KR101905061B1 - Lithium ion secondary battery - Google Patents

Abstract

The present invention provides a lithium ion secondary battery in which improvement of durability against high rate charging and discharging is intended. The lithium ion secondary battery of the present invention is a lithium ion secondary battery comprising a negative electrode active material layer comprising a negative electrode active material composed of a graphite carbonaceous material having a graphite structure at least in part and a conductive carbonaceous material composed of conductive amorphous carbon as a carbonaceous material different from the graphite carbonaceous material Material. The negative electrode active material has a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less. The conductive carbon material has a bulk density of 0.4 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a lithium ion secondary battery,

The present invention relates to a lithium ion secondary battery.

BACKGROUND ART [0002] In recent years, a lithium ion secondary battery has been becoming more and more important as a power source for a vehicle or a power source for a personal computer and a portable terminal because a lithium ion secondary battery has a light weight and a high energy density. In particular, the use of a power source for a driving motor mounted on a vehicle (hereinafter referred to as " power source for driving a vehicle ") is being widened.

In a lithium ion secondary battery mounted on a vehicle such as an automobile and used as a power source for driving a vehicle, a so-called high rate charge / discharge in which charging and discharging are performed with a large current in a short time is performed. During the execution of such high rate charging and discharging, in the electrodes (positive electrode and negative electrode) of the lithium ion secondary battery, the structure of the electrode active material changes due to abrupt insertion or detachment of lithium ion as the charge carrier, (Hereinafter referred to as " expansion / contraction " by combining expansion and contraction). Such expansion / contraction of the electrode active material layer can be a factor that the non-aqueous electrolyte contained in the electrode flows out from the electrode (specifically, the electrode active material layer).

Particularly, in a lithium ion secondary battery comprising a graphite-based carbon material as a negative electrode active material, the degree of expansion / contraction of the negative electrode active material layer during high rate charging and discharging tends to be large and leakage of the nonaqueous electrolyte tends to occur. The outflow of the non-aqueous electrolyte from the negative electrode means that the lithium salt contained in the discharged electrolyte flows out together with the negative active material layer, and the lithium salt concentration in the negative electrode active material layer is likely to decrease.

Further, the outflow of the electrolytic solution due to the expansion / contraction of the negative electrode active material layer and the decrease of the lithium salt concentration are factors causing the unevenness of the lithium salt concentration on the surface and inside of the negative electrode (negative electrode active material layer). If such unevenness of the lithium salt concentration (in particular, concentration unevenness in the direction of the surface of the negative electrode) occurs, a spot having a large resistance in the negative electrode active material layer is formed in a spot, and the battery performance (durability) Is undesirably low.

In this regard, for example, Patent Document 1 below discloses a lithium ion secondary battery characterized in that a low crystalline carbon material having a lattice plane spacing doo2 of 0.372 nm to 0.400 nm is used as a negative electrode active material . In Patent Document 1, the low-crystalline carbon material has a crystal structure of 0.372 nm or more, which is the lattice plane spacing d002 in the case where lithium is inserted into graphite, from the beginning. Therefore, (The low-crystalline carbon material) does not repeatedly expand / shrink, and as a result, good cycle characteristics are obtained.

Japanese Unexamined Patent Application Publication No. 2002-231316

However, the lithium ion secondary battery having the low-crystalline carbon material described in the above-mentioned Patent Document 1 as a negative electrode active material has been developed on the assumption that charging and discharging are carried out at a relatively low rate, The durability of charging and discharging at a particularly high rate required for a lithium ion secondary battery, for example, charging at an extremely high rate, such as 5 C or more, can not be sufficiently improved.

SUMMARY OF THE INVENTION Accordingly, the present invention has been made to solve the above-mentioned conventional problems in a lithium ion secondary battery, and it is an object of the present invention to provide a lithium ion secondary battery which has durability (high rate cycle characteristics) And to provide a lithium ion secondary battery which is more effectively improved.

The present inventors have focused on the relationship between the properties of the negative electrode active material and the conductive material contained in the negative electrode active material layer of the lithium ion secondary battery and the battery performance.

When the bulk density of amorphous carbon (hereinafter also referred to as " conductive carbon material ") as the conductive material contained in the negative electrode active material layer is smaller than the bulk density of the graphite carbon material as the negative electrode active material, It has been found that the void volume in the negative electrode active material layer tends to be large and the diffusion rate of lithium ions in the negative electrode active material layer increases. In the negative electrode active material layer having such a constitution, lithium ions are smoothly supplied to the electrode body and lithium ions are easily diffused in the electrode body, so that even when high rate charge / discharge is performed, unevenness of lithium salt concentration is suppressed . Therefore, improvement in durability against charging and discharging at a high rate can be expected.

