JP2012049151A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which is excellent in immersion of an electrolytic solution into an electrode, can be manufactured in a short time and is small in initial irreversible capacity, that is, which has high capacity density as the battery capacity lost in the initial charge/discharge is reduced and has the excellent low temperature discharge efficiency, that is, which can discharge at high current density in a low temperature environment and is well-balanced in various kinds of battery characteristics such as a high cycle maintenance rate.SOLUTION: The lithium secondary battery includes: a positive electrode and negative electrode capable of storing/releasing lithium ions; an electrolyte; and a nonaqueous electrolytic solution. In the lithium secondary battery, (1) a negative electrode material in the negative electrode includes a negative electrode material (A) of which the circularity is 0.85 or more and the BET specific surface area is 4.1 m/g or less, and (2) propylene carbonate and carbonate having an unsaturated bonding are contained in the nonaqueous electrolytic solution.

Description

  The present invention relates to a lithium secondary battery.

In recent years, with the miniaturization of electronic devices, it has been desired to increase the capacity of secondary batteries. Therefore, lithium secondary batteries with higher energy density are attracting attention as compared to nickel / cadmium batteries and nickel / hydrogen batteries.
At first, it was attempted to use lithium metal as the negative electrode active material, but during repeated charging and discharging, dendritic lithium precipitated and penetrated the separator, reaching the positive electrode, and could cause a short circuit. It turned out to be. Therefore, at present, in the charge / discharge process, attention is focused on the use of a carbon material that allows lithium ions to enter and exit between the layers and prevent the precipitation of lithium metal as the negative electrode active material.

As the carbon material, when graphite is used, particularly when graphite having a high degree of graphitization is used as a negative electrode active material for a lithium secondary battery, a capacity close to 372 mAh / g, which is the theoretical capacity of lithium storage of graphite, is obtained. And is known to be preferable as an active material.
On the other hand, as the solvent for the non-aqueous electrolyte, cyclic carbonates represented by high-permittivity solvents such as ethylene carbonate, propylene carbonate, and butylene carbonate, and low-viscosity solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate Chain carbonates, cyclic esters such as γ-butyrolactone, cyclic ethers such as tetrahydrofuran and 1,3-dioxolane, and chain ethers such as 1,2-dimethoxyethane are used alone or in combination. Yes. In particular, a mixture of a high dielectric constant solvent and a low viscosity solvent is often used, and for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) _2, etc. are used alone or in combination of two or more. Used.

  In particular, a battery using propylene carbonate as an electrolyte is known to be useful for high current density discharge in a low temperature environment because of its high dielectric constant. However, since propylene carbonate reacts violently on the surface of the negative electrode material during charging, there is a problem in that charge / discharge efficiency is lowered, and technical improvements for using propylene carbonate in the electrolytic solution have been made.

For example, in Patent Document 1, a coating layer made of any one of an ion conductive polymer, a water-soluble polymer, and an alkali metal salt is formed on the surface of the carbon negative electrode to suppress decomposition of the carbon negative electrode and the nonaqueous electrolyte layer. A method for suppressing the deposition of decomposition products of the nonaqueous electrolyte layer on the negative electrode surface is described.
Further, in Patent Document 2, the graphite particles for negative electrode are composed of a lump graphite particle group composed of phosphorus-like or flake-like natural graphite particles, and the lump graphite particle group has a cumulative 50% diameter (D50 by the laser light diffraction method). Diameter), specific surface area by nitrogen gas adsorption method, apparent density by static method and apparent density by tap method are in a specific range, apparent density by tap method and apparent density by static method are in a specific ratio, and Using graphite particles for negative electrodes of non-aqueous secondary batteries in which the area intensity ratio (ID / IG) of the D peak appearing near 1350 cm ⁻¹ and the G peak appearing near 1580 cm ^{−1 in the} Raman spectroscopic analysis is in a specific range. Thus, it is described that propylene carbonate and γ-butyrolactone are decomposed on the surface of graphite during charging and the charge / discharge efficiency is prevented from being lowered. Further, in Patent Document 2, the irreversible capacity is reduced by adsorbing or coating 0.1 to 5% by weight of a surfactant derived from the basic structure of C ₆ H ₁₀ O ₅ on the graphite particle group. Is described.

  In Patent Document 3, a part or all of the surface of the carbon material is coated with binder powder particles by mixing the carbon material of the negative electrode active material and the binder powder particles while applying pressure and shearing action. A method is described, and it is described that it is excellent in the discharge characteristics and cycle life characteristics of a battery by being excellent in the binding strength etc. of negative electrode active materials.

JP-A-11-129992 JP 2003-168432 A JP 2002-42787 A

  However, according to our study, the methods described in Patent Document 1 and Patent Document 2 cannot sufficiently suppress the reactivity of the carbon negative electrode surface itself to propylene carbonate, and the initial irreversible capacity is not sufficiently reduced. At the same time, since the immersion of the electrolytic solution into the electrode is insufficient, there remains a problem that a time loss is caused in the manufacturing process of the lithium ion secondary battery. This is probably because carbonates having unsaturated bonds are not used in the electrolyte solution, and the raw graphite is not spheroidized. On the other hand, in the method described in Patent Document 3, the discharge characteristics and cycle life characteristics of the obtained battery are obtained by mixing the coating of the binder powder particles on the carbon material of the negative electrode active material while applying pressure and shearing action. Is improved, but is insufficient in terms of initial irreversible capacity and low-temperature discharge efficiency. This is presumably because the raw graphite is not spheroidized and the non-aqueous electrolyte is not specially devised.

  The present invention has been made in view of the above problems, and its purpose is that the electrolyte has good immersion in the electrode, can be manufactured in a short time, and the initial irreversible capacity is small. Since the battery capacity lost by the first charge / discharge is reduced, the capacity is high, the low temperature discharge efficiency is good, that is, the high current density discharge is possible in a low temperature environment, the cycle maintenance rate is high, etc. The object is to provide a lithium secondary battery having excellent balance of battery characteristics.

  The present inventors paid attention to the fact that there is a porous portion where a polymer material can be brought into contact with and intruded into graphite as the negative electrode active material particles, and as a result of intensive investigations to solve the above-mentioned problems, spheroidization was performed. By using a spheroidized material in which a polymer material is attached to graphite particles as the negative electrode active material as a negative electrode material, and combining an electrolyte containing carbonate and propylene carbonate with an unsaturated bond, a very good electrolyte immersion It has been found that a lithium ion secondary battery exhibiting liquidity and having excellent low temperature and high current density charge / discharge characteristics and low irreversible capacity, that is, a lithium secondary battery excellent in various battery characteristics can be achieved. Reached.

