may be a calcined coke obtained by calcining raw calcined cokes such as the coal-based calcined cokes or coal calcined alone or in combination. The firing process is performed for raw coke or calcined coke to further adjust the crystal state, or for surface control or surface modification. In the firing process, a phosphorus compound or a boron compound may be added to prepare the crystallization of the carbon material, or the firing may be performed a plurality of times. In addition, in the firing process, even if one or more processes such as a shape control process such as a coarse powder, a process of modifying the surface with other organic or inorganic components, a coating process, or a process of uniformly or dispersing other metal components on the surface good.

On the other hand, the firing treatment is preferably carried out at a maximum temperature of 900 ° C or higher and 1500 ° C or lower. Preferably 950 ° C to 1450 ° C, and more preferably 1000 to 1400 ° C. If the sintering temperature is too high, the crystal growth of the coke material is excessively promoted and the true specific gravity becomes excessively high, so that the crystal structure of the coke is oriented like graphite and the deformability is deteriorated. As a result, there is a fear that the orientation of graphite can not be controlled at the time of consolidation in an electrode in which graphite and low crystalline carbon are mixed. If the firing temperature is too low, the crystal structure becomes undeveloped and the true specific gravity is lowered. In addition, a functional group (OH group or COOH group) derived from the raw material remains on the surface of the coke, And the initial efficiency may be lowered. In addition, this side reaction may cause a surface coating to be formed, which may deteriorate the input / output characteristics, and the metal lithium precipitation resistance may decrease. On the other hand, the holding time at the maximum attained temperature of the firing treatment is not particularly limited, but is preferably 30 minutes or more, and the firing atmosphere is preferably an inert gas atmosphere such as argon or nitrogen. Further, depending on the conditions of the calcination treatment, it is possible to carry out the same treatment as the calcination treatment, and in this case, only the calcination treatment may be performed.

The low-crystalline carbon particles preferably have a BET specific surface area of 1.0 to 10.0 m 2 / g. And more preferably 2.0 to 10.0 m < 2 > / g. This BET specific surface area is determined by the shape at the time of pulverization due to the crystal state of the carbon material and the particle size distribution after pulverization. If the BET specific surface area is too small, the charging / discharging rate of the lithium ion is delayed. If the BET specific surface area is excessively large, the tap density is not increased and the electrode density is not increased. Since the BET specific surface area affects the rate of surface reaction when lithium ions enter and exit the carbon structure, it is preferable to control the BET specific surface area to an appropriate value.

The amount of the graphite particles (A) and the low-crystalline carbon particles (B) in the negative electrode active material used for the negative electrode for a lithium ion secondary battery of the present invention is preferably from 95 to 70 mass% ) Is set in the range of 5 to 30 mass%. That is, the mass ratio of (A) :( B) is 70:30 to 95: 5, preferably 80:20 to 90:10. When the amount of the low-crystalline carbon particles is too small, the effect of suppressing the deformation of the graphite particles due to the incorporation of the low-crystalline carbon particles becomes insufficient and the charge / discharge efficiency is lowered due to the lowered surface area of the reaction due to deformation of the graphite. Since the graphite is folded in a plane, it causes electrode expansion due to charging and discharging, resulting in deterioration of cycle characteristics. On the other hand, when the amount of the low-crystalline carbon particles is excessive, the graphite particles are deformed and fractured rather than causing degradation of the cycle characteristics. In addition to this, the capacity density per weight of the negative electrode active material decreases. Charge / discharge efficiency as a mixture is lowered because the charging / discharging efficiency is lower than that of particles.

The negative electrode active material particle of the negative electrode for a lithium secondary battery of the present invention is composed of a mixture containing graphite particles (A) and low-crystalline carbon particles (B), but the mixture as a whole has a true specific gravity of 2.13 to 2.23 g / A specific surface area of 3 to 6 m < 2 > / g, and an average particle diameter D50 of 5 to 20 mu m. The average particle size (D50) is the median diameter. When the true specific gravity of the negative electrode active material particles is too low, a side reaction occurs at the time of charging and discharging, leading to a decrease in efficiency. On the contrary, when the negative active material particle content is excessively high, the ratio of graphite particles is excessively increased. It is not sufficiently obtained. If the BET specific surface area is too small, it is difficult to secure a reaction area as the negative electrode active material, and the input / output characteristics at low temperatures may be lowered. On the other hand, if the BET specific surface area is excessively large, lithium ion consumption at the surface of the negative electrode active material There is a fear that reduction decomposition of one electrolytic solution occurs and the positive electrode capacity is reduced. Further, since the BET specific surface area is large, necessary binders necessary for preparing the slurry are increased, and the active material content in the negative electrode is likely to decrease. If the average particle diameter D50 is too small, the tap density is lowered. On the contrary, if the average particle diameter is too large, the BET specific surface area becomes small and the desired range is deviated.