On the other hand, however, even when the bulk density of the conductive carbon material is smaller than the bulk density of the anode active material (graphite carbon material), when the specific surface area of the conductive carbon material is too large, (Decomposition reaction of the electrolyte solution in the negative electrode active material layer at the time of charging, for example) is promoted, and durability (high rate cycle characteristics) against high rate charge and discharge is lowered.

The present invention provides a novel approach to solve or alleviate the unevenness of the lithium salt concentration on the basis of various knowledge about the characteristics of the negative electrode active material and the conductive material contained in the negative electrode active material layer of the lithium ion secondary battery, Thereby improving the durability of the lithium ion secondary battery for charging and discharging.

In order to achieve the above object, according to the present invention, there is provided an electrode assembly comprising a positive electrode having a positive electrode active material layer on a positive electrode collector, an electrode body having a negative electrode having a negative electrode active material layer on the negative electrode collector, Ion secondary battery is provided.

In the lithium ion secondary battery disclosed herein, the negative electrode active material layer includes a negative electrode active material composed of a graphite carbon material having a graphite structure at least in part and a negative electrode active material composed of conductive amorphous carbon as a carbon material different from the graphite carbon material It contains conductive carbon material. Here, the negative electrode active material has a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less, and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less.

The conductive carbon material has a bulk density of 0.4 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less.

In the lithium ion secondary battery having such a constitution, it is possible to suppress the occurrence of a side reaction (e.g., a decomposition reaction of the electrolyte solution in the negative electrode active material layer at the time of charging) at the time of high rate charging and discharging in the negative electrode active material layer, It is possible to secure a volume (public rate).

Therefore, even when the structure of the graphite carbonaceous material, which is the negative electrode active material, changes at high rate charging / discharging accompanied by abrupt insertion or desorption of lithium ions, the degree of expansion / contraction of the negative electrode active material layer can do. Further, since the diffusion of lithium ions in the negative electrode active material layer is promoted, the non-uniformity of the lithium salt concentration in the surface direction of the negative electrode can be suppressed.

Therefore, according to the lithium ion secondary battery of this constitution, it is possible to realize high durability (high rate cycle characteristic) for high rate charging and discharging. For example, according to the lithium ion secondary battery of the present configuration, it is possible to suppress the rise of the internal resistance even in the use mode in which the charging and discharging at the high rate is repeated.

In a preferred embodiment of the lithium ion secondary cell disclosed herein, the content ratio of the conductive carbon material in the negative electrode active material layer is in the range of 2 to 10 parts by mass based on 100 parts by mass of the entire solid content of the negative electrode active material layer .

When the content ratio (mixing ratio) of the conductive carbonaceous material composed of the conductive amorphous carbon in the negative electrode active material layer is in the above range, it is possible to suppress the nonuniformity of the lithium salt concentration and to suppress the battery capacity due to the addition of the conductive carbonaceous material to the negative electrode active material layer (Discharging capacity) can be made compatible with good balance.

Further, in a preferred embodiment, the conductive carbon material is at least one kind of carbon black.

Various carbon blacks have good conductivity, and their volume densities and BET specific surface areas can be adjusted within a suitable range by controlling the particle diameter and the structure (lumps linked with a plurality of particles). For this reason, the carbon black is a conductive carbonaceous material composed of a conductive amorphous carbon preferable for realizing the object of the present invention.

When the conductive carbon material is carbon black, it is preferable that the conductive carbon material is a furnace black, and the bulk density of the furnace black is 0.02 g / cm3 or more and 0.04 g / cm3 or less.

When a furnace black having a bulk density in the above range is used as the carbon black which is the conductive carbon material, the site where the furnace black surface and the electrolyte are in contact with each other specifically increases so that the function as the anode active material is exhibited. That is, the furnace black having such a range of volume density exhibits not only its function as a conductive material (conductive carbon material) but also its function as a negative electrode active material. Therefore, the deterioration of the battery capacity due to the addition of the conductive carbon material can be suppressed to a small extent.

Further, in the furnace black having such a range of the bulk density, the particles tend to be connected to each other. Therefore, a large structure in which the furnace black particles are connected to each other is formed, and this structure functions as a skeleton in the anode active material layer. As a result, the coating film density is lowered and the void volume of the cathode is increased. As a result, the diffusion of lithium ions in the cathode surface is promoted, and the occurrence of unevenness in salt concentration of the electrolytic solution is suppressed. Therefore, the increase of the resistance can be suppressed to a high degree, and the high rate cycle characteristic can be further improved.

Further, in a preferred embodiment, the positive electrode collector and the negative electrode collector are a long sheet-like positive electrode sheet and negative electrode sheet, respectively, and the electrode body is formed by overlapping the positive electrode sheet and the negative electrode sheet with a separator interposed therebetween And is a wound electrode in a wound state.

