JP2015187973A - Negative electrode active material for lithium ion secondary battery, lithium ion secondary battery negative electrode using the same and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode active material for lithium ion secondary battery having a high capacity per unit volume (weight), provided with practical characteristics capable of dealing even with the automotive applications for HEV, PHEV, and the like, of the discharge capacity, initial efficiency, input characteristics, capacity retention rate, and the like, and to provide a lithium ion secondary battery negative electrode using the same and a lithium ion secondary battery.SOLUTION: A negative electrode active material for lithium ion secondary battery is formed of a carbon material having a true specific gravity of 2.00-2.16 g/cm, where the D, D, Dand D-Dare in the range of 2-5 μm, 8-12 μm, 16-26 μm and 5-10 μm, respectively, in the particle size distribution of particles in volume reference, the tap density is 0.4 g/cc or more, and the BET specific surface area by nitrogen gas adsorption distribution method is 5.1-9.0 m/g. A lithium ion secondary battery negative electrode using the same, and a lithium ion secondary battery are also provided.

Description

  The present invention relates to a negative electrode active material for a lithium ion secondary battery, a lithium ion secondary battery negative electrode and a lithium ion secondary battery using the same.

  Lithium ion secondary batteries take advantage of superior features such as high operating potential, large battery capacity, and long cycle life, and low environmental pollution, so the conventional nickel-cadmium batteries and It is widely used in place of nickel metal hydride batteries.

  In addition, in order to respond to energy and environmental problems, electric vehicles, hybrid electric vehicles (HEVs) that combine a nickel-hydrogen battery-driven motor and a gasoline engine, and mobile electronic devices such as handy video cameras, etc. It is widely used as a power source, and its demand is expected to increase further in the future.

  As a negative electrode active material constituting a negative electrode of a lithium ion secondary battery, a carbon material is generally used in terms of safety and life. Among carbon materials, the graphite material is an excellent material having a high energy density, which is obtained at a high temperature of at least about 2000 ° C., usually about 2600 to 3000 ° C. However, there are problems with high input / output characteristics and cycle characteristics. Have. For this reason, for example, graphite materials are not suitable for high input / output applications such as power storage and electric vehicles, and input / output characteristics applications at low temperatures, and the use of carbon materials with other structures has been studied. ing.

  In recent years, from the viewpoint of further improving the performance of HEV, further improvement in performance has been demanded for lithium ion secondary batteries, and improvement of the performance has become an urgent task. Specifically, the discharge capacity of the lithium ion secondary battery is raised as an important characteristic so that a current that is an energy source of HEV can be sufficiently supplied. In addition, the ratio of the charge capacity to the discharge capacity, that is, the initial efficiency is required to be high so that the discharge current amount is sufficiently higher than the charge current amount. Furthermore, in order to enable charging in a short time, the lithium ion secondary battery preferably maintains a high charge capacity up to a high current density, and is also required to have a high capacity maintenance rate. In other words, it is required to improve such output characteristics, discharge capacity, initial efficiency, and capacity retention ratio in a well-balanced manner.

  In order to provide such a lithium ion secondary battery, many carbon materials such as coke and graphite have been studied as a negative electrode active material. However, although the discharge capacity described above can be increased, the initial efficiency is not sufficient. In addition, the actual battery voltage is insufficient, the high output characteristics in recent years cannot be satisfied, and the requirements for the capacity maintenance ratio cannot be satisfied.

  Therefore, in place of the above graphite material, coal-based and / or petroleum-based (hereinafter referred to as “coal-based”) raw coke or coal-based calcined coke is singly or mixed and fired. There has been proposed a negative electrode active material for lithium ion secondary batteries.

  For example, Patent Document 1 discloses that high input / output characteristics are exhibited by an active material having a larger crystal layer and a fine pore volume than graphite by firing at a temperature of 2000 ° C. or less and modifying the active material surface. It has been shown. Patent Document 2 proposes that a catalyst be used during firing in order to widen the crystal layers. By treating at a firing temperature lower than that during graphite production, an active material having a wider crystal layer than graphite is proposed. It has been shown that the material can be produced.

Coal-based raw coke and coal-based calcined coke that have the advantages mentioned above, but because the firing temperature is lower than that of graphite material, the carbon crystallinity is low and the unit volume when used as an electrode There is a problem that the capacity per (weight) becomes low. That is, an electrode using a general graphite material has a capacity of 360 mAh / g and a volume density of 1.4 to 1.8 g / cm ³ , whereas an electrode using the above material has a capacity of 240 to 340 mAh / g. g and the volume density are 1.0 to 1.2 g / cm ³ , so that the capacity as an electrode is lowered. Therefore, problems such as increase in the capacity of the active material and increase in volume density at the electrode are inherent in coal-based raw coke and coal-based calcined coke.