The negative electrode active material for a lithium secondary battery according to the present invention preferably has an average particle diameter D50 of the graphite particles (A) and the low-crystalline carbon particles (B) within a range of 5 to 20 mu m, respectively. If the particle size is outside this range, the low-crystalline carbon particles are not well mixed with the particles of the graphite particles when slurried, and are entangled with the negative-electrode active material layer, It is preferable to adjust the particle size.

When the graphite particles and the low-crystalline carbon particles are mixed at a predetermined ratio in the negative electrode active material for a lithium secondary battery of the present invention, the mixture has a tap density of 0.5 g / cm 3 or more, preferably 0.5 to 1.2 g / Cm < 3 >. Wherein the tap density is measured as a mixture of the binders before mixing. When the tap density is low, contact between the particles at the time of electrode production becomes insufficient and the conduction path is reduced. Therefore, battery performance is deteriorated. When the press pressure is increased to increase the density, As a result, the surface area is increased and the adhesion of the electrode is lowered, leading to further deterioration of the cell performance due to reduction of the conduction pass. Therefore, it is preferable to set the tap density to 0.5 g / cm < 3 > as an index in order to increase the packing density before pressing.

In order to further increase the tap density, for example, it is necessary to increase the proportion of the fine powder having D10 of less than 1 mu m or to increase the ratio of coarse grains around D90, and as a result, It is not necessary to increase the tap density to 1.2 g / cm 3 because the uniformity or performance of the electrode is disturbed by the influence of the large particles or the coarse particles, leading to deterioration of the battery performance. On the other hand, the tap density of the powder can be measured by using a device of a tap capacitor KYT-400 (manufactured by Seishin Kogyo Co., Ltd.), a cylinder volume of 100 cc, a tapping distance of 38 mm, and a tap count of 300 times.

The negative electrode active material layer used in the negative electrode for a lithium ion secondary battery of the present invention can suppress the breakdown of the graphite particles and thereby control the orientation by adding the specific low crystalline carbon particles to the graphite particles. A film of crystalline carbon may be provided. The surface of the negative electrode active material layer is coated with a low crystalline carbon film to improve the adsorption / desorption performance of lithium ions, and the effect of improving the deformability of the negative electrode active material particles during consolidation can be expected. On the other hand, the formation of the low-crystalline carbon film is preferably carried out both on the graphite particles and the low-crystalline carbon particles, and may be carried out both at the same time by a conventionally known method such as vapor phase method by CVD or liquid phase method by pitch, Alternatively, the compounding may be carried out after separately treating.

The negative electrode for a lithium ion secondary battery of the present invention is obtained by forming a negative electrode active material layer formed by mixing a negative electrode active material for a lithium secondary battery and a binder on a collector such as a copper foil.

The negative electrode active material layer is formed on the current collector by forming a slurry by using a negative electrode active material and a binder as a solvent, coating on the current collector, drying and then consolidating under an arbitrary condition. The negative electrode active material layer may include an additive such as a thickener in addition to the negative electrode active material and the binder.

More specifically, for example, the negative electrode active material and the binder are kneaded in a weight ratio of 93: 7 to 99: 1 (negative electrode active material: binder), and the slurry is coated on a copper foil of a predetermined thickness to form The solvent may be dried under the drying conditions, and thereafter, the electrode may be consolidated to obtain an electrode (negative electrode) having a predetermined negative electrode active material layer. The density of the negative electrode active material may be adjusted in the range of 1.10 to 1.40 g / cm 3 after consolidation. If the electrode density is not sufficiently raised, conduction between the active materials is not achieved, electrons are not easily transferred during charging and discharging, and the rapid charge / discharge characteristics are deteriorated. If the electrode is too compacted, the volume density of the electrode is increased, but the graphite is oriented and broken to lower the charging / discharging efficiency and the cycle characteristics. Further, since the voids in the particles decrease, the amount of the electrolytic solution retained in the electrode decreases, which also causes the rapid charging performance to deteriorate. In order to improve the rapid charging performance, it is necessary that the amount of the electrolytic solution present in the electrode is equal to or more than a certain level, and both surfaces of the reaction area for reacting with lithium ions present in the electrolytic solution are required. Therefore, it is preferable to set the pressing condition to give the above electrode density.