Since the wound electrode body is a structure (in particular, a wound electrode body formed in a flat shape) in which the non-aqueous electrolyte easily flows out from the electrode body due to the expansion / contraction of the electrode (particularly, the negative electrode) (And a lithium ion secondary battery having the wound electrode body) to which the invention is applied.

As described above, the lithium ion secondary battery disclosed here is provided with excellent battery characteristics such as high rate cycle characteristics (for example, suppression of increase in internal resistance). Therefore, by taking advantage of these features, it can be suitably used as a power source (power source for driving a vehicle), such as a hybrid vehicle or a plug-in hybrid vehicle.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram for schematically explaining an internal configuration of a lithium ion secondary battery according to an embodiment. FIG.

Hereinafter, a preferred embodiment of the lithium ion secondary battery disclosed here will be described. In addition, items other than those specifically mentioned in this specification can be identified as design matters of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and the technical knowledge in the field.

Hereinafter, a lithium ion secondary battery in which a flat wound electrode body and a nonaqueous electrolyte solution are accommodated in a corresponding flat (box-shaped) container will be described as an example of a preferred embodiment of the present invention. However, (Length, width, thickness, etc.) do not necessarily reflect actual dimensional relationships.

As used herein, the term " lithium ion secondary battery " refers to, for example, lithium ion as an electrolyte ion (charge carrier), and charge / discharge is realized by movement of charge accompanying lithium ion between positive and negative electrodes. And a general rechargeable battery. Generally, a lithium ion battery or a lithium secondary battery is a typical example of the lithium ion secondary battery in this specification.

In the present specification, the "bulk density (g / cm 3)" of the conductive carbon material (conductive material) and the graphite carbon material (negative electrode active material) is measured by a measurement method conforming to "JIS K6219-2: 2015" It refers to the measurement result.

In the present specification, the "BET surface area (m 2 / g)" of the conductive carbon material (conductive material) and the graphite carbon material (negative electrode active material) is measured by a measurement method conforming to "JIS K6217-7: 2013" It refers to the measurement result.

As shown in Fig. 1, the lithium ion secondary battery 100 according to the present embodiment is provided with a case 50 made of metal (suitable for a resin or laminate film system). This case (outer container) 50 includes a case body 52 having a flat rectangular parallelepiped shape with an open upper end and a lid 54 covering the opening.

The positive electrode terminal 70 electrically connected to the positive electrode 10 of the wound electrode body 80 and the positive electrode terminal 70 electrically connected to the negative electrode 20 are formed on the upper surface (i.e., the cover body 54) A terminal 72 is provided. A long sheet-shaped separator (separator sheet) 40 and a long-sheet-shaped separator (negative electrode sheet) The wound electrode body 80, which is laminated together and wound, is accommodated together with the nonaqueous electrolyte solution.

The cover body 54 is provided with a gas discharging mechanism such as a safety valve for discharging the gas generated inside the case 50 to the outside of the case 50 in the same manner as the conventional lithium ion secondary battery of this type , And therefore, the description and the illustration are omitted.

The positive electrode sheet 10 is provided with a positive electrode active material layer 14 composed mainly of a positive electrode active material (a substance capable of absorbing and desorbing lithium ions) on both surfaces of a long-sheet-like positive electrode current collector 12. However, the positive electrode active material layer 14 is not provided at one side edge (i.e., one end in the winding axis direction) in the width direction perpendicular to the longitudinal direction of the positive electrode sheet 10 , And the positive electrode active material layer non-formed portion (16) exposing the positive electrode collector (12) with a constant width is formed.

In the lithium ion secondary battery disclosed here, the content of the positive electrode active material is not particularly limited. For example, one or two or more compounds conventionally used in lithium ion secondary batteries can be used. For example, particles of a lithium transition metal composite oxide having a layered crystal structure or a spinel crystal structure and a lithium transition metal compound of a polyanion type (e.g., olivine type) can be used. A typical example of a suitable cathode active material is a lithium transition metal composite oxide containing Li and at least one transition metal element (preferably at least one of Ni, Co, and Mn). For example, LiNiO _2, a composite oxide, or the positive electrode active material (particles) made of _{LiNi 1/3 Co 1/3} Mn 1/3 O 2 , such as a ternary positive electrode active material of (NCM lithium composite oxide) are used, such as LiCoO ₂ do. It may also contain at least one element selected from the group consisting of W, Zr, Nb, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F.

Although not particularly limited, it is preferable that the ratio of the positive electrode active material in the total solid content of the positive electrode active material layer is 70 to 97 mass% (for example, 75 to 95 mass%).