For example, Patent Document 3 discloses that two types of graphite materials having different fracture strengths and specific surface areas, that is, a scale having a median diameter (D ₅₀ ) of 13 μm or more and 15 μm or less in order to cope with higher energy density of a lithium ion secondary battery. After mixing the artificial spherical graphite pseudo-spheroidized graphite particles and the mesophase small spherulite graphitized product having a median diameter (D ₅₀ ) of 12 μm or more and 19 μm or less, high density even with a small press pressure It has been shown that a negative electrode active material layer that is filled and has an appropriate gap is obtained.

Patent Document 4 discloses an average particle as a negative electrode active material for a lithium ion secondary battery having high electrode density, excellent electrolyte permeability, low capacity loss due to charge / discharge, and good cycle performance. An example is shown in which three types of graphite powders having different relationships between the diameter (D ₅₀ ) and D ₉₀ / D ₁₀ are used.

JP 2009-224322 A JP 2011-9185 A JP 2009-164013 A JP 2007-324067 A

  The present invention per unit volume (weight) with practical characteristics that can be used for in-vehicle applications such as HEV and PHEV, such as discharge capacity, initial efficiency, input characteristics, capacity maintenance rate, etc. of lithium ion secondary batteries It aims at providing the negative electrode active material which can obtain a lithium ion secondary battery with high capacity | capacitance. Another object of the present invention is to provide a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery using the negative electrode active material for a lithium ion secondary battery.

  As a result of intensive studies to achieve the above problems, the present inventors have controlled the particle size distribution of the active material based on a specific raw material within a certain range, while setting the surface area of the active material to a predetermined value. The inventors have found that this can be solved, and have completed the present invention.

That is, the present invention is formed from a carbon material having a true specific gravity of 2.00 to 2.16 g / cm ³ , and D ₁₀ in a particle size distribution of particles on a volume basis is 2 to 5 μm, D ₅₀ is 8 to 12 μm, D ₉₀ is in the range of 16 to 26 μm, D ₅₀ -D ₁₀ is in the range of 5 to ₁₀ μm, the tap density is 0.4 g / cc or more, and the BET specific surface area (hereinafter referred to as BET specific surface area by the nitrogen gas adsorption flow method). Is a negative electrode active material for a lithium ion secondary battery, characterized in that it is 5.1 to 9.0 m ² / g.

  As such an active material, it is preferable to use a coal-based and / or petroleum-based (coal-based) raw coke or a coal-based calcined coke obtained by firing alone or in combination. .

The present invention also provides a negative electrode having a mixture layer formed on the current collector formed by mixing the negative electrode active material for a lithium ion secondary battery and a binder, and the cross section of the negative electrode is observed. In the active material shape, 80% or more of the number of observed active material particles has an ellipse equivalent length / short ratio (ellipse equivalent short axis length / ellipse equivalent long axis length) of 0.05 to 0.70, and A volume density of the composite material layer is 1.10 to 1.25 g / cm ³ , wherein the negative electrode is a lithium ion secondary battery negative electrode.

  Furthermore, the present invention is a lithium ion secondary battery, wherein the lithium ion secondary battery negative electrode and the positive electrode are opposed to each other with a separator interposed therebetween.

  According to the present invention, for example, while satisfying the discharge capacity, initial efficiency, input characteristics, capacity maintenance rate required for in-vehicle applications such as HEV and PHEV, the volume density when the electrode (negative electrode) is increased, A negative electrode active material capable of obtaining a lithium ion secondary battery excellent in performance balance can be provided.

  Hereinafter, embodiments of the present invention will be described in detail.

The negative active material for a lithium ion secondary battery of the present invention has a true specific gravity in the range of 2.00 to 2.16 g / cm ³ . The negative electrode active material for a lithium ion secondary battery that provides such true specific gravity can be obtained by mixing or mixing coal-based and / or petroleum-based (coal-based) raw coke, or coal-based calcined coke, alone or in combination. It can be obtained by calcination (in the present specification, “coal-based etc.” may be “coal-based and / or petroleum-based”, that is, either coal-based or petroleum-based, both It may be a mixed system of If the true specific gravity is less than 2.00 g / cm ³ , when applied to a lithium ion secondary battery, a side reaction occurs during charge and discharge, leading to a decrease in capacity and efficiency. On the other hand, when the true specific gravity exceeds 2.16 g / cm ³ , when applied to a battery, the input / output characteristics and capacity retention characteristics are degraded. Coal-based raw coke is petroleum-based and / or coal-based heavy oil, for example, using a coking facility such as a delayed coker, and the highest temperature is about 400 ° C. to 700 ° C. for about 24 hours. It means the one obtained by carrying out the decomposition and polycondensation reaction, and the coal-based calcined coke means the one obtained by calcining the coal-based raw coke and the maximum temperature reached 800 ° C. It means petroleum-based and / or coal-based coke calcined at about ˜1500 ° C.