The negative electrode for a lithium ion secondary battery generally uses graphite particles as a negative electrode active material. Since graphite particles have a structure in which a large number of flat plate-like crystals are laminated, slip occurs between layers due to external force, and they are easily deformed. In order to increase the capacity density of the negative electrode, compaction is performed by pressing the active material coated on the current collector as described above. At this time, the graphite particles are deformed by the press pressure and the graphite crystal layer is oriented in the planar direction.

The press pressure is appropriately adjusted to a predetermined electrode density, but the maximum line pressure is, for example, about 600 kg / cm. On the other hand, orientation ratio (I ₁₁₀ / I of the graphite peak intensity of the (110) plane of the crystal (I ₁₁₀₎ and (004) peak intensity of the face (I ₀₀₄₎ is obtained from XRD (X-ray diffraction) analysis of the negative electrode active material layer ₀₀₄ ), and when the peak intensity ratio (I ₁₁₀ / I ₀₀₄ ) is small, the graphite crystal layer is not oriented so much, and when it is large, the graphite crystal layer is oriented in the plane direction with respect to the electrode .

Lithium-ion secondary battery negative electrode of the present invention, ratio of the graphite (110) of crystal peak intensity of the face (I ₁₁₀₎ and (004) peak intensity of the face (I ₀₀₄₎ obtained from the XRD measurement of the negative electrode (I ₁₁₀ / I ₀₀₄ ) is 0.70 to 1.10. This peak intensity ratio is measured by XRD after consolidation of the negative electrode active material layer to a density of 1.10 to 1.40. If the peak intensity ratio is excessively high, the orientation of the graphite in the negative electrode active material in the plane direction becomes too large due to consolidation of the negative electrode active material, the expansion and shrinkage in the thickness direction during repetitive charging and discharging becomes large, and the cycle characteristics of the lithium ion secondary battery produced from the negative electrode It worsens. Also, when the peak intensity ratio is too low regardless of whether the electrode density is adjusted, it means that graphite is broken in the electrode, and as a result, charge / discharge efficiency is lowered. The preferred peak intensity ratio (I ₁₁₀ / I ₀₀₄ ) is 0.80 to 1.05.

Further, by the press pressure in the above-mentioned compaction, the graphite particles are deformed and not only the graphite crystal layer is oriented in the plane direction, but also the graphite particles are deformed and the internal voids are closed. The reaction area in the electrode is related to the sum of the porosity of the negative electrode active material layer per unit area observed by an electron microscope of the electrode cross section and the porosity of the graphite particles.

Therefore, when the end face of the negative electrode active material layer of the present invention is observed by an electron microscope, the void perimeter field of the negative electrode active material layer per unit area is 1.30 to 2.00 mu m / mu m < ² > The negative electrode active material layer has the voids in the portions where the binder is not filled and the voids in the particles as the voids between the particles, so that the perimeter of the void per unit area becomes the total sum of these.

The electrode perimeter is prepared by forming an electrode having a thickness of 50 mu m or more in the active material layer and by means of a mechanical spun magic, a microtome method, a cross-section polisher (CP) method, a focused ion beam (FIB) And observed by a method such as SEM. At this time, it is preferable to select one in which the pores of the particles are not changed by the processing method. The observed electrode image is binarized so as to block the particles and the gap, and the surrounding field including the hole is measured. It is preferable to observe the observation at 20 or more because there are variations in particle distribution in the observation section. For the measurement of the surrounding field, it is preferable to use an image analysis software (WinROOF: manufactured by Mitani Shoji Co., Ltd.) or the like. The detailed measurement conditions are as shown in the examples.