The positive electrode active material (particle) to be used may be a so-called hollow cathode active material (hollow particle) having a shell portion and a hollow portion formed therein, or may be a so-called solid positive electrode active material having no hollow portion ). The hollow cathode active material particles are preferable because they can more efficiently perform mass exchange (for example, migration of Li ions) with respect to the nonaqueous electrolyte, as compared with the positive electrode active material particles having a solid structure.

The average particle diameter (measured value based on the laser diffraction scattering method) of the cathode active material particles (secondary particles) is preferably 1 mu m or more and 25 mu m or less. With such a positive electrode active material particle having an average particle diameter, good battery performance can be exhibited more stably.

The positive electrode active material layer 14 can be prepared by mixing the above-described positive electrode active material (NCM lithium composite oxide or the like) with various additive materials (for example, a slurry-like composition prepared by adding a nonaqueous solvent, To the cathode current collector 12 at a predetermined thickness. The anode current collector 12 may be formed of a conductive material.

An example of the additive is a conductive material. As the conductive material, a carbon material such as carbon powder or carbon fiber is preferably used. As other additives, various polymer materials capable of functioning as a binder (binder) can be mentioned. For example, polymers such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC) can be preferably employed. Alternatively, styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene (PE), polyacrylic acid (PAA), or the like may be used.

Like the positive electrode sheet 10, the negative electrode sheet 20 has a structure in which a negative electrode active material layer 24 composed mainly of a negative electrode active material (graphite carbon material) is provided on both sides of a long electrode sheet negative electrode collector. However, the negative electrode active material layer 24 is provided at one lateral edge of the negative electrode sheet 20 in the width direction (i.e., one end in the winding axis direction opposite to the positive electrode active material layer non-forming portion 16) The negative electrode active material layer non-formed portion 26 is formed in which the negative electrode collector 22 is exposed at a constant width.

In the lithium ion secondary battery disclosed herein, a graphite based carbon material having a graphite structure (graphite structure) is used at least in part as the negative electrode active material. By having a graphite structure, it has a good function as a lithium ion storage (insertion) and releasable active material. As the graphite carbon material, various graphite materials such as natural graphite and artificial graphite may be molded into spherical or flake-like shapes. It is preferable to spheroidize.

Further, in order to improve the insertion efficiency of the lithium ion into the negative electrode active material (graphite carbon material), it is preferable to use an amorphous carbon-coated graphite carbon material having at least a part of the surface of the graphite carbon material Use is preferred. The amorphous carbon-coated graphite carbonaceous material is obtained by kneading grains of graphite carbonaceous material and a material capable of forming an amorphous carbon layer (for example, pitches such as petroleum pitch) into a high-temperature region (for example, Lt; 0 > C or less).

The bulk density of the negative electrode active material (graphite carbon material) to be used is suitably 0.5 g / cm3 or more and 0.7 g / cm3 or less. Such a bulk density is preferably 0.56 g / cm3 or more and 0.62 g / cm3 or less, more preferably 0.6 g / cm3 and the use of a black carbonaceous carbon material having a bulk density before and after (for example, 0.6 ± 0.1 g / desirable.

The BET specific surface area of the negative electrode active material (graphite carbon material) to be used is suitably from 2 m 2 / g to 6 m 2 / g. The BET specific surface area is preferably not less than 3 m 2 / g and not more than 5 m 2 / g, more preferably not less than 4 m 2 / g and the use of a black carbon material having a BET specific surface area before and after (for example, about 4 ± 0.1 m 2 / g) Is particularly preferable.

There is no particular limitation on an appropriate size (particle size of secondary particles) of the graphite carbon material used as the negative electrode active material. For example, the average particle size of the measured value based on the laser diffraction / scattering method is 1 탆 or more and 50 탆 (Typically, not less than 5 탆 and not more than 20 탆, preferably not less than 8 탆 and not more than 12 탆) can be preferably used.

Although not particularly limited, it is preferable that the ratio of the negative electrode active material to the total solid content of the negative electrode active material layer is 85 to 98 mass% (for example, 90 to 95 mass%).

The negative electrode active material layer may contain an additive such as a conductive material, a binder, and a boehmite material, if necessary, in addition to the negative electrode active material (particles) made of a graphite carbon material. The conductive material will be described later. As the binder, those which can be contained in the above-mentioned positive electrode active material layer can be preferably used. Carboxymethylcellulose (CMC) or methylcellulose (MC) can be suitably used as the thickening material.

In the lithium ion secondary battery disclosed herein, a conductive carbon material composed of conductive amorphous carbon is contained as a conductive material in combination with a negative electrode active material composed of a graphite carbon material in the negative electrode active material layer. Here, the amorphous carbon is a generic name of carbon which does not exhibit a clear crystal state, and includes coal, charcoal, soot, and the like. Amorphous carbon is a typical example of amorphous carbon.