  The method for obtaining a negative electrode active material for a lithium ion secondary battery in which the true specific gravity falls within the above range will be described in detail. First, a coal-based heavy oil is used, for example, using a coking facility such as a delayed coker, and the highest temperature reached. Coal-based raw coke is obtained by advancing the thermal decomposition and polycondensation reaction at a temperature of about 400 ° C. to 700 ° C. for about 24 hours. Thereafter, the obtained coal-based raw coke mass is pulverized to a predetermined size as necessary. An industrially used pulverizer can be used for the pulverization. Specific examples include atomizers, Raymond mills, impeller mills, ball mills, cutter mills, jet mills, hybridizers, orient mills, but are not particularly limited thereto. In the pulverization step, one type or two or more types of these devices may be used, or a single type of device may be used by pulverizing a plurality of times.

  The heavy coal oil used here may be a heavy petroleum oil or a heavy coal oil, but the heavy heavy oil is richer in aromatic properties, Since there are few impurities, such as S, V, and Fe, and there is also little volatile matter, it is more preferable to use heavy coal oil.

  In order to produce coal-based calcined coke, the coal-based raw coke obtained as described above is calcined at a maximum attained temperature of 800 ° C. to 1500 ° C. in a low oxygen atmosphere. The treatment temperature for calcination is preferably in the range of 1000 ° C to 1500 ° C, more preferably 1200 ° C to 1500 ° C. For calcination of raw coke such as coal when producing calcined coke, equipment such as reed hammer furnace, shuttle furnace, tunnel furnace, rotary kiln, roller hearth kiln or microwave capable of mass processing can be used. However, it is not particularly limited to these. Further, these calcination facilities may be either continuous type or batch type. Next, the obtained coal-based calcined coke lump is pulverized to a predetermined size using a pulverizer such as an atomizer used industrially, as in the case of raw coke. The pulverized coke powder can be sized to a predetermined particle size by cutting fine powder by classification or removing coarse powder with a sieve or the like.

The raw coke and calcined coke obtained above are preferably further subjected to a firing treatment. The firing temperature is preferably 800 ° C. or higher and 1500 ° C. or lower at the highest temperature reached. When the firing temperature exceeds the upper limit, the crystal growth of the coke material is excessively promoted, and it becomes difficult to make the true specific gravity 2.16 g / cm ³ or less. When the true specific gravity exceeds 2.16 g / cm ³ , the crystal structure of the coke is oriented like graphite at the time of firing, and the distance between the crystal layers becomes narrow. As described above, the input / output characteristics, the capacity retention ratio, etc. Therefore, the characteristic due to the structure will be deteriorated. Moreover, when the firing temperature is below the lower limit, the crystal structure becomes undeveloped and the true specific gravity is not more than 2.00 g / cm ³ , and raw material-derived functional groups (OH groups, COOH groups, etc.) are present on the coke surface. As described above, a side reaction occurs when the battery is charged and discharged as described above, leading to a decrease in capacity and efficiency. Further, the holding time at the highest temperature of the baking treatment is not particularly limited, but is preferably 30 minutes or more. Further, the firing atmosphere is preferably an inert gas atmosphere such as argon or nitrogen.

  In addition, the calcining treatment may be performed by burning coal-based raw coke or calcined coke alone or in combination, and in the process, the firing may be divided into multiple firings. In addition, as long as the various conditions of the active material in the present invention are satisfied, a step of shape control such as granulation may be included as necessary, or the surface of the active material may be modified with an organic or inorganic component or coated. Further, the metal component may be formed uniformly or dispersed on the surface.

  The true specific gravity of the negative electrode active material is measured by a liquid phase replacement method (also called a pycnometer method). Specifically, powder (active material) is put into a pycnometer, a solvent liquid such as distilled water is added, and air and solvent liquid on the sample surface are replaced by a method such as vacuum deaeration to obtain an accurate sample weight and volume. Thus, the true specific gravity value is calculated.