If the void perimeter is too short, it means that the graphite in the negative electrode active material is deformed by consolidation and the internal void is excessively closed, and the rapid charging property of the lithium ion secondary battery produced in the future is deteriorated. On the other hand, if the void perimeter is excessively long, the negative electrode density is lowered and the cycle performance and the DCR performance are deteriorated. The preferred pore circumference is 1.30 to 1.40 占 퐉 / 占 퐉 ² .

In the negative electrode for a lithium ion secondary battery of the present invention, it is preferable that the negative electrode active material layer satisfies a predetermined range while the peak strength ratio (I ₁₁₀ / I ₀₀₄ ) satisfies a predetermined range.

On the other hand, the characteristics such as specific gravity, density, average particle size, and aspect ratio of the graphite particles, the low-crystalline carbon particles and the negative electrode active material are values in the state of the raw material or the mixture before the negative electrode, And the void perimeter are the values in the state after the negative electrode.

As the positive electrode used in the lithium ion secondary battery of the present invention, a positive electrode active material, a binder, a conductive material or the like, which is made into a slurry with an organic solvent or water, is applied to a current collector and dried to form a sheet Is used. The positive electrode active material contains a transition metal and lithium, and is preferably a material containing one kind of transition metal and lithium, and examples thereof include a lithium transition metal composite oxide and a lithium-containing transition metal phosphate compound. They may be mixed and used. The transition metal of the lithium transition metal composite oxide is preferably vanadium, titanium, chromium, manganese, iron, cobalt, nickel, copper and the like. Specific examples of the lithium transition metal composite oxide include lithium cobalt complex oxides such as LiCoO ₂ , lithium nickel complex oxides such as LiNiO ₂ , lithium manganese composite oxides such as LiMnO ₂ , LiMn ₂ O ₄ and Li ₂ MnO ₃ , Those obtained by substituting a part of the transition metal atoms which are the main components of the metal composite oxide with other metals such as aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, magnesium, gallium and zirconium have. Substituted but a concrete example, for example, LiNi _{0. 5} Mn _{0 . 5} O ₂ , LiNi _{0 . 80} Co _{0 . 17} Al _{0 . 03} O ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMn _{1 . 8} Al _{0 . 2} O ₄ , LiMn _{1 . 5} Ni _{0 . 5} O ₄ , and the like. As the transition metal of the lithium-containing transition metal phosphate compound, vanadium, titanium, manganese, iron, cobalt and nickel are preferable. Specific examples thereof include iron phosphate such as LiFePO ₄ , cobalt phosphate such as LiCoPO ₄ A part of the transition metal atoms which are the main components of the lithium transition metal phosphate compound may be replaced with another metal such as aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, magnesium, gallium, zirconium, , And the like.

The binder for the positive electrode and the solvent for slurrying may be the same as those used for the negative electrode. The amount of the binder (binder) used for the positive electrode is preferably 0.001 to 20 parts by mass, more preferably 0.01 to 10 parts by mass, and most preferably 0.02 to 8 parts by mass with respect to 100 parts by mass of the positive electrode active material. The amount of the solvent of the positive electrode is preferably 30 to 300 parts by mass, more preferably 50 to 200 parts by mass, per 100 parts by mass of the positive electrode active material.

As the conductive material of the positive electrode, carbon nanofibers and the like such as graphite fine particles, carbon black such as acetylene black and Kechenblack, fine particles of amorphous carbon such as needle coke, and the like are used, but not limited thereto. The amount of the conductive material used for the positive electrode is preferably 0.01 to 20 parts by mass, more preferably 0.1 to 10 parts by mass, per 100 parts by mass of the positive electrode active material.

As the collector of the positive electrode, aluminum, stainless steel, nickel-plated steel or the like is generally used.

The negative electrode and the positive electrode thus prepared can be used as the lithium ion secondary battery of the present invention. The lithium ion secondary battery of the present invention is disposed so that a separation membrane exists between the above-described negative electrode and positive electrode.