As the conductive carbon material, various carbon materials composed of amorphous carbon such as soot can be employed. However, from the viewpoint of easy adjustment of conductivity, fine shape (particle diameter, structure and the like), various carbon blacks are suitable for use. For example, carbon black excellent in conductivity such as furnace black and acetylene black can be suitably used as the conductive carbon material in the practice of the present invention.

The bulk density of the conductive carbon material to be used is not more than 0.4 g / cm 3 (for example, not less than 0.05 g / cm 3 and not more than 0.4 g / cm 3) and less than the bulk density of the negative electrode active material (graphite carbonaceous material) small. As a result, the void volume in the negative electrode active material layer can be relatively secured. The bulk density is preferably 0.3 g / cm 3 or less (for example, 0.05 g / cm 3 or more and 0.3 g / cm 3 or less), more preferably 0.2 g / cm 3 or less (for example, 0.05 g / desirable.

The BET specific surface area of the conductive carbon material to be used is suitably not more than 50 m 2 / g (for example, not less than 14 m 2 / g and not more than 50 m 2 / g). This suppresses the occurrence of side reactions during high rate charging and discharging (for example, decomposition reaction of the electrolyte solution in the anode active material layer at the time of charging), thereby further improving the high rate cycle characteristics The BET specific surface area is preferably 35 m 2 / g or less (for example, 14 m 2 / g or more and 35 m 2 / g or less), more preferably 20 m 2 / g or less Or less (for example, 14 m2 / g or more and 20 m2 / g or less).

The preferable size of the conductive carbon material used as the conductive material (the particle diameter of the primary particles) is not particularly limited. For example, the average particle diameter of the primary particles based on the electron microscope (TEM or SEM) Carbon black of about 500 nm or less (typically 20 nm or more and 200 nm or less) can be preferably used.

Although not particularly limited, it is preferable that the content ratio of the conductive carbon material in the negative electrode active material layer is 2 parts by mass or more and 10 parts by mass or less in 100 parts by mass of the entire solid content of the negative electrode active material layer. According to this mixing ratio, suppression of unevenness of the lithium salt concentration and suppression of reduction of the battery capacity (discharge capacity) due to the addition of the conductive carbon material to the negative electrode active material layer can be balanced with good balance. Particularly, when the content of the conductive carbon material in the anode active material layer is 100 parts by mass as a whole, the content of the conductive carbon material is 5 parts by mass or more and 10 parts by mass or less (for example, 5 parts by mass or more and 7 parts by mass or less, or 7 parts by mass or more and 10 parts by mass Or less).

When carbon black is selected as the conductive carbon material, it is preferable that the conductive carbon material is a furnace black, and the bulk density of the furnace black is 0.02 g / cm 3 or more and 0.04 g / cm 3 or less.

In the case of using the furnace black having such a bulk density, the mass ratio of the negative electrode active material to the furnace black (the cathode ratio of the negative electrode active material to the negative electrode active material) Active material / furnace black) is preferably in the range of 96.6 / 2 to 88.6 / 10, more preferably in the range of 95.6 / 3 to 93.6 / 5.

The negative electrode active material layer 24 is formed by mixing a composition prepared by mixing the above-described negative electrode active material (graphite carbonaceous material) and a conductive material (conductive carbonaceous material) together with various other additives (for example, by adding an aqueous solvent or a non- And then adhering the prepared slurry composition to the negative electrode collector at a predetermined thickness.

An example of the additive is a binder. For example, the same materials as those contained in the above-described positive electrode active material layer 14 can be used. As other additives, a thickening material, a dispersing material and the like may be suitably used. For example, carboxymethylcellulose (CMC) or methylcellulose (MC) can be suitably used as the thickening material.

The separator 40 which is laminated with the positive electrode sheet 10 on which the positive electrode active material layer 14 is formed and the negative electrode sheet 20 on which the negative electrode active material layer 24 is formed are stacked on the positive electrode sheet 10 and the negative electrode sheet 20).

Typically, the separator 40 is formed of a sheet material having a predetermined width and a plurality of small holes. As the separator 40, for example, a separator having a single-layer structure made of a porous polyolefin-based resin or a separator having a laminated structure can be used. Further, a layer of insulating particles may be further formed on the surface of the sheet member made of such a resin. Examples of the particles having insulating properties include inorganic fillers having insulating properties (for example, fillers such as metal oxides and metal hydroxides) or resin particles having insulating properties (for example, particles such as polyethylene and polypropylene) .