The negative electrode active material for a lithium ion secondary battery of the present invention has D ₁₀ in the particle size distribution of the negative electrode active material of 2 to 5 μm, D ₅₀ of 8 to 12 μm, D ₉₀ of 16 to 26 μm, and D ₅₀ -D. It is necessary that ₁₀ is in the range of 5 to ₁₀ μm. Preferably, D ₁₀ is 2 to 4 μm, D ₅₀ is 8 to 12 μm, D ₉₀ is 18 to 24 μm, and D ₅₀ -D ₁₀ is in the range of 6 to ₁₀ μm. This is because the pulverized particles of raw coke such as coal-based coke and coal-based calcined coke alone or mixed and calcined have the particle size distribution as described above. It means having. In addition, the BET specific surface area of the negative electrode active material at this time is set to 5.1 to 9.0 m ² / g. The negative electrode active material having the particle size distribution as described above may be subjected to coarse pulverization with an orient mill or the like, either alone or mixed with a calcined coal-based raw coke or coal-based calcined coke, and then a hammer. It can be obtained by finely pulverizing with a mill or jet mill and removing fine powder by air classification as required. These pulverization methods and classification methods are not particularly limited, and general methods can be used.

Regarding the particle size distribution of the negative electrode active material described above, if D ₁₀ is less than 2 μm, the specific surface area is excessively increased, and the initial efficiency of the obtained secondary battery is lowered. D ₉₀ of it is difficult to obtain an electrode of uniform and smooth surface texture to the electrode during the production due to the presence of coarse powder exceeds 26 .mu.m. If D ₅₀ -D ₁₀ is less than 5 μm, the particle size distribution of the particles becomes sharp, the proportion of fine powder having a small particle size becomes large, and it becomes difficult for the particles to form a close-packed structure at the time of electrode preparation. Will drop. When D ₅₀ -D ₁₀ exceeds 10 μm, there is a high possibility that coarse particles having D ₉₀ exceeding 26 μm will be present. There is a concern that coarse particles exceeding 26 μm may adversely affect the smoothness of the electrode surface, resulting in poor adhesion to the current collector, damage on the separator side, and coarse particles falling off. Further, when D ₁₀ is larger than 5 μm, the proportion of fine powder having D ₁₀ of 2 μm or less becomes small, and it becomes difficult for the particles to form a close-packed structure. For these reasons, the particle size distribution described above is required in the present invention. D ₅₀ -D ₁₀ represents the spread of the distribution shape in the particle size distribution of the active material particles. Not define the extent of the D ₅₀ particle size distribution which is a conventional center value has been found that it is possible to produce a superior electrode filling properties by having a spread of the distribution shown in the present invention. Further, the coke powder that is the raw material of the negative electrode active material having the above particle size distribution may be obtained by using any one of the above-mentioned coal-based raw coke powder, coal-based calcined coke powder, and the like. Alternatively, or a mixture of both may be used.

Here, regarding the particle size distribution measurement of the negative electrode active material (carbon material), in the present invention, measurement was performed using an apparatus of LMS-30 (manufactured by Seishin Enterprise Co., Ltd.) and the dispersion medium using water + active agent. As the standard of the abundance ratio of the particles, the volume distribution was measured using a laser diffraction / scattering method, and the particle size distribution was evaluated using a cumulative distribution based on the volume. That is, the particle size distribution of the negative electrode active material measured by a laser diffraction scattering method, cumulative 10% by volume particle diameter in the particle size distribution was D _10. Similarly, a cumulative 50% by volume particle diameter and D _50, a cumulative 90% by volume particle diameter as D _90, also the difference between D ₅₀ and D ₁₀ was D ₅₀ -D _10.

  In the process of pulverizing and controlling the particle size distribution, the negative electrode active material in the present invention becomes flat and flake shaped. As the shape of the negative electrode active material, 80% or more of the number of active material particles observed when the cross section of the produced electrode is observed has an ellipse equivalent length / short ratio (ellipse equivalent short axis length / ellipse equivalent long axis length) of 0. 05 to 0.70. When the ellipse equivalent length / short ratio is more than 0.70, the negative electrode active material has a more spherical shape, and even in the same particle size distribution, fine packing and tap density change, and electrode density and battery performance change. Further, when the elliptical equivalent length ratio is less than 0.05, the negative electrode active material has a more needle-like shape, not only the filling method and tap density change, but also the surface area of the negative electrode active material becomes too large, Phenomena such as side reactions that lower battery performance occur. Therefore, in the present invention, the number of negative electrode active material particles observed in the shape of the negative electrode active material when observed from the cross section of the electrode (negative electrode) having a mixture layer formed by mixing with a binder on the current collector The negative electrode active material is used such that 80% or more of the ratio is 0.05 to 0.70 in terms of the ellipse-equivalent length-to-short ratio (ellipse-equivalent minor axis length / elliptical equivalent major axis length).

  As a method for observing the electrode cross section, an electrode having a composite layer thickness of 50 μm or more is manufactured, and a method such as a mechanical polishing method, a microtome method, a CP (Cross-section Polisher) method, or a focused ion beam (FIB) method is used. A cross section of the electrode is prepared by observing all particle sizes having a minimum particle size of 1 μm or more by a method such as SEM. For the observed particles, the ellipse equivalent length / short ratio (ellipse equivalent short axis length / ellipse equivalent long axis length) is measured. Since there are variations in the distribution of particles in the observation cross section, observation of 20 fields or more is preferable. The particle size may be measured by using image analysis software (WinRooF: manufactured by Mitani Corporation).