That is, the gist of the present invention is a lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium ions, an electrolyte, and a non-aqueous electrolyte,
(1) The negative electrode material in the negative electrode includes a negative electrode material (A) in which one or more polymer materials are attached to graphite as a negative electrode active material, and the circularity is 0.85 or higher,
(2) containing propylene carbonate and carbonate having an unsaturated bond in the non-aqueous electrolyte,
The present invention resides in a lithium secondary battery characterized by that.

  The lithium secondary battery of the present invention exhibits a very excellent electrolyte immersion property, has high production efficiency, is excellent in low temperature and high current density charge / discharge characteristics, and has a low initial irreversible capacity, and is practically effective.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail. However, the description of constituent elements described below is an example (representative example) of an embodiment of the present invention, and is not specified by these contents.
[Lithium secondary battery]
The basic structure of the lithium secondary battery of the present invention is the same as that of a conventionally known lithium secondary battery. Usually, a positive electrode and a negative electrode capable of occluding and releasing lithium ions, an electrolyte, and a nonaqueous solvent for dissolving the electrolyte are included. Prepare.

[Positive electrode]
The positive electrode is obtained by forming a positive electrode active material layer containing a positive electrode active material and a binder on a current collector.
(Active material)
Examples of the positive electrode active material include metal chalcogen compounds that can occlude and release alkali metal cations such as lithium ions during charge and discharge. Examples of metal chalcogen compounds include vanadium oxide, molybdenum oxide, manganese oxide, chromium oxide, titanium oxide, tungsten oxide and other transition metal oxides, vanadium sulfide, molybdenum sulfide. things, sulfides of titanium, transition metal sulfides such as CuS, NIPS _3, FEPS phosphorus transition metals, such as ₃ - sulfur compounds, VSe _2, NbSe ₃ selenium compounds of transition metals, such as, Fe _0.25 V _0.75 S _2, Examples thereof include composite oxides of transition metals such as Na _0.1 CrS ₂ and composite sulfides of transition metals such as LiCoS ₂ and LiNiS ₂ .

Among these, V ₂ O ₅ , V ₅ O ₁₃ , VO ₂ , Cr ₂ O ₅ , MnO ₂ , TiO, MoV ₂ O ₈ , LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , TiS ₂ , V ₂ S ₅ , Cr _0.25 V _0.75 S ₂ , Cr _0.5 V _0.5 S _2, etc. are preferable. LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, and lithium transitions in which some of these transition metals are replaced with other metals are particularly preferable. It is a metal complex oxide. These positive electrode active materials may be used alone or in combination.

(Binder)
A known binder can be arbitrarily selected and used as the binder for binding the positive electrode active material. Examples include inorganic compounds such as silicate and water glass, and resins having no unsaturated bond such as Teflon (registered trademark) and polyvinylidene fluoride. Among these, a resin having no unsaturated bond is preferable. If a resin having an unsaturated bond is used as the resin for binding the positive electrode active material, the resin may be decomposed during the oxidation reaction. The weight average molecular weight of these resins is usually 10,000 or more, preferably 100,000 or more, and usually 3 million or less, preferably 1 million or less.

  The positive electrode active material layer may contain a conductive material in order to improve the conductivity of the electrode. The conductive agent is not particularly limited as long as it can be mixed with an active material in an appropriate amount to impart conductivity, but is usually carbon powder such as acetylene black, carbon black, and graphite, various metal fibers, powder, and foil. Etc.

(Positive electrode)
The positive electrode is formed by slurrying a positive electrode active material or a binder with a solvent, and applying and drying on a current collector. Examples of the solvent include N-methylpyrrolidone and water. Examples of the current collector for the positive electrode include aluminum, nickel, and SUS. The thickness of the current collector is usually 5 μm or more, preferably 9 μm or more, and usually 30 μm or less, preferably 20 μm or less.

  After applying the slurry on the current collector, it is usually 60 ° C. or higher, preferably 80 ° C. or higher, and usually 250 ° C. or lower, preferably 195 ° C. or lower, from the viewpoint of preventing time efficiency and unnecessary input of energy. Dry at a temperature under dry air or an inert atmosphere to form an active physical layer. The thickness of the active material layer obtained by applying and drying the slurry is usually 5 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and usually 200 μm or less, preferably 100 μm or less, more preferably 75 μm or less. .

[Negative electrode]
The negative electrode includes a current collector and an active material layer formed on the current collector, and the active material layer contains a negative electrode material and a normal binder. The negative electrode material according to the present invention contains the negative electrode material (A) of the present invention described below.
(Negative electrode material)
The negative electrode material (A) of the present invention is graphite capable of occluding and releasing lithium ions used as a negative electrode active material of a lithium secondary battery, and the graphite is impregnated with one or more kinds of polymer materials. The circularity of graphite with the polymer material attached is 0.85 or more. As graphite, either natural graphite or artificial graphite may be used, but natural graphite is preferred in terms of raw material price and graphite surface characteristics.

  The degree of circularity of graphite on which the polymer material is spotted is usually 0.85 or more, preferably 0.90 or more, more preferably 0.92 or more. The theoretical upper limit of the circularity is 1, but the circularity is usually 0.99 or less, preferably 0.97 or less, more preferably 0.95 or less. If the value is below the lower limit, the shape becomes almost flat, and the high-density charge / discharge characteristics tend to deteriorate. If the circularity is too large, the contact resistance between the particles increases, and the charge / discharge cycle characteristics tend to deteriorate.

The circularity of graphite to which graphite and a polymer are attached is defined by the following formula, and when the circularity is 1, a theoretical sphere is obtained.
Circularity = (perimeter of equivalent circle) / (perimeter of circle with particle projection area)
As the value of the circularity, for example, a flow type particle image analyzer (for example, FPIA manufactured by Sysmex Industrial Co.) is used, and a measurement target (here, negative electrode material which is graphite or graphite to which a polymer material is attached). 2 g was mixed with a 0.2 vol% aqueous solution (about 50 mL) of polyoxyethylene (20) sorbitan monolaurate as a surfactant, and 28 kHz ultrasonic waves were irradiated at an output of 60 W for 1 minute. A value measured for particles having a particle size in the range of 10 to 40 μm can be used.

  The graphite having a circularity of 0.85 or more according to the present invention can be obtained by performing mechanical or physical treatment, chemical treatment such as oxidation treatment, plasma treatment, or the like, if necessary, using natural and artificial graphite. Specifically, for example, as described in JP-A-2005-149792, the carbonaceous material can be obtained by surface grinding. For example, using a crusher equipped with a high-speed rotating rotor with a large number of blades installed inside the casing, mechanical treatment such as impact compression, friction, shearing, etc. is applied to the carbonaceous material to perform surface treatment while crushing. Just do it. The peripheral speed of the rotor is preferably 30 to 100 m / sec, particularly 50 to 100 m / sec. The classification after pulverization is generally obtained by using an airflow classifier such as a forced vortex centrifugal classifier such as a micron separator or turboplex, or an inertia classifier such as an elbow jet. It can also be obtained using a centrifugal sedimentator.