The initial capacity ratio N / P between the initial capacity N (mAh / cm2) of the negative electrode and the initial capacity P (mAh / cm2) of the positive electrode is preferably 1.0 to 1.5 and more preferably 1.0 to 1.2. Generally, in the lithium ion secondary battery, a large number of negative electrode receiving lithium is mounted on the positive electrode having lithium. That is, a negative electrode having a larger negative electrode active material than the positive electrode is used. This is a measure to prevent lithium ions from being taken in when the negative electrode is charged at low temperature and to prevent lithium metal from precipitating on the electrode. However, if the N / P is over 1.5 and the negative electrode is mounted excessively, the thickness of the negative electrode is increased and the output and input characteristics of the electrode itself are lowered. Also, There is a problem that the density is lowered. On the other hand, if the excess amount of the negative electrode is too small, there is a possibility that metal lithium is precipitated on the negative electrode at the time of filling under the positive electrode as the effective capacity when the negative electrode deteriorates under various circumstances. .

Between the positive electrode and the negative electrode, an electrolytic solution containing an electrolyte and a non-aqueous electrolytic solution is usually filled. Examples of the electrolyte include conventionally known electrolytes such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiB (CF ₃ SO ₃ ) ₄ , LiB (C ₂ O ₄ ) ₂ , LiBF ₂ (C ₂ O ₄ ), LiSbF ₆ , LiSiF ₅ , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, LiAlF ₄ , LiAlCl _4, and derivatives thereof. Among these, _{LiPF 6, LiBF 4, LiClO 4} , LiAsF 6, LiCF 3 SO 3, LiC (CF 3 SO 2) derivatives and LiC ₃ and _{LiCF 3 SO 3 (CF 3 SO} 2) selected from the group consisting of ₃ derivative Is preferable because it is excellent in electric characteristics.

Examples of the nonaqueous electrolytic solution include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, 1,1-dimethoxyethane, Ethane, 1,2-diethoxyethane, gamma -butyrolactane, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, The organic solvent is preferably selected from the group consisting of diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, trimethyl borate, tetramethyl silicate, nitromethane, dimethyl formamide, N- methyl pyrrolidone, Methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, isobutyl, isobutyl, isobutyl, isobutyl, isobutyl, It can be used.

In the lithium ion secondary battery of the present invention, it is preferable to use a separation membrane between the positive electrode and the negative electrode. As the separation membrane, a commonly used microporous film of a polymer can be used without particular limitation.

Examples of the polymer include polyolefins such as polyethylene, polypropylene, polyvinylidene fluoride, polyvinylidene chloride, polyacrylonitrile, polyacrylamide, polytetrafluoroethylene, polysulfone, polyethersulfone, polycarbonate, polyamide, polyimide, Polyether compounds such as polyethylene oxide and polypropylene oxide, polymer compounds or derivatives thereof mainly containing various cellulose derivatives such as carboxymethylcellulose and hydroxypropylcellulose, poly (meth) acrylic acid and various esters thereof, A film composed of a combination or a mixture, and the like. These films may be used singly or as a multilayer film by superimposing these films. In addition, various additives may be used for these films, and the kind and content thereof are not particularly limited. Of these films, a film made of polyethylene, polypropylene, polyvinylidene fluoride or polysulfone is preferably used for the lithium ion secondary battery of the present invention.

In these films, micropores are formed so that the electrolytic solution seeps in and the ions easily permeate. As the method of imidaclopentesis, there are a phase separation method in which a solution is formed by subjecting a solution of a polymer compound and a solvent to micro-phase separation, a solvent is extracted and removed, and a molten polymer compound is extruded and extruded in a high- And a stretching method in which a gap is formed between the crystals by further stretching to obtain a multi-grain structure, and the like are suitably selected depending on the film to be used.

The shape of the lithium ion secondary battery of the present invention is not particularly limited, and may be various shapes such as a coin shape, a cylindrical shape, and a prism shape.

[Example]

Hereinafter, the present invention will be described concretely with reference to Examples, but the present invention is not limited by these Examples.

Examples 1 to 5 and Comparative Examples 1 to 6

Spherical natural graphite having a true specific gravity of 2.24 g / cm3, an average particle size (D50) of 8.5 占 퐉 and a ratio of particles having an aspect ratio of 0.5 or more of an oval phase of not less than 0.5 was used as the graphite particles (A) (B1 to 5) shown in Table 1 obtained by firing coal-based raw coke at a maximum attainable temperature of 800 ° C, 1300 ° C, and 1600 ° C, respectively, in a nitrogen gas atmosphere were compounded at the blending ratios shown in Table 2 , And negative electrode active materials (C1 to 10) were prepared. Table 2 shows the true specific gravity, average particle size (D50), compounding ratio and kinds of low-crystalline carbon particles of the negative electrode active material.