The negative electrode active material for a lithium ion secondary battery of the present invention has a tap density of 0.4 g / cc or more in order to increase the initial density at the time of electrode preparation, and is in the range of 0.4 to 0.8 g / cc. It is preferable. If the tap density is less than 0.4 g / cc, contact between the negative electrode active materials at the time of electrode preparation becomes insufficient and the conduction path is reduced, so that the battery performance is lowered, and the press pressure is increased to increase the density. If this is done, the amount of deformation becomes large and the negative electrode active material breaks, leading to an increase in surface area and further reduction of the conduction path due to a decrease in electrode adhesion, leading to a decrease in battery performance. Therefore, in order to increase the packing density before pressing, it is necessary to set the tap density as an index to at least 0.4 g / cc. In addition, in order to achieve more than 0.8 g / cc, for example, it is necessary to increase the proportion of fine powder with D ₁₀ less than 1 μm, or increase the proportion of coarse particles in the vicinity of D ₉₀ . It is not necessary to make the tap density more than 0.8 g / cc because the surface area increases or the uniformity and performance of the electrode are disturbed by the influence of coarse particles, leading to a decrease in battery performance.

  In the present invention, for the tap density of the negative electrode active material, the measured value at a cylinder volume of 100 cc, a tapping distance of 38 mm, and the number of taps of 300 times was used using a tap denser KYT-400 (manufactured by Seishin Enterprise).

The negative electrode active material for a lithium ion secondary battery of the present invention has a BET specific surface area of 5.1 to 9.0 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 less than 5.1 m ² / g, the charge / discharge rate of lithium ions is slow, which is undesirable. If it is greater than 9.0 m ² / g, the tap density does not increase and the electrode density does not increase, which is not preferable. Since the BET specific surface area affects the speed of surface reaction when lithium ions enter and exit the carbon structure, it is important to control the BET specific surface area to an appropriate value.

  In the present invention, the BET specific surface area was determined by a nitrogen gas adsorption flow method, and an apparatus of BELSORP-mini II (manufactured by Nippon Bell Co., Ltd.) was used.

  The present invention is also a lithium ion secondary battery negative electrode using the negative electrode active material for lithium ion secondary battery, and the negative electrode is disposed on the current collector (generally copper foil). It consists of a composite layer formed by mixing a substance and a binder.

  Generally, a water-soluble binder such as a fluorine resin powder such as polyvinylidene fluoride (PVDF) or a polyimide (PI) resin, styrene butadiene rubber (SBR), or carboxymethyl cellulose (CMC) is used for the binder.

  The composite material layer is formed on the current collector by preparing a slurry of the negative electrode active material and the binder described above using a solvent, and applying and drying on the current collector (generally copper foil), and then It can be performed by pressing under any condition. The solvent used is not particularly limited, and N-methylpyrrolidone (NMP), dimethylformamide, water, alcohol, or the like is used.

More specifically, for example, the negative electrode active material and the binder are kneaded at a weight ratio of 93 to 97: 7 to 3 (negative electrode active material: binder), and this slurry is applied onto a copper foil having a predetermined thickness. Then, the solvent can be dried under a drying condition of 60 to 120 ° C., and then pressed at a linear pressure of 100 to 600 kg / cm to obtain a negative electrode. By making the manufacturing conditions at this time within the above range, An electrode having a volume density in the range of 1.10 to 1.25 g / cm ³ is obtained. Here, if the linear pressure at the time of pressing is increased too much, the volume density of the electrode is increased, but the active material is deformed and destroyed, resulting in poor contact within the electrode, leading to a decrease in capacity and efficiency. It is desirable to set the press conditions for the volume density.

The lithium ion secondary battery of the present invention can be obtained using the negative electrode thus produced. The lithium ion secondary battery of this invention is arrange | positioned so that a separator may exist between an above-described negative electrode and a positive electrode. The negative electrode and the positive electrode are opposed to each other via a separator, and as a positive electrode facing each other, a lithium-containing transition metal oxide LiM (1) xO ₂ (wherein x is a numerical value in the range of 0 ≦ x ≦ 1, In the formula, M (1) represents a transition metal and is composed of at least one of Co, Ni, Mn, Ti, Cr, V, Fe, Zn, Al, Sn, and In), or LiM (1) yM (2) _2- yO ₄ (wherein y is a numerical value in the range of 0 ≦ y ≦ 1, where M (1) and M (2) represent transition metals, Co, Ni, Mn, Ti, Cr, Transition metal chalcogen compounds (Ti, S ₂ , NbSe, etc.), vanadium oxides (V ₂ O ₅ , V ₆ O ₁₃ , V ₂ O) comprising at least one of V, Fe, Zn, Al, Sn, and In ₄ , V ₃ O ₆ , etc.) and lithium compounds, the general formula MxMo ₆ Ch ₆ -y, where x is 0 ≦ x ≦ 4, y is a numerical value in the range of 0 ≦ y ≦ 1, wherein M represents a metal including a transition metal and Ch represents a chalcogen metal) or activated carbon, A positive electrode active material such as activated carbon fiber can be exemplified.