The positive electrode active material layer non-forming portion 16 of the positive electrode sheet 10 and the negative electrode active material layer non-forming portion 26 of the negative electrode sheet 20 are respectively projected from both sides in the width direction of the separator sheet 40, The sheet 10 and the negative electrode sheet 20 are superimposed in a slightly shifted manner in the width direction and wound in the sheet longitudinal direction. As a result, in the transverse direction with respect to the winding direction of the wound electrode body 80, the active material layer non-formed portions 16 and 26 of the positive electrode sheet 10 and the negative electrode sheet 20 are wound on the wound core portion 10), the negative electrode active material layer forming portion of the negative electrode sheet 20, and the two separator sheets 40 are tightly wound). The positive electrode lead terminal 74 and the negative electrode lead terminal 76 are connected to the positive electrode side protruding portion 16 (i.e., the non-forming portion of the positive electrode active material layer) 16 and the negative electrode side protruding portion And is electrically connected to the positive electrode terminal 70 and the negative electrode terminal 72, respectively.

As the electrolytic solution (nonaqueous electrolytic solution), the same nonaqueous electrolytic solution conventionally used for a lithium ion secondary battery can be used without particular limitation. Such a nonaqueous electrolytic solution typically has a composition containing a supporting salt (lithium salt) in a suitable nonaqueous solvent.

Examples of the nonaqueous solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, Dioxolane, and the like can be used.

In addition, as the supporting salt, _{e.g., LiPF 6, LiBF 4, LiAsF} 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (CF 3 SO 2) 2, LiC (CF 3 SO 2) 3 , etc. Can be used. As an example, a nonaqueous electrolytic solution containing LiPF ₆ at a concentration of about 1 mol / L in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (for example, a volume ratio of 3: 4: 3) .

When the lithium ion secondary battery is assembled, the wound electrode body 80 is housed in the body 52 from the upper opening of the case body 52, and a suitable non-aqueous electrolyte is placed in the case body 52 (casting liquid). Thereafter, the opening is sealed by welding or the like with the lid 54 to complete assembly of the lithium ion secondary battery 100 according to the present embodiment. The sealing process of the case 50 and the process of disposing the electrolyte solution (liquid injection process) may be the same as those performed in the production of a conventional lithium ion secondary battery, and do not characterize the present invention. Thus, the construction of the lithium ion secondary battery 100 according to the present embodiment is completed.

Hereinafter, some test examples related to the present invention will be described, but it is not intended to limit the present invention to the test examples.

≪ Preparation of Lithium Ion Secondary Battery (Sample Battery for Evaluation)

≪ Example 1 >

The following materials were used to fabricate the lithium ion secondary battery of Example 1.

(1) Preparation of positive electrode sheet:

A mean particle size of the ternary positive electrode active material of about _{10㎛ (LiNi 1/3 Co 1} /3 Mn 1/3 O 2) and a conductive agent (carbon black) and a positive electrode active material with a binder (PVDF),: the conductive material: (Hereinafter referred to as " binder composition ") was mixed at a mass ratio of 90: 8: 2 and further mixed with N-methylpyrrolidone (NMP) such that the solid content became 56% Positive electrode mixture "). Subsequently, this positive electrode composite material was applied to both surfaces of a positive electrode current collector made of an aluminum foil sheet having a thickness of 15 mu m, followed by drying and pressing, whereby the total thickness was 65 mu m, Lt; RTI ID = 0.0 > mm < / RTI >

(2) Production of negative electrode sheet:

A sample obtained by adding and mixing an appropriate amount of pitch to 100 g of the graphite material is sintered in a high temperature region of 500 DEG C or more and 800 DEG C or less and is crushed and classified to obtain an average particle diameter of about 10 mu m, An amorphous carbon-coated black carbon material having a bulk density of 0.6 g / cm 3 and a BET specific surface area of 4 m 2 / g was prepared.

Carbon black (furnace black) having a bulk density of 0.05 g / cm 3 and a BET specific surface area adjusted to 14 m 2 / g and an average particle diameter (primary particle diameter) of 100 nm or less was used as the conductive material Prepared.

The mass ratio of the prepared negative electrode active material (graphite carbon material), the conductive material (conductive carbon material), the styrene butadiene rubber (SBR) as the binder and the carboxymethyl cellulose (CMC) The negative active material layer and the conductive material were first mixed by stirring, and then the binder, thickening material and water were added and mixed so that the binder material: thickening material = 93.6: 5.0: 0.7: 0.7, (Hereinafter referred to as " negative electrode mixture "). Subsequently, the negative electrode composite material was applied to both surfaces of a negative electrode current collector made of a copper foil sheet having a thickness of 14 占 퐉, followed by drying and pressing to form a total thickness of 150 占 퐉 and a width of 102 mm and a length of 3100 Mm long negative electrode active material layer. As is apparent from the above description, in this embodiment, the content ratio of the conductive carbon material in the negative electrode active material layer is 5 parts by mass of the total amount of the solid content of the negative electrode active material layer as 100 parts by mass.