Firing temperature
℃ True weight
g / cm3 D50
㎛ Abstraction
% To less than 0.5% B1 1300 2.14 10.2 75 B2 1300 2.14 8.0 82 B3 1300 2.14 10.0 54 B4 800 1.74 10.3 80 B5 1600 2.20 10.1 83

True weight
g / cm3 D50
㎛ Tap density
g / cm3 Mixing ratio
(A / B) Low-crystalline carbon particles C1 2.23 8.6 0.65 95/5 B1 C2 2.23 8.7 0.65 90/10 B1 C3 2.23 8.4 0.65 90/10 B2 C4 2.23 8.4 0.65 90/10 B3 C5 2.22 8.8 0.62 80/20 B1 C6 2.21 9.1 0.60 70/30 B1 C7 2.20 9.2 0.60 60/40 B1 C8 2.24 8.5 0.65 100/0 - C9 2.19 8.7 0.65 90/10 B4 C10 2.24 8.7 0.65 90/10 B5

1.0 part by weight of acetylene black as a conductive material, 2.0 parts by weight of styrene butadiene rubber as a binder and 1.5 parts by weight of carboxymethyl cellulose as a thickener were mixed in 94.5 parts by weight of each of the negative electrode active materials C1 to 9 and dispersed in 50 parts by weight of water Thereby obtaining a slurry. This slurry was applied to a negative electrode current collector made of copper, and after drying, the negative electrode active material layer was pressed (pressed) using a double-sided roll. Thereafter, the negative electrode was cut into a predetermined size to produce a negative electrode.

As the positive electrode active material, Li (NiMnCo) O ₂ , 5 parts by mass of acetylene black as a conductive material and 7 parts by mass of polyvinylidene fluoride as a binder were mixed and dispersed in 50 parts by mass of N-methylpyrrolidone to obtain a slurry. This slurry was applied to a current collector made of aluminum, dried, and press molded. Thereafter, the positive electrode was cut into a predetermined size to produce a positive electrode.

LiPF ₆ was dissolved in a mixed solvent of 30 vol% of ethylene carbonate, 40 vol% of ethylmethyl carbonate and 30 vol% of dimethyl carbonate to prepare an electrolytic solution.

The negative electrode and the positive electrode thus obtained were held in an aluminum pack with a microporous film (separator) made of polypropylene having a thickness of 25 mu m interposed therebetween. Thereafter, the nonaqueous electrolyte solution adjusted as described above was poured into the aluminum pack, and the pack was sealed and sealed to fabricate the lithium ion secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 6.

On the other hand, various measurements and evaluations are made as follows, unless there is an emblem.

The average particle size D50 was measured using an apparatus of LA-920 (manufactured by HORIBA), and the dispersion medium was measured by using water and an activator. As a criterion of the existence ratio of the particles, the volume distribution was measured using the laser diffraction scattering method.

An electrode cross section was prepared by a cross-section polisher (CP) method and observed at a magnification of 500 times using a scanning electron microscope (FE-SEM S4700: manufactured by Hitachi Hi-tech Co., Ltd.). The number of observed particles was 100 or more. The measurement of the stratum corneum ratio of the particles was carried out by using image analysis software (WinROOF: manufactured by Mitani Shoji Co., Ltd.).

True specific gravity was measured by the liquid phase substitution method (pycnometer method). Specifically, the powder of the negative electrode active material is placed in a pycnometer, a solvent such as distilled water is added, and the air and the solvent liquid on the surface of the powder are substituted by a method such as vacuum degassing and the accurate powder weight and volume are determined. Value.

The density (after consolidation) of the negative electrode active material layer was calculated from the thickness of the active material layer and the weight of the active material layer per unit area. Specifically, the obtained negative electrode was cut into a predetermined size, and the weight was measured with a precision flat pile and the thickness was measured with a micro gauge. The current collector used for the coated substrate of the negative electrode was also measured in the same area and calculated by the following formula.