Further, Examples of the electrolyte filling the space between the positive electrode and the negative electrode, and any known ones can be used, for example _{LiClO 4, LiBF 4, LiPF 6} , LiAsF 6, LiB (C 6 H 5), LiCl LiBr, Li ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ ) ₃ SO ₂ ) ₃ C, Li) CF ₃ CH ₂ OSO ₂ ) ₂ N, Li (CF ₃ CF ₂ CH ₂ 1 type such as OSO ₂ ) ₂ N, Li (HCF ₂ CF ₂ CH ₂ OSO ₂ ) ₂ N, Li ((CF ₃ ) ₂ CHOSO ₂ ) ₂ N, LiB [C ₆ H ₃ (CF ₃ ) ₂ ] ₄ Or the mixture of 2 or more types can be mentioned.

  Examples of the non-aqueous electrolyte include propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,1-dimethoxyethane, 1,2-dimethoxyethane, 1,2 -Diethoxyethane, γ-butyrolactan, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, anisole, diethyl ether, sulfolane, methylsulfolane, acetonitrile, chloronitrile, propionitrile, Trimethyl borate, tetramethyl silicate, nitromethane, dimethylformamide, N-methylpyrrolidone, ethyl acetate, trimethyl orthoformate, nitrobenzene Benzoyl chloride, benzoyl bromide, tetrahydrothiophene, dimethyl sulfoxide, 3-methyl-2-oxazolidone, ethylene glycol, sulfite, a single solvent or a mixture of two or more solvents such as dimethyl sulfite may be used.

  Hereinafter, the present invention will be specifically described based on examples. However, the content of this invention is not restrict | limited by these Examples. The true specific gravity, particle size distribution, tap density, and BET specific surface area were measured by the methods described above.

(Example 1)
Using a refined pitch from which heavy quinoline insolubles have been removed from coal-based heavy oil, bulk coke produced by heat treatment at a temperature of 500 ° C. for 24 hours by a delayed coking method (raw coke) is obtained using an orientation mill and a jet mill. Fine pulverization gave raw coke pieces (fine pulverized raw coke) having an average particle size (D ₅₀ ) of 10.5 μm.

The raw coke pieces obtained as described above are calcined with a rotary kiln in a low oxygen atmosphere at an inlet temperature of 700 ° C. to an outlet temperature of 1500 ° C. (maximum temperature reached) for 1 hour or longer to obtain calcined coke. It was. The calcined coke is finely pulverized by appropriately adjusting the processing amount per unit time and the gas flow rate at the time of processing in the same jet mill as above, and then most of the fine powder of 3 μm or less is removed by air classification. Thus, the true specific gravity is 2.15 g / cm ³ , D ₁₀ is 3.7 μm, D ₅₀ is 10.2 μm, D ₉₀ is 19.7 μm, and D ₅₀ -D ₁₀ is 6.5 μm. A negative electrode active material for a secondary battery was obtained. The tap density of this negative electrode active material was 0.52 g / cm ³ , and the BET specific surface area determined by the nitrogen gas adsorption flow method was 6.7 m ² / g.

Next, styrene-butadiene rubber (SBR, manufactured by JSR Corporation) and carboxymethyl cellulose (CMC, manufactured by Nippon Paper Industries Co., Ltd.) as a binder are 5% by mass with respect to the negative electrode active material for the lithium ion secondary battery. And kneaded to prepare a slurry. The obtained slurry was applied uniformly on the surface of a 15 μm thick copper foil, dried at a temperature of 60 to 120 ° C., and then pressed at a linear pressure of 300 kg / cm to form a sheet-like negative electrode. Obtained. The volume density of this electrode was 1.15 g / cm ³ . A negative electrode for a test was prepared by cutting out from this sheet into a circle having a diameter of 15 mmφ. In order to evaluate the electrode characteristics of the single negative electrode for this test, metallic lithium cut into about 15.5 mmφ was used as the counter electrode. In addition, when the produced electrode was cut | disconnected by CP method and the cross section was observed by FE-SEM (magnification 1500 times), among the active material particles observed within the range of a viewing angle of 75 micrometers x 30 micrometers, the ellipse equivalence length ratio It was confirmed that the active material particles in the range of 0.05 to 0.70 was 88%. In addition, about the observation visual field, in order to reduce dispersion | variation, the average value of 20 visual fields was used.