In addition, as the graphite, the amount of voids in the particle corresponding to a diameter of 1 μm or less and the unevenness due to the step on the particle surface, which is obtained by Hg porosimetry (mercury intrusion method), is preferably 0.05 mL / g or more, Those having a concentration of 0.1 mL / g or more are preferable because an adhesion effect due to the contact of the polymer material with the voids in the particles is easily obtained. Further, the total pore volume is preferably 0.1 mL / g or more, and more preferably 0.25 mL / g or more because it is easy to attach. Further, the average pore diameter is preferably 0.05 μm or more, more preferably 0.1 μm or more, because it is easy to attach to an effective site and an attachment effect is easily obtained. Further, the average pore diameter is preferably 80 μm or less, more preferably 50 μm or less, since the attachment is effectively performed.
Specific examples of the Hg porosimetry apparatus and procedure include the following apparatuses and procedures.

  As a device for Hg porosimetry, a mercury porosimeter (Autopore 9520: manufactured by Micromeritex Corporation) is used, and a sample (negative electrode material) is weighed to a value of about 0.2 g and enclosed in a powder cell. Then, deaeration and pretreatment are performed at room temperature under vacuum (50 μmHg or less) for 10 minutes. Subsequently, the pressure is returned to 4 psia (about 28 kPa), mercury is introduced, the pressure is increased from 4 psia (about 28 kPa) to 40000 psia (about 280 MPa), and then the pressure is reduced to 25 psia (about 170 kPa). During this time, after the equilibration time of 10 seconds, the measurement operation of measuring the mercury amount is performed at 80 points or more. The pore distribution is calculated from the mercury intrusion curve thus obtained using the Washburn equation. Note that the surface tension (γ) of mercury is calculated as 485 dyne / cm and the contact angle (ψ) is 140 °.

Natural graphite is preferably one having few impurities, and is subjected to various purification treatments as necessary. Further, those having a high degree of graphitization are preferred, and specifically, those having a (002) plane spacing (d ₀₀₂ ) of less than 3.37 mm (0.337 nm) by X-ray wide angle diffraction method are preferred. The natural graphite has a BET specific surface area of usually 3.5 m ² / g or more, preferably 4.5 m ² / g or more, and usually 8 m ² / g or less, preferably 6 m ² / g or less.

The volume average particle diameter of graphite is usually 5 μm or more, and usually 50 μm or less, preferably 25 μm or less, and most preferably 18 μm or less. Note that the graphite may be secondary particles in which a plurality of particles are aggregated. In this case, the average particle diameter of the secondary particles is preferably within the above-mentioned range, and the average particle diameter of the primary particles is preferably in the range of usually 15 μm or less. When the particle size is too small, the specific surface area increases, the reaction surface with the electrolyte increases, and the irreversible capacity tends to increase. On the other hand, if the particle size is too large, so-called streaks due to large lumps occur when applying a slurry of the active material and binder to the current collector, making it difficult to form an active material layer with a uniform thickness. It becomes.
The circularity of the graphite means the circularity of secondary particles when the graphite is agglomerated.

(Polymer material)
In the present invention, graphite to which a polymer material is attached is used as the negative electrode material. The type of the polymer material is not particularly limited, but an electrolytic solution in which 1M LiPF ₆ is dissolved in a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 3: 7 (hereinafter referred to as “appropriately” The polymer material (C-1) that is easily soluble in the reference electrolyte solution (B) according to the following method (hereinafter referred to as “polymer material (C- 1) ") and a polymer material (C-2) that is difficult to dissolve in the reference electrolyte solution (B) (hereinafter referred to as" polymer material (C-2) "as appropriate). The graphite to which the polymer material of the present invention is attached has at least one of at least one polymer material (C-1) and / or one or more polymer materials (C-2) attached. Graphite.

  The solubility of the polymer material is determined by the following method. First, the target polymer material is dissolved in a good solvent, cast on a peelable substrate so that the thickness after drying is about 100 μm, dried under an inert gas, and then 12.5 mm in diameter. To produce a sample for evaluation. The obtained sample was immersed in the reference electrolyte (B) and allowed to stand in a sealed container under an Ar gas atmosphere under normal temperature and normal pressure conditions, and the areas of the samples after 1 day and 90 days were measured respectively. The ratio value (area reduction rate after 90 days with respect to 1 day later) is obtained. If the area reduction rate is 3% or more, it is determined that the polymer material (C-1) is easily dissolved in the reference electrolyte (B), and the area reduction rate is less than 3% (the area increases). In other words, the polymer material (C-2) is difficult to dissolve in the reference electrolyte solution (B). In the present invention, the polymer material (C-1) preferably has an area reduction rate of 4% or more, and the polymer material (C-2) has an area reduction rate of 2.5%. The following are preferred.

  Using the above-mentioned method, typical polymer materials such as polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene About oxide (PEO), the solubility with respect to the reference | standard electrolyte solution (B) was measured. The results are shown in Table 1.

  As the polymer material (C-1), in addition to carboxymethylcellulose (CMC), polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO) listed in Table 1, polystyrene, polymethyl acrylate An acrylic acid ester polymer such as polymethyl methacrylate (PMMA), polypropylene oxide, and a cross-linked product thereof can be used. Among these, carboxymethyl cellulose, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate are preferable because they are inexpensive and easily available.

  As the polymer material (C-2), in addition to polyvinyl alcohol (PVA) and styrene-butadiene rubber (SBR) listed in Table 1, styrene / isoprene / styrene rubber, acrylonitrile / butadiene rubber, butadiene rubber, ethylene Propylene / diene copolymer, polyvinyl chloride, polytetrafluoroethylene, polyethylene, polypropylene, isobutylene, polyethylene terephthalate and other polyesters, nylon, and cross-linked products thereof can be used. Among them, polyvinyl alcohol is preferable because it has high flexibility, exhibits hot water solubility, and is easy to handle.

  The molecular weight of the polymer material (C-1) and the polymer material (C-2) varies depending on the polymer chain bonding method, the branching ratio, the intramolecular functional group, and the steric conformation, and cannot be uniquely determined. Generally, a polymer material having a lower molecular weight has a higher solubility in the reference electrolyte solution (B), and conversely, a polymer material having a higher molecular weight tends to have a lower solubility in the reference electrolyte solution (B). It is in.

  The preferred particle size of the polymer material varies depending on the method of attaching these to graphite. In the case of using a so-called wet attachment method in which a polymer material is dissolved and then attached using a solvent, the particle size of the polymer material is not particularly limited. However, in the case of using a so-called dry attachment method in which a microcrystalline domain that does not pass through a solvent or is not completely dissolved in the solvent remains, the size of the polymer material particles is important. Specifically, the particle size of the polymer material when using the dry impregnation method is usually 5 μm or less, preferably 0.5 μm or less. If the particle size is too large, the adhesion to graphite tends to deteriorate.