Density of negative electrode active material layer = (negative electrode weight - current collector weight) / volume of active material layer

The crystal orientation degree of the graphite particles was measured by XRD measurement. X-ray diffractometer Model RINT-TTR III, X-ray tube: CuK alpha, tube current: 300 mA, tube voltage: 50 kV were used. Was obtained from the ratio of the peak intensity near 77.2 ° corresponding to the (110) plane of the graphite particle crystal after the background removal and the peak intensity near 54.0 ° corresponding to the (004) plane. Removal of the background was done by drawing the baseline by linear approximation and subtracting the baseline value at that peak. The spectrum of the current collector (Cu) is observed in the XRD spectrum, but the peak intensity ratio is not affected.

Orientation degree = peak intensity of (110) plane / peak intensity of (004) plane

= Peak intensity ratio (I ₁₁₀ / I ₀₀₄ )

The electrode perimeter was fabricated by cross-section polisher (CP) method, and the electrode cross-section was fabricated using a scanning electron microscope (FE-SEM S4700 Hitachi Hi-Tech Co., Ltd.) And observed with an electron microscope.

The observed electrode image is binarized so as to block the particles and the binder and the void, and the surrounding field including the hole is measured. Observation at 20 or more viewing angles was performed because there was a variation in the distribution of particles in the observation section. The measurement of the perforation field was analyzed using image analysis software (WinROOF).

The initial capacity ratio N / P is a value obtained by separately measuring the charge capacity with respect to the positive electrode and the negative electrode, and dividing the charge capacity N (mAh) of the negative electrode by the charge capacity P (mAh) of the positive electrode. Specifically, the charging capacity P (mAh) of the positive electrode and the charging capacity N (mAh) of the negative electrode were calculated as follows. First, the charging capacity p (mAh / g) of the positive electrode active material and the charging capacity n (mAh / g) of the negative electrode active material are measured. The charging capacity p (mAh / g) of the positive electrode active material is the charging capacity per 1 g of the active material (lithium metal oxide) when the counter electrode is lithium metal and charged from 2.5 V to 4.2 V at a constant current of 30 mA / cm 2. The charging capacity n (mAh / g) of the negative electrode active material is the charging capacity per 1 g of the active material when the counter electrode is made of lithium metal and charged at 1.5 V to 0 V at a constant current of 30 mA / cm 2 and at a constant voltage for 90 minutes.

Initial capacity ratio N / P = n (mAh / g) / p (mAh / g)

The charging / discharging efficiency was measured by charging the counter electrode with lithium metal from 1.5 V to 0 V at a constant current of 30 mA / cm 2 and then charging it for 90 minutes under constant voltage. Discharge rate from 0 V to 1.5 V at a constant current of 30 mA / cm 2 after stopping for 30 minutes, and the ratio of the initial discharge capacity to the initial charge capacity. On the other hand, the charging / discharging efficiency was set at a threshold of 88.0%.

Charging / discharging efficiency = 100 × initial discharging capacity / initial charging capacity

Using the lithium ion secondary batteries of Examples and Comparative Examples, a test of cycle performance was carried out. The test was carried out in a constant temperature bath at 45 ° C. First, at the 1C rate, the charge termination voltage was set to 4.2 V, and then the discharge was repeated up to 2.5 V at the 1 C rate. The first discharge capacity was 100%, and the 200th discharge capacity was 80% or less.

The DC resistance (DCR) at the time of charging was measured using the lithium ion secondary batteries of Examples and Comparative Examples. The DCR measurement was carried out by constant-voltage charging at a charging rate of 60% at a discharging rate of 1 C rate at 23 DEG C for 90 minutes, and thereafter, a constant-temperature- The discharge rate was charged at a constant current of 1 C rate or 5 C rate for 10 seconds, and the change in the battery voltage in these cases was measured. From the obtained results, the DCR value was obtained from the following equation.

DCR =? Voltage /? Current

= (1C after 10 seconds voltage-5C voltage after 10 seconds) / (1C rate-5C rate)

DC resistance ratio at charging (DCR ratio) = (DCR _{-20 ° C} / DCR _{23 ° C} )

The case where the DC resistance ratio is 6.0 or less is indicated by?, And the case where the DC resistance ratio exceeds 6.0 is indicated by?.

Table 3 shows the types of the negative electrode active material, the density of the negative electrode, the peak intensity ratio, the pore surrounding length, the N / P, and the battery performance.