In addition, a solution of LiPF ₆ dissolved at a concentration of 1 mol / l in a mixed solvent of ethylene carbonate and diethyl carbonate (1: 1 volume ratio) as an electrolytic solution, and a porous membrane of propylene as a separator, A coin cell was prepared using the test negative electrode, and a lithium ion secondary battery was produced. The capacity of the produced battery was 1 mA / cm ² . When a constant current discharge of 1 mA / cm ^{2 and} a constant current discharge of 20 mA / cm ² were performed in a voltage range where the lower limit charge voltage of the terminal voltage was 0 V and the upper limit voltage of the discharge was 1.5 V at a constant temperature of 25 ° C. The charge maintenance rate was calculated from the ratio. The results are shown in Table 1.

In addition, using the above lithium ion secondary battery, the battery was charged from 1.5 V to 0 V at a constant current of 30 mA / g current density per weight of the negative electrode active material, and then charged at a constant voltage for 90 minutes to obtain the initial discharge capacity. After measuring and resting for 30 minutes, discharging from 0 V to 1.5 V at a constant current of 30 mA / g current density, measuring the initial charge capacity, the ratio of the initial discharge capacity to the initial charge capacity represented by the following formula From this, the initial efficiency was calculated. The results are shown in Table 1.
Initial efficiency (%) = 100 × initial discharge capacity / initial charge capacity Note that the determination in Table 1 is an evaluation of the capacity per unit volume of the negative electrode active material, rapid chargeability, and initial efficiency. When the initial efficiency was more than 1.10 to 1.25 g / cm ³ and the initial efficiency exceeded 80% and the charge retention rate exceeded 40%, it was marked as ◯, and otherwise it was marked as x.

(Examples 2-3, Comparative Examples 1-3)
A negative electrode active material for a lithium ion secondary battery having a different particle size distribution as shown in Table 1, except that the conditions at the time of jet mill pulverization after obtaining calcined coke were changed. Got. The characteristics of the obtained negative electrode active material are shown in Table 1. Moreover, when the cross section of the electrode produced similarly to Example 1 was observed, as for the active material particle which is the range of ellipse equivalent length ratio 0.05-0.70, all samples are 85-89%. confirmed. Using them, the negative electrode and the lithium ion secondary battery were obtained in the same manner as in Example 1, and the charge retention rate and the initial efficiency were examined, respectively. The results are shown in Table 1.

Example 4
Example 1 except that the raw coke pieces were calcined by rotary kiln in a low oxygen atmosphere at an inlet temperature of 700 ° C. to an outlet temperature of 1000 ° C. (maximum temperature reached) for 1 hour or more to obtain calcined coke. The same operation was performed to obtain a lithium ion secondary battery. The characteristics of the obtained negative electrode active material are shown in Table 1. Moreover, when the cross section of the electrode produced in the same manner as in Example 1 was observed, it was confirmed that 90% of the active material particles were in the range of the ellipse-equivalent length-to-short ratio of 0.05 to 0.70. Further, the charge maintenance ratio and the initial efficiency were examined in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 4)
Except that the raw coke pieces were heat-treated for 1 hour or more at a temperature near the inlet temperature of 700 ° C. to a temperature near the outlet temperature of 1800 ° C. (maximum temperature reached) in a low oxygen atmosphere using a rotary kiln to obtain calcined coke. The same operation was performed to obtain a lithium ion secondary battery. The characteristics of the obtained negative electrode active material are shown in Table 1. Moreover, when the cross section of the electrode produced similarly to Example 1 was observed, it was confirmed that the active material particle which is in the range of ellipse equivalent length ratio 0.05-0.70 is 87%. Further, the charge maintenance ratio and the initial efficiency were examined in the same manner as in Example 1. The results are shown in Table 1.

(Example 5)
Using a refined pitch from which quinoline insolubles have been removed from coal-based heavy oil, a coal-based massive coke produced by heat-treating at a temperature of 500 ° C. for 24 hours by a delayed coking method is obtained. Lump coke (raw coke) was calcined for 1 hour or more at a temperature near the inlet temperature of 800 ° C. to a temperature near the outlet temperature of 1500 ° C. (maximum temperature reached) in a low-oxygen atmosphere using a rotary kiln to obtain coal-based massive calcined coke.