Among the polymer materials, one or more polymer materials (C-1) are preferably attached to graphite from the viewpoint of improving the battery performance in a balanced manner.
In addition, two or more types of polymer materials, particularly one type, are used from the viewpoints of high electrode plate strength, good immersion, low initial irreversible capacity, high current density charge / discharge characteristics, and high cycle retention. The above polymer material (C-1) and one or more polymer materials (C-2) are preferably attached to graphite.

  The negative electrode material (A) of the present invention has a structure in which two or more different polymer materials (C-1) and (C-2) are attached to different positions of particles on graphite as negative electrode active material particles. You may have taken. Here, “each at a different position of the particles” means that these polymer materials (C-1) and (C-2) are selectively attached to different positions of graphite. . Whether the polymer materials (C-1) and (C-2) are “position-selectively” attached to the graphite can be determined by the amount of decrease in pore volume having a diameter of 1 μm or less, which will be described later. .

The specific manner of attachment is not particularly limited, but when polymer materials (C-1) and (C-2) are attached, the polymer material (C- The aspect in which 1) is attached and the polymer material (C-2) is attached to the outer surface (outer peripheral portion) of graphite is preferable from the viewpoint of reducing irreversible capacity and improving high current density charge / discharge characteristics.
The method for producing the negative electrode material (A) of the present invention is not particularly limited as long as the polymer material can be attached to graphite. However, in order to satisfy the scope of the present invention, the circularity of the graphite to which the polymer material is attached preferably uses graphite having the preferable circularity described above as the raw material graphite. In addition, when two kinds of polymers having different solubilities are attached, in order to efficiently and reliably manufacture the negative electrode material having the above-described structure with few steps, first, one or more kinds of polymer materials are used with respect to graphite. A method comprising at least two steps of attaching (C-1) first (first attaching step) and then attaching the polymer material (C-2) (second attaching step) is preferable. Note that the first attachment step and the second attachment step do not have to be clearly separated steps, and may be performed continuously.

For both the step of attaching the polymer material (C-1) to graphite (first attachment step) and the step of attaching the polymer material (C-2) (second attachment step), the specific attachment method is as follows: Although not particularly limited, the following three are typical examples.
(I) A method of simply mixing graphite and the polymer material (C-1) or (C-2) in a particle state.
(Ii) A technique in which graphite and polymer material (C-1) or (C-2) are mixed and attached or fused by mechanical impact.
(Iii) A technique in which the polymer material (C-1) or (C-2) is swollen, dispersed, or dissolved in a solvent, and this is attached to graphite and then dried.
Any one of these methods (i) to (iii) may be performed alone, or two or more methods may be combined appropriately.

  In the case of the method (i) or (ii), the specific mixing method is not particularly limited as long as it does not depart from the gist of the present invention. For example, either dry mixing or wet mixing may be used. The mixer used for mixing is not particularly limited, but examples include mechano-fusion, hybridizer, ong mill, mechano-micros, micros, jet mill, hybrid mixer, kneader, fluidized bed granulator, and radige. A mixer, a spray dryer, a disperser, etc. are mentioned. Any one of these mixers may be used alone, or two or more thereof may be used in any combination.

  These specific mixing methods may be appropriately selected according to the type of graphite or polymer material to be used. In general, when mechanical dry mixing such as mechanofusion and hybridizer is used, a polymer material is easily attached to the step part of the surface of the negative electrode active material particles, and when wet mixing is used, the negative electrode active material When using a powder mixer that has a weak shear between the particles, such as fluidized bed and paint shaker, the polymer material can easily enter the voids inside the particles. It is thought that what has adhered is obtained.

  On the other hand, when the method (iii) is used, the details of the method are not particularly limited unless departing from the gist of the present invention. Any solvent can be used as long as it can disperse, swell, or dissolve the polymer material (C-1) or (C-2), but any solvent that can dissolve can be easily manufactured. is there. Specifically, water and alcohols such as ethanol, and other organic solvents such as benzene, toluene, and xylene are not a problem, but those that can be dispersed, swollen, and dissolved by water have an environmental impact. It is suitable because it is small and the process is inexpensive. The polymer material (C-1) or (C-2) is dissolved, dispersed or swelled by using a mixing and dispersing device such as a disperser or a kneader to bring the polymer material and the solvent into contact with each other. At the time of mixing, there is no problem even if the liquid is added to the particles or the particles are added to the liquid, and the impregnation method may be any device that can mix liquid and powder, such as a disperser and a kneader. There are no particular restrictions. During mixing, a low solid content concentration to a high solid content concentration can be freely selected. In addition to this, it is possible to impregnate by a spraying method such as spray drying. Methods for drying these materials include spray drying such as spray drying, shelf drying that heats in a stationary state, a method of drying by introducing heat energy while mixing and stirring, vacuum drying, etc. There is no particular problem as long as the technique can reduce the content.

  The weight ratio of the polymer material to graphite (weight of polymer material: graphite weight) is usually 0.01: 99.99 or more, preferably 0.05: 99.95 or more, and usually 10:90 or less, Especially, it is the range of 2:98 or less. If the ratio of the polymer material is too small, sufficient polymer material is not attached in the pores, and the initial irreversible capacity may be inferior. On the other hand, when the ratio of the polymer material is too large, the weight of the active material that contributes to charging / discharging of lithium is reduced, and the battery capacity may be reduced.

  When two different types of soluble polymers are attached, the weight ratio of the polymer material (C-1) to the polymer material (C-2) (weight of the polymer material (C-1): polymer) The weight of the material (C-2) is usually 1: 1 or more, and usually 1000: 1 or less, preferably 100: 1 or less. If the ratio of the polymer material (C-1) is too small, the high current density discharge characteristics tend to be lowered. If the ratio of the polymer material (C-2) is too small, the initial charge / discharge efficiency tends to be lowered. There is.

(Negative electrode material after polymer material attachment)
The negative electrode material (A) of the present invention in which a polymer material is attached to graphite as described above has a circularity of 0.85 or more, preferably 0.90 or more, more preferably 0.92 or more, Usually, it is 0.99 or less, preferably 0.97 or less, more preferably 0.95 or less.
If the value is below the lower limit, the shape is almost flat, and the low-temperature discharge efficiency is lowered, which is not preferable. Moreover, when too large, the contact resistance between particle | grains will increase and it exists in the tendency for charging / discharging cycling characteristics to fall.
The circularity of the negative electrode material (A) of the present invention to which the polymer material is attached is the circularity in the state of the secondary particles to which the polymer material is attached when the negative electrode material is aggregated. means.