Negative
Active material Negative
density peak
Intensity ratio air gap
Circumference N / P Charge / discharge efficiency% Cycle Performance DCR
Performance Example 1 C1 1.30 0.90 1.31 1.2 89.7 ○ ○ Example 2 C2 1.35 1.05 1.33 1.2 89.4 ○ ○ Comparative Example 1 C2 1.60 1.20 1.25 1.2 88.0 × × Comparative Example 2 C2 1.00 0.50 1.50 1.2 89.7 × × Example 3 C3 1.30 1.00 1.34 1.2 88.3 ○ ○ Comparative Example 3 C4 1.30 0.52 1.12 1.2 87.7 ○ ○ Example 4 C5 1.32 0.94 1.32 1.2 89.0 ○ ○ Example 5 C6 1.31 0.82 1.30 1.2 88.2 ○ ○ Comparative Example 4 C7 1.35 0.65 1.15 1.2 87.6 ○ × Comparative Example 5 C8 1.30 1.15 1.20 1.2 88.0 × × Comparative Example 6 C9 1.28 1.00 1.20 1.2 88.0 × × Comparative Example 7 C10 1.39 1.25 1.20 1.2 89.6 × ×

The present invention can provide a lithium secondary battery anode that can be practically satisfactory in terms of energy density and cycle characteristics required for various applications. Therefore, the present invention can be applied not only to power sources for portable electronic devices represented by notebooks or smart phones, A hybrid electric vehicle in which an electric vehicle or a motor and a gasoline engine are combined, a large battery of a plug-in hybrid electric vehicle, and a battery in a stationary direction that is used in combination with a generator such as a solar power generator or a wind power generator. It can be used for applications.

Claims (9)

A negative electrode for a lithium ion secondary battery having a negative electrode active material layer on a current collector, wherein the negative electrode active material layer comprises a negative electrode active material and a binder, the negative electrode active material comprises graphite particles having a ratio of particles having an elongation percentage of 0.5 or more, Characterized in that low-crystalline carbon particles having a specific gravity of 1.90 to 2.16 g / cm 3 and a ratio of particles having an aspect ratio of less than 0.5 of the steric phase ratio of 60% or more are mixed in a mass ratio of 95: 5 to 70:30. . The method according to claim 1,
The negative electrode active material is a peak intensity of 110 of the graphite particles determined by the XRD measurement surface of the layer (I ₁₁₀₎ and the ratio (I ₁₁₀ / I ₀₀₄₎ between the peak intensity (I ₀₀₄₎ of face (004) is 0.70 ~ 1.10 Negative electrode for lithium ion secondary battery. The method according to claim 1,
Wherein the negative electrode perimeter area per unit area observed by an electron microscope of the end face of the negative electrode active material layer is 1.30 to 2.00 mu m / mu m < ² >. The method according to claim 1,
The ratio (I ₁₁₀ / I ₀₀₄₎ between the peak intensity (I ₀₀₄₎ of the negative electrode active material and the peak intensity of the (110) plane of the graphite particles crystal (I ₁₁₀₎ (004) by XRD measurement of the floor surface while 0.70 ~ 1.10 , And the periperal space per unit area observed by an electron microscope of the end face of the negative electrode active material layer is 1.30 to 2.00 mu m / mu m < ² >. The method according to claim 1,
Wherein the negative electrode active material has a true specific gravity of 2.13 to 2.23 g / cm3, a tap density of 0.5 g / cm3 or more, and an average particle diameter D50 of 5 to 20 占 퐉. The method according to claim 1,
Wherein the low-crystalline carbon particles in the negative electrode active material have an average particle diameter D50 of 5 to 20 占 퐉. The method according to claim 1,
Wherein the graphite particles in the negative electrode active material have a true specific gravity of 2.23 to 2.24 g / cm3 and an average particle size D50 of 5 to 20 占 퐉. The method of claim 3,
Wherein the low-crystalline carbon particles in the negative electrode active material are coke, coal or petroleum calcined coke obtained by calcining coal or petroleum raw coke at 900 to 1500 占 폚, or calcined coke calcined at 900 to 1500 占 폚 And a negative electrode for a lithium ion secondary battery. The lithium ion secondary battery according to any one of claims 1 to 8, wherein the negative electrode for a lithium ion secondary battery is opposed to the positive electrode through a separator, wherein the initial capacity N (mAh / cm2) of the negative electrode and the initial capacity P (mA / cm < 2 >) of the lithium ion secondary battery is in the range of 1.0 to 1.5.