The coal-based massive calcined coke obtained as described above is finely pulverized by appropriately adjusting the throughput per unit time and the gas flow rate at the time of treatment with a jet mill, and then by air classification to 3 μm or less. Most of the fine powder was removed to obtain coal-based calcined coke. The obtained coal-based calcined coke is calcined with a roller hearth kiln in a nitrogen gas atmosphere at a maximum temperature of 1500 ° C. for 1 hour or longer, so that the true specific gravity is 2.15 g / cm ³ and D ₁₀ is 3. A negative electrode active material for a lithium ion secondary battery having 0 μm, D ₅₀ of 10.2 μm, D ₉₀ of 20.0 μm, and D ₅₀ -D ₁₀ of 7.2 μm was obtained. The tap density of this negative electrode active material was 0.55 g / cm ³ , and the BET specific surface area determined by the nitrogen gas adsorption flow method was 6.6 m ² / g.

  Moreover, when the cross section of the electrode produced similarly to Example 1 was observed, it was confirmed that the active material particle which is in the range of ellipse equivalent length ratio 0.05-0.70 is 89%. Further, the charge maintenance ratio and the initial efficiency were examined in the same manner as in Example 1. The results are shown in Table 1.

(Examples 6 to 7)
Except for changing the conditions at the time of jet mill pulverization after obtaining coal-based massive calcined coke, the same operation as in Example 5 was performed, and the lithium ion secondary battery having a different particle size distribution as shown in Table 1 was used. A negative electrode active material was obtained. The characteristics of the obtained negative electrode active material are shown in Table 1. Moreover, when the cross section of the electrode produced similarly to Example 1 was observed, as for the active material particle which is the range of ellipse equivalent length ratio 0.05-0.70, all samples are 82-90%. confirmed. Using these, a negative electrode and a lithium ion secondary battery were obtained in the same manner as in Example 1, and the charge retention rate and initial efficiency were examined. The results are shown in Table 1.

(Example 8)
A lithium ion secondary battery was obtained by performing the same operation as in Example 5 except that the calcined coke was baked with a roller hearth kiln at a maximum temperature of 1000 ° C. for 1 hour or longer in a nitrogen gas atmosphere. The characteristics of the obtained negative electrode active material are shown in Table 1. Moreover, when the cross section of the electrode produced similarly to Example 1 was observed, it was confirmed that the active material particle which is the range of ellipse equivalent length ratio 0.05-0.70 is 88%. Further, the charge maintenance ratio and the initial efficiency were examined in the same manner as in Example 1. The results are shown in Table 1.

As is apparent from Table 1, the lithium ion secondary battery using the negative electrode active material for lithium ion secondary battery that satisfies the requirements of the present invention exhibits a high charge retention rate and has a rapid charge characteristic while remaining in volume. It can be seen that the density exceeds 1.10 g / cm ^3, and the initial efficiency which is a concern due to the increase in surface area can be maintained at a high value.

  It can also be seen that the rapid charge characteristics are impaired when the true specific gravity of the negative electrode active material is increased. This is considered to be one of the reasons that the crystallization of carbon progresses due to the high firing temperature and the interlayer distance becomes narrow like graphite.

  While the present invention has been described in detail based on the above specific examples, the present invention is not limited to the above specific examples, and various modifications and changes can be made without departing from the scope of the present invention.

Claims (4)

It is formed from a carbon material having a true specific gravity of 2.00 to 2.16 g / cm ³ , and D ₁₀ in a particle size distribution on a volume basis is 2 to 5 μm, D ₅₀ is 8 to 12 μm, and D ₉₀ is 16 to 26 μm. And D ₅₀ -D ₁₀ are in the range of 5 to ₁₀ μm, the tap density is 0.4 g / cc or more, and the BET specific surface area by the nitrogen gas adsorption flow method is 5.1 to 9.0 m ² / g. A negative electrode active material for a lithium ion secondary battery.   The active material is obtained by calcining either a coal-based and / or petroleum-based raw coke or a coal-based and / or petroleum-based calcined coke alone or a mixture of both. The negative electrode active material for lithium ion secondary batteries according to claim 1. A negative electrode having a mixture layer formed on the current collector by mixing the negative electrode active material for a lithium ion secondary battery according to claim 1 or 2 and a binder, and a cross section of the negative electrode was observed. In the shape of the active material, 80% or more of the number of observed active material particles has an ellipse equivalent length / short ratio (ellipse equivalent short axis length / ellipse equivalent long axis length) of 0.05 to 0.70. And the volume density of the said composite material layer is 1.10-1.25 g / cm < ³ >, The lithium ion secondary battery negative electrode characterized by the above-mentioned.   A lithium ion secondary battery according to claim 3, wherein the lithium ion secondary battery negative electrode and the positive electrode are opposed to each other with a separator interposed therebetween.