(3) Construction of battery

(Here, alumina particles) were formed on one surface of a separator base material having a three-layer structure of polypropylene (PP) / polyethylene (PE) / polypropylene (PP) having a thickness of 20 占 퐉 and a pore diameter of 0.1 占 퐉. And a heat-resistant layer (HRL layer) having a thickness of 4 占 퐉 including a binder were formed on a separator sheet having a total thickness of 24 占 퐉.

The positive electrode sheet and the negative electrode sheet were laminated via two separator sheets and wound in an elliptical shape. A pressure of about 4 kN / cm 2 was applied to the side surface of the winding body at a room temperature with a flat plate or the like for about 2 minutes (Pressed), thereby producing a wound electrode body having a flat shape.

The positive electrode lead terminal and the negative electrode lead terminal were laid by ultrasonic welding means on the positive electrode active material layer non-forming portion and the negative electrode active material layer non-forming portion of the wound electrode body, respectively. Thereafter, this wound electrode body was housed in a box-shaped battery container together with the nonaqueous electrolyte solution, and the opening of the battery container was hermetically sealed. As the non-aqueous electrolyte, 41 g of a nonaqueous electrolyte solution containing LiPF ₆ as a supporting salt at a concentration of about 1 mol / liter was mixed in a mixed solvent containing EC, DMC and EMC at a volume ratio of 3: 4: 3.

The sealed square type lithium ion secondary battery constructed as described above was subjected to initial charging and discharging treatment by a conventional method to obtain a lithium ion secondary cell for evaluation (Example 1) having a rated capacity of 3.6 Ah.

≪ Examples 2 to 8 and Comparative Examples 1 and 2 >

As the conductive material (conductive carbon material), any one of carbon black (furnace black) having an average particle diameter (primary particle diameter) of 100 nm or less and adjusted to have a bulk density and a BET specific surface area, Lithium ion secondary batteries for evaluation (Examples 2 to 8 and Comparative Examples 1 and 2) were produced for each conductive material by the same materials and the same process as those in Example 1 except that one was used.

≪ Example 9 >

Carbon black (furnace black) having an average particle diameter (primary particle diameter) of 100 nm or less adjusted to a bulk density of 0.4 g / cm 3 and a BET specific surface area of 50 m 2 / g was used as a conductive material (conductive carbon material) , And an amorphous carbon-coated graphite carbon material having an average particle diameter of about 10 μm adjusted to a bulk density of 0.6 g / cm 3 and a BET specific surface area of 4.0 m 2 / g was used as a negative electrode active material (graphite carbon material) A lithium ion secondary cell for evaluation (Example 9) was produced by the same process and by the same process as in Example 1 except for the above.

≪ Examples 10 to 13 and Comparative Examples 3 to 6 >

Examples 9 and 10 were prepared in the same manner as in Example 9 except that any one of the amorphous carbon-coated graphite carbon materials having an average particle diameter of about 10 mu m adjusted to have a bulk density and a BET specific surface area shown in the corresponding column of Table 2 below was used as the negative electrode active material. The lithium ion secondary batteries for evaluation (Examples 10 to 13 and Comparative Examples 3 to 6) were produced for each of the negative electrode active materials by the same materials and the same process.

≪ Example 14 >

A negative electrode material was prepared in the same manner as in Example 9 except that the amount of the conductive carbon material in the negative electrode active material layer was adjusted so that the amount of the conductive carbon material added was 2 parts by mass in the 100 parts by mass of the total solid content of the negative electrode active material layer , And a lithium ion secondary cell for evaluation (Example 14) was produced by the same process.

≪ Example 15 >

A negative electrode material was prepared in the same manner as in Example 9 except that the content of the conductive carbon material in the negative electrode active material layer was adjusted to such an amount that the amount of the conductive carbon material added was 7 parts by mass in the 100 parts by mass of the entire solid material in the negative electrode active material layer , And a lithium ion secondary battery for evaluation (Example 15) was produced by the same process.

≪ Example 16 >

A negative electrode material was prepared in the same manner as in Example 9 except that the content of the conductive carbonaceous material in the negative electrode active material layer was adjusted so that the amount of the conductive carbonaceous material added was adjusted to 10 parts by mass in 100 parts by mass of the total solid content of the negative electrode active material layer , And a lithium ion secondary battery for evaluation (Example 16) was produced by the same process.

≪ Evaluation of high rate cycle characteristic (resistance increase rate) >

Each of the lithium ion secondary cells for evaluation (Examples 1 to 16 and Comparative Examples 1 to 6) prepared as described above was evaluated relative to the rate of increase in resistance after the high rate cycle test.

More specifically, under the temperature condition of 60 占 폚, each battery is charged with a constant current of 36A (corresponding to 10C) until the terminal voltage becomes 4.1 V, (Equivalent to 0.5 C) was repeated 1000 times in total.