(Crosslinking materials, etc.)
For the negative electrode material (A) of the present invention, a cross-linking material may be used in addition to the above graphite and polymer materials. In particular, when the polymer material (C-2) is attached to graphite, by bonding some functional groups of the side chain and the main chain of the polymer material (C-2) with a crosslinking agent, The network structure of the polymer material (C-2) can be developed and the molecular weight can be changed after the attachment. This effect has the effect of improving the electrolytic solution resistance and improving the initial charge / discharge efficiency. The kind in particular of a crosslinking material is not restrict | limited, What is necessary is just to select an appropriate thing according to the kind of polymeric material (C-2) used together. Specifically, when polyvinyl alcohol is used as the polymer material (C-2), examples of preferable cross-linking materials include glyoxazal, organometallic complexes such as Ti and Zr, and derivatives thereof. However, the type of the cross-linking material that can be used is not limited to this. For example, other cross-linking materials can be used depending on the type of the functional group of the polymer material (C-2). In addition, any 1 type may be used individually for a crosslinking material, and 2 or more types may be used together by arbitrary combinations and a ratio.

(Other)
In addition, it is preferable that the reduction | decrease of the specific surface area measured by BET method is seen in the step which attached polymer material to graphite. Specifically, it is preferable that the BET specific surface area after the addition of the polymer material is usually reduced by 10% or more, especially 25% or more, compared with the BET specific surface area of the graphite before the application of the polymer material. The absolute value of the specific surface area by BET method of the graphite before impregnation of the polymeric material is usually 1 m ² / g or more, preferably 2m ² / g or more and usually 20 m ² / g or less, preferably 10 m ² / It is the range below g. Further, the absolute value of the specific surface area of the graphite after the polymer material attachment by the BET method is usually 0.5 m ² / g or more, preferably 1 m ² / g or more, and usually 8 m ² / g or less, preferably 6 m ^2. / G or less.

(Mixed with other negative electrode materials)
The negative electrode material (A) of the present invention described above can be suitably used as a negative electrode material for a lithium secondary battery by using any one of them alone or in combination of two or more of them in any composition and combination. As long as the effects of the present invention are not impaired, one or more of the above-described negative electrode materials (A) of the present invention are mixed with other one or two or more negative electrode materials (E), You may use as a negative electrode material of a secondary battery.
When the negative electrode material (E) is mixed with the negative electrode material (A) described above, the mixing ratio of the negative electrode material (E) to the total amount of the negative electrode material (A) and the negative electrode material (E) is usually 80% by weight or less, preferably It is in the range of 50% by weight or less, more preferably 20% by weight or less. If the mixing ratio of the negative electrode material (E) is too large, the characteristics of the negative electrode material (A) tend to hardly appear.

(Binder)
As the binder, one having an olefinically unsaturated bond in the molecule is usually used. The type is not particularly limited, and specific examples include styrene-butadiene rubber, styrene / isoprene / styrene rubber, acrylonitrile-butadiene rubber, butadiene rubber, and ethylene / propylene / diene copolymer. By using such a binder having an olefinically unsaturated bond, the swellability of the active material layer with respect to the electrolytic solution can be reduced. Of these, styrene-butadiene rubber is preferred because of its availability.

  By using a binder having such an olefinically unsaturated bond in combination with the above active material, the strength of the negative electrode plate can be increased. When the strength of the negative electrode is high, deterioration of the negative electrode due to charge / discharge is suppressed, and the cycle life can be extended. In addition, since the negative electrode according to the present invention has high adhesive strength between the active material layer and the current collector, even when the binder content in the active material layer is reduced, the negative electrode is wound to produce a battery. It is speculated that the problem that the active material layer peels from the current collector does not occur.

As the binder having an olefinically unsaturated bond in the molecule, one having a large molecular weight or one having a large proportion of unsaturated bonds is desirable. Specifically, in the case of a binder having a large molecular weight, it is desirable that the molecular weight is usually 10,000 or more, preferably 50,000 or more, and usually 1,000,000 or less, preferably 300,000 or less. In the case of a binder having a large proportion of unsaturated bonds, the number of moles of olefinically unsaturated bonds per gram of all binders is usually 2.5 × 10 ⁻⁷ or more, preferably 8 × 10 ⁻⁷ or more, Further, it is usually 1 × 10 ⁻⁴ or less, preferably 5 × 10 ⁻⁶ or less. The binder only needs to satisfy at least one of these regulations regarding molecular weight and regulations regarding the proportion of unsaturated bonds, but it is more preferable to satisfy both regulations simultaneously. When the molecular weight of the binder having an olefinically unsaturated bond is too small, the mechanical strength is inferior, and when it is too large, the flexibility is inferior. Moreover, if the ratio of the olefinically unsaturated bond in the binder is too small, the effect of improving the strength is reduced, and if it is too large, the flexibility is poor.

The binder having an olefinically unsaturated bond has a degree of unsaturation of usually 15% or more, preferably 20% or more, more preferably 40% or more, and usually 90% or less, preferably 80% or less. Is desirable. The degree of unsaturation represents the ratio (%) of the double bond to the repeating unit of the polymer.
In the present invention, a binder that does not have an olefinically unsaturated bond can also be used in combination with the above-described binder that has an olefinically unsaturated bond as long as the effects of the present invention are not lost. The mixing ratio of the binder having no olefinically unsaturated bond to the binder having an olefinically unsaturated bond is usually 150% by weight or less, preferably 120% by weight or less. By using a binder that does not have an olefinically unsaturated bond, the coatability can be improved. However, if the combined amount is too large, the strength of the active material layer is lowered.

  Examples of the binder having no olefinic unsaturated bond include thickening polysaccharides such as methylcellulose, carboxymethylcellulose, starch, carrageenan, pullulan, guar gum, xanthan gum (xanthan gum), polyethers such as polyethylene oxide and polypropylene oxide, Vinyl alcohols such as polyvinyl alcohol and polyvinyl butyral, polyacids such as polyacrylic acid and polymethacrylic acid, or metal salts of these polymers, fluorine-containing polymers such as polyvinylidene fluoride, alkane polymers such as polyethylene and polypropylene, and these A copolymer etc. are mentioned.

  In the present invention, the negative electrode material (A) of the present invention in which the polymer material (C-1) and / or (C-2) is attached to graphite is combined with the above-mentioned binder having an olefinically unsaturated bond. When used, the ratio of the binder used for the active material layer can be reduced as compared with the conventional case. Specifically, the negative electrode material of the present invention and a binder (in some cases, it may be a mixture of a binder having an unsaturated bond and a binder having no unsaturated bond as described above). The weight ratio of each is usually 90/10 or more, preferably 95/5 or more, and usually 99.9 / 0.1 or less, preferably 99.5 / 0.5 or less, and more preferably, in each dry weight ratio. Is in the range of 99/1 or less. If the binder ratio is too high, the capacity is likely to decrease and the resistance is increased. If the binder ratio is too small, the electrode plate strength is inferior.

(Negative electrode)
The negative electrode of the present invention is prepared by dispersing the negative electrode material (A) of the present invention, a binder, and a negative electrode material (E) used as necessary in a dispersion medium, and applying the slurry to a current collector. It is formed by. As the dispersion medium, an organic solvent such as alcohol or water can be used. If necessary, a conductive agent may be added to the slurry. Examples of the conductive agent include carbon black such as acetylene black, ketjen black, and furnace black, and fine powder made of Cu, Ni having an average particle diameter of 1 μm or less, or an alloy thereof. The addition amount of the conductive agent is usually about 10% by weight or less with respect to the negative electrode material of the present invention.

A conventionally well-known thing can be used as a collector which apply | coats a slurry. Specific examples include metal thin films such as rolled copper foil, electrolytic copper foil, and stainless steel foil. The thickness of the current collector is usually 5 μm or more, preferably 9 μm or more, and usually 30 μm or less, preferably 20 μm or less.
After applying the slurry on the current collector, the slurry is usually dried at a temperature of 60 ° C. or higher, preferably 80 ° C. or higher, and usually 200 ° C. or lower, preferably 195 ° C. or lower, in dry air or an inert atmosphere. A physical layer is formed.

  The thickness of the active material layer obtained by applying and drying the slurry is usually 5 μm or more, preferably 20 μm or more, more preferably 30 μm or more, and usually 200 μm or less, preferably 100 μm or less, more preferably 75 μm or less. . If the active material layer is too thin, it is not practical as a negative electrode due to the balance with the particle size of the active material.

(Immersion)
The upper limit of the immersion property of the negative electrode for a lithium secondary battery of the present invention is usually 25 seconds or less, preferably 22 seconds or less, more preferably 20 seconds or less.
When this upper limit is exceeded, there is a problem in that the stage of liquid injection becomes rate-determining in the manufacturing process of the lithium ion secondary battery, resulting in a large amount of capital investment and time loss in manufacturing.
However, the immersion property described here is measured by the evaluation method described in the examples of the present specification.

[Electrolytes]
As the electrolyte, a lithium salt is usually used, and it can be appropriately selected from known lithium salts known to be usable for this application. For example, halides such as LiCl and LiBr, perhalogenates such as LiClO ₄ , LiBrO ₄ and LiClO ₄ , inorganic lithium salts such as inorganic fluoride salts such as LiPF ₆ , LiBF ₄ and LiAsF ₆ , LiCF ₃ SO ₃ , Fluorine-containing organic lithium salts such as perfluoroalkanesulfonic acid salts such as LiC ₄ F ₉ SO ₃ and perfluoroalkanesulfonic acid imide salts such as Li trifluorosulfonimide ((CF ₃ SO ₂ ) ₂ NLi) . Lithium salts may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte is usually in the range of 0.5M to 2.0M.

  Further, as the electrolyte, a polymer solid electrolyte which is a conductor of an alkali metal cation such as lithium ion can be used. Examples of the polymer solid electrolyte include a polymer in which a Li salt is dissolved in a polyether polymer compound such as polyethylene oxide and polypropylene oxide, and a polymer in which the terminal hydroxyl group of the polyether is substituted with an alkoxide.

[Non-aqueous electrolyte]
The nonaqueous electrolytic solution according to the present invention is characterized by containing propylene carbonate and a carbonate having an unsaturated bond.
Examples of the carbonate having an unsaturated bond include cyclic carbonates such as vinylene carbonate compounds, vinyl ethylene carbonate compounds, and methylene ethylene carbonate compounds, or chain carbonates such as allyl methyl carbonate compounds, vinyl methyl carbonate, and diallyl carbonate. .

Examples of vinylene carbonate compounds include vinylene carbonate, methyl vinylene carbonate, ethyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4,5-diethyl vinylene carbonate, and the like.
Examples of the vinyl ethylene carbonate compound include vinyl ethylene carbonate, 4-methyl-4-vinyl ethylene carbonate, 4-ethyl-4-vinyl ethylene carbonate, 4-n-propyl-4-vinyl ethylene carbonate, 5-methyl-4- Examples include vinyl ethylene carbonate, 4,4-divinyl ethylene carbonate, 4,5-divinyl ethylene carbonate, and the like.

Examples of the methylene ethylene carbonate compound include methylene ethylene carbonate, 4,4-dimethyl-5-methylene ethylene carbonate, 4,4-diethyl-5-methylene ethylene carbonate, and the like.
Among these, vinylene carbonate and vinyl ethylene carbonate are preferable from the viewpoint of improving cycle characteristics, and vinylene carbonate is particularly preferable.

These may be used alone or in combination of two or more.
Since the carbonate having an unsaturated bond forms a stable protective film on the surface of the negative electrode, the cycle characteristics of the battery can be improved. In the battery containing the negative electrode material of the present invention, the electrolyte is propylene carbonate. Moreover, by having a carbonate having an unsaturated bond, the initial irreversible capacity and the low-temperature discharge efficiency are improved.

When using 2 or more types together, it is preferable to use together vinylene carbonate and vinyl ethylene carbonate.
The content of propylene carbonate in the non-aqueous electrolyte is usually 5% by volume or more, preferably 10% by volume or more, more preferably 20% by volume or more, and usually 95% by volume or less, preferably 70% by volume or less, more preferably. Is preferably 50% by volume or less. If the amount is too small, the effect of improving the high current density charge / discharge characteristics at low temperatures due to the inclusion of propylene carbonate is reduced. If the amount is too large, the decomposition of propylene carbonate cannot be ignored, and an increase in irreversible capacity becomes a problem.

  The content of the carbonate having an unsaturated bond with respect to the nonaqueous electrolytic solution is usually 0.5% by volume or more, preferably 1% by volume or more, more preferably 2% by volume or more, and usually 10% by volume or less, preferably 7%. .5% by volume or less, more preferably 5% by volume or less is desirable. If the amount is too small, the synergistic effect of improving the immersion property due to the presence of VC becomes thin, leading to an increase in lead time in the production process, which is not preferable, and the initial irreversible capacity and the low-temperature discharge efficiency are inferior. If the amount is too large, the reaction between the carbonate having an unsaturated bond and the positive electrode results in a high resistance of the coating film formed on the positive electrode, resulting in a problem that the high current density charge / discharge characteristics deteriorate.