After such a high-rate charging / discharging cycle treatment, the battery was charged under a temperature environment of 25 캜 and adjusted to a charged state of SOC of 60%. Thereafter, a pulse discharge was performed for 10 seconds at a current of 10 C, and the IV resistance value (m?) After 1000 cycles of high-rate charging and discharging of each lithium ion secondary battery for evaluation was obtained from the voltage drop amount after 10 seconds from the start of discharge . The IV resistance value (m?) After the same cycle in the reference lithium ion secondary battery set in advance as the target value is set as the reference IV resistance value = 1, and the IV resistance value of each evaluation lithium ion secondary battery is set to As a relative value. That is, the relative value of the IV resistance of each evaluation lithium ion secondary battery was calculated by the IV resistance measurement value (m?) / Reference IV resistance value (m?) After 1000 cycles. The results are shown in the corresponding columns of Tables 1 and 2.

As shown in Tables 1 and 2, a negative electrode active material (graphite carbonaceous material) having a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less, With respect to the lithium ion secondary batteries (Examples 1 to 16) including the negative electrode active material layer formed using the conductive material (conductive carbon material) having a bulk density of 0.4 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less , It was confirmed that the relative value of all the IV resistances was less than the reference IV resistance value of 1, and that the device had a high high rate cycle characteristic (durability). Specifically, it is as follows.

In Examples 1 to 3 shown in Table 1, the bulk density of the conductive material is particularly low, particularly 0.1 g / cm 3 or less. By combining this point with a low BET specific surface area of 50 m < 2 > / g or less, the durability was particularly high for high rate charge and discharge. Further, it can be said that the smaller the BET specific surface area, the lower side reaction in the negative electrode (for example, the decomposition reaction of the electrolyte in the negative electrode active material layer at the time of charging) was suppressed and the better results were obtained.

Further, in Examples 5 to 6, the BET specific surface area of the conductive material is particularly low as 20 m 2 / g or less. By combining this point with a low bulk density of 0.4 g / cm < 3 > or less, it showed good durability against high rate charge and discharge. Also, the smaller the bulk density, the lower the relative value of IV resistance.

Also in Examples 7 to 8, since the BET specific surface area of the conductive material is about 30 +/- 1 m < 2 > / g and the bulk density is as low as 0.2 to 0.21 g / cm < 3 >, good durability against high rate charge and discharge is exhibited.

On the other hand, Comparative Example 1 using a conductive material having a low bulk density but having a BET specific surface area of more than 50 m 2 / g and Comparative Example 2 using a conductive material having a low BET specific surface area but a bulk density exceeding 0.4 g / , It was confirmed that the relative value of the IV resistance exceeded the reference IV resistance value of 1 and the high rate cycle characteristic (durability) was lowered. In Comparative Example 1, the reason is that the specific surface area is too large and the side reaction is promoted. Further, in Comparative Example 2, the reason why the void volume of the negative electrode active material layer could not be secured is a factor.

In Examples 9 to 13 shown in Table 2, since both the bulk density and the BET specific surface area of the negative electrode active material were in an appropriate range, they showed good durability against high rate charge and discharge.

From the results of Example 9 and Examples 14 to 16, the content ratio of the conductive carbonaceous material in which the total solid content of the negative electrode active material layer was 100 parts by mass was within a range of 2 parts by mass to 10 parts by mass, The higher the content of the material, the better the durability.

On the other hand, in Comparative Examples 3 to 6, it was confirmed that the relative value of the IV resistance exceeded the reference IV resistance value of 1, and the high rate cycle characteristic (durability) was lowered. In Comparative Example 3, since the BET specific surface area of the negative electrode active material is too small, it is a factor that the reaction area is small. In Comparative Example 4, since the BET specific surface area of the negative electrode active material is too large, the side reaction is promoted. In Comparative Example 5, since the bulk density of the negative electrode active material is too small, the charge / discharge capacity is small, and it can be considered that the IV resistance when the large current is caused to flow due to the increase of the negative electrode load. In Comparative Example 6, the bulk density of the negative electrode active material was too large, and the void volume of the negative electrode active material layer was small. As a result, it can be considered that the IV resistance was increased due to diffusion inhibition of lithium ion and expansion of lithium salt concentration unevenness.

≪ Example 17 >

A furnace sheet was produced by using a furnace black having a bulk density of 0.02 g / cm 3 and a BET specific surface area adjusted to 13 m 2 / g as a conductive material (conductive carbon material), and a mass ratio of each component , A total amount of the negative electrode active material and the conductive material: binder: thickening material = 98.6: 0.7: 0.7 (negative electrode active material: conductive material (mass ratio) = 95: 5) , And the total thickness of