The content (volume ratio) of the carbonate having an unsaturated bond with respect to propylene carbonate is usually 0.005 or more, preferably 0.01 or more, and usually 2 or less, preferably 1 or less. If the amount is too small, the effect of reducing the increase in irreversible capacity tends not to be obtained, and if the amount is too large, the high current density charge / discharge characteristics tend to decrease.
The non-aqueous electrolyte of the present invention may be appropriately selected from known non-aqueous solvents conventionally proposed as solvents for non-aqueous electrolytes in addition to propylene carbonate and carbonates having an unsaturated bond. good. Examples of the solvent that may be used in combination include chain carbonates such as diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate; cyclic carbonates such as ethylene carbonate and butylene carbonate (excluding propylene carbonate); 1,2- Chain ethers such as dimethoxyethane; cyclic ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, 1,3-dioxolane; chain esters such as methyl formate, methyl acetate, and methyl propionate; γ-butyrolactone, γ -Cyclic esters such as valerolactone.

Among these, it is preferable to contain a chain carbonate and a cyclic carbonate (excluding propylene carbonate) from the viewpoint of the balance of dielectric constant, viscosity and the like of the electrolytic solution. In this case, the volume ratio of the chain carbonate to the cyclic carbonate (chain carbonate: cyclic carbonate) is usually 5:95 or more, preferably 10:90 or more, more preferably 50:50 or more, and usually 95: 5 or less. , Preferably 90:10 or less, more preferably 80:20 or less. If the content of the chain carbonates is too large, there is a problem that lithium ions that can contribute to charge / discharge decrease due to a decrease in the dielectric constant, and if it is too small, the viscosity increases and the properties of the liquid injection tend to deteriorate. .
The content (volume%) of the above-mentioned solvent that may be used in combination with the nonaqueous electrolytic solution is usually 95.5% or less, preferably 90% or less, and usually 4.5% or more. When the amount is too large, there is a problem that the charge / discharge characteristics at a low temperature and high current density are deteriorated. When the amount is too small, the effect of combined use is hardly recognized.

(Others / Propylene carbonate / Carbonates with unsaturated bonds / Reason why the negative electrode material of the present invention is good)
As described in Patent Document 1, by adding a resin to the surface of the negative electrode active material, the wettability (immersability) between the carbon negative electrode and the nonaqueous electrolyte layer is improved, and the decomposition of propylene carbonate is suppressed. As is known, this immersion property is still not sufficient in an actual production line. On the other hand, it is possible to improve the immersion property by adding vinylene carbonate in place of propylene carbonate in the electrolytic solution. However, in this case as well, the immersion property is still not sufficient. On the other hand, although the details of the reason are unclear, when graphite is used as the active material, the circularity is set to 0.85 or more by resin attachment, and propylene carbonate and vinylene carbonate are contained in the non-aqueous electrolyte. By including such unsaturated carbonate, the lithium secondary battery in which the immersion property is synergistically improved, and the increase in the initial irreversible capacity is suppressed while the decomposition of propylene carbonate is suppressed and the low temperature discharge efficiency is maintained. It becomes.

(Separator)
In order to prevent a short circuit between the electrodes, a porous separator such as a porous film or a nonwoven fabric is usually interposed between the positive electrode and the negative electrode. In this case, the nonaqueous electrolytic solution is used by impregnating a porous separator. As a material for the separator, polyolefin such as polyethylene and polypropylene, polyethersulfone, and the like are used, and polyolefin is preferable.

  The form of the lithium secondary battery of the present invention is not particularly limited. Examples include a cylinder type in which a sheet electrode and a separator are spiral, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked, and the like. In addition, by storing batteries of these forms in an optional outer case, the battery can be used in an arbitrary shape such as a coin shape, a cylindrical shape, or a square shape.

  The procedure for assembling the lithium secondary battery of the present invention is not particularly limited, and may be assembled by an appropriate procedure according to the structure of the battery.For example, the negative electrode is placed on the outer case, and the electrolyte and A battery can be formed by providing a separator and placing a positive electrode so as to face the negative electrode and caulking together with a gasket and a sealing plate.

  EXAMPLES Next, although an Example demonstrates this invention further more concretely, this invention is not limited to description of a following example, unless the summary is exceeded.

<Example 1>
(Negative electrode material)
As a polymer material (C-1), 1 g of carboxymethylcellulose (BSH6 manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) was added to 199 g of pure water and dissolved. Into this solution, 200 g of spherical natural graphite particles having a specific surface area of 5.8 m ² / g, an average particle size of 23 μm, and a circularity (0.93) were added as natural graphite. It stirred and mixed for 2 hours using the disperser. The obtained mixture was placed in a stainless steel vat so as to have a height of 1.5 cm, and dried in N ₂ gas at 110 ° C. for 10 hours. This was sieved to obtain negative electrode active material particles with one layer of polymer material. This was used as the negative electrode material of Example 1.
About the obtained negative electrode material, the specific surface area by BET method was measured with the following procedures. These results are shown in Table 2.

<BET method>
The BET specific surface area was measured by a BET one-point method (nitrogen gas adsorption) using an automatic surface area measuring apparatus (AMS8000: manufactured by Okura Riken). The sample (negative electrode material) was accurately weighed so as to have a value of about 0.8 g, and placed in a dedicated cell and attached to the apparatus. Pretreatment was performed for 30 minutes by heating to 100 ° C. and flowing a measurement gas (nitrogen 30%, helium balance). After completion of the pretreatment, the cell was cooled to liquid nitrogen temperature, the gas was saturated and adsorbed, and then the sample was heated to room temperature and the amount of gas desorbed by TCD was measured. From the amount of gas obtained and the sample weight after measurement, the specific surface area was calculated using the BET one-point method.

(Creation of negative electrode)
10 g of an aqueous dispersion of carboxymethylcellulose (carboxymethylcellulose concentration 1% by weight) and an aqueous dispersion of styrene-butadiene rubber having an unsaturation degree of 75% (styrene-butadiene rubber concentration 50%) as the negative electrode material and binder. %, Styrene-butadiene rubber molecular weight 120,000) 0.2 g was mixed using a high speed mixer to prepare a slurry. This slurry was applied onto a copper foil (current collector) by a doctor blade method and dried. This was pressed at a linear density of 20 to 300 kg / cm by a roll press to form an active material layer. The weight of the active material layer after drying was 10 mg / cm ² , the density was 1.6 g / mL, and the average electrode thickness was 68 μm. The negative electrode (negative electrode for lithium secondary battery) produced by the above procedure is used as the negative electrode of the example.

<Evaluation of immersion speed>
The coating electrode for a lithium secondary battery prepared in the above (Preparation of negative electrode) was punched into a disk shape having a diameter of 12.5 mm and dried under reduced pressure at 110 ° C. to prepare a measurement sample. This sample was fixed to be horizontal, and 5 μL of an electrolyte solution having a volume ratio of propylene carbonate, ethylene carbonate, and vinylene carbonate of 50: 45: 5 was dropped on the sample with a microsyringe. The time from dripping to the disappearance of the droplet was measured visually, and the immersion quality was evaluated based on the shortness of this time. The results are shown in Table 2.

<Evaluation of initial battery characteristics (initial irreversible capacity) and low temperature high current density charge / discharge characteristics>
To 100 parts by weight of the negative electrode material, 2 parts by weight of a 50% aqueous dispersion of styrene-butadiene rubber and 100 parts by weight of a 1% aqueous solution of carboxymethylcellulose were added and kneaded to form a slurry. This slurry was applied on a copper foil by a doctor blade method. After drying at 110 ° C., it was consolidated by a roll press so that the negative electrode layer had a thickness of 65 μm and a density of 1.63 g / mL. This was punched into a disk shape with a diameter of 12.5 mm and dried under reduced pressure at 190 ° C. to obtain a negative electrode. This negative electrode and a lithium metal plate (counter electrode 0.5 mm thickness 14φ) were mixed in a 2: 3: 5 (volume%) mixture of propylene carbonate: ethylene carbonate: diethylene carbonate in an amount of 1% by weight. A half-cell for a charge / discharge test was fabricated by stacking them with a separator impregnated with an electrolyte mixed with vinylene carbonate and 0.8 M LiPF ₆ . This half-cell is charged to 0.01 V (Li / Li ⁺ ) with a current of 0.2 mA (= intercalation of lithium ions to the negative electrode), and at this voltage, the current capacity per 1 g of the negative electrode layer is 350 mAhr. Charged until Subsequently, it discharged to 1.5V with the electric current of 0.4 mA, and made the initial irreversible capacity (mAh / g) the difference of the amount of charge and the amount of discharge to 0.8V.

Next, after standing for 6 hours in a thermostatic bath at 0 ° C., the following low-temperature high-density discharge efficiency measurement was performed.
The battery was charged to 0.005 V at a current of 0.2 mA, further charged at 0.005 V until the current reached 0.02 mA, and then discharged to 1.0 V at 0.215 mA. The discharge amount at this time was 0.2 C discharge capacity. Subsequently, the battery was charged to 0.005 V at a current of 0.2 mA, further charged at 0.005 V until the current became 0.02 mA, and then discharged to 1.0 V at 0.86 mA. The discharge amount at this time was 0.5 C discharge capacity. The high current density discharge efficiency was calculated from the following equation.
Low temperature high current density discharge efficiency (%) = 0.5C discharge capacity / 0.2C discharge capacity

<Comparative Example 1>
In Example 1, a negative electrode active material was prepared in the same manner as described above except that the polymer material was not attached to the spheroidized natural graphite and was used as it was as the negative electrode active material. In addition, the electrolyte solution whose volume ratio of propylene carbonate and ethylene carbonate is 50:50 was used for the electrolyte solution for immersion rate measurement. Further, a negative electrode and a lithium secondary battery were prepared by the same procedure except that vinylene carbonate was not added to the electrolytic solution in Example 1. The evaluation results are shown in Table 2.
As shown in Table 2, there is a problem that the immersion of the electrolytic solution into the electrode is insufficient because the resin attachment of the present invention is not performed and vinylene carbonate is not used.

<Comparative example 2>
In Example 1, a negative electrode and a lithium secondary battery were prepared by the same procedure except that the polymer material was not attached to the spheroidized natural graphite and was used as it was as the negative electrode active material. In addition, the electrolyte solution whose volume ratio of propylene carbonate, ethylene carbonate, and vinylene carbonate is 50: 45: 5 was used for the electrolyte solution for immersion rate measurement.
The evaluation results are shown in Table 2.
As shown in Table 2, since the graphite of the negative electrode material is not resin-added, there is a problem of initial irreversible capacity due to decomposition of propylene carbonate and a problem of insufficient immersion of the electrolyte in the electrode. .

<Comparative Example 3>
As the electrolyte solution for measuring the immersion speed in Example 1, an electrolyte solution having a ratio of propylene carbonate and ethylene carbonate of 50:50 was used. A negative electrode and a lithium secondary battery were prepared by the same procedure except that vinylene carbonate was not added to the electrolytic solution in Example 1. The evaluation results are shown in Table 2.
As shown in Table 2, since the non-aqueous electrolyte does not contain vinylene carbonate, the problem of initial irreversible capacity due to the decomposition of propylene carbonate and the problem of insufficient immersion of the electrolyte in the electrode There is.

<Comparative example 4>
A negative electrode and a lithium secondary battery were prepared by the same procedure as in Example 1 except that flake graphite (circularity 0.82) was used in place of the spherical natural graphite particles having a circularity of 0.93. The evaluation results are shown in Table 2.
As shown in Table 2, since the negative electrode material is not spherical, there is a problem of initial irreversible capacity due to propylene carbonate decomposition.

<Comparative Example 5>
In Example 1, a negative electrode and a lithium secondary battery were prepared in the same procedure except that an electrolytic solution having a volume ratio of propylene carbonate and ethylene carbonate of 55:45 was used as the electrolytic solution for measuring the immersion rate. . The evaluation results are shown in Table 2.
As shown in Table 2, since propylene carbonate is added instead of the vinylene carbonate of the present invention, there is a problem that the immersion of the electrolytic solution into the electrode is insufficient.

  The lithium secondary battery of the present invention is a lithium secondary battery having good immersion characteristics, small initial irreversible capacity, excellent high current density charge / discharge characteristics, high cycle maintenance ratio, that is, excellent various battery characteristics. Realized. In addition, due to the immersion property of the lithium secondary battery of the present invention, the negative electrode material having the above-described advantages can be manufactured in a simple process in the manufacturing process. Therefore, the present invention can be suitably used in various fields such as an electronic device in which a lithium secondary battery is used.

Claims (7)

A lithium secondary battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium ions, an electrolyte, and a non-aqueous electrolyte,
(1) The negative electrode material in the negative electrode includes a negative electrode material (A) having a circularity of 0.85 or more and a BET specific surface area of 4.1 m ² / g or less,
(2) containing propylene carbonate and carbonate having an unsaturated bond in the non-aqueous electrolyte,
A lithium secondary battery characterized by that. ^2. The lithium secondary battery according to claim 1, wherein the negative electrode material has a BET specific surface area of 0.5 m ² / g or more and 4.1 m ² / g or less.   The lithium secondary battery according to claim 1, wherein the negative electrode material is graphite.   The lithium secondary battery according to claim 3, wherein the graphite is natural graphite.   The lithium secondary battery according to claim 4, wherein the natural graphite is spheroidized.   The lithium secondary battery according to any one of claims 1 to 5, wherein the content of propylene carbonate in the non-aqueous electrolyte is 5% by volume or more and 95% by volume or less.   7. The content of carbonate having an unsaturated bond in the non-aqueous electrolyte is 0.5% by volume or more and 10% by volume or less. 7. Lithium secondary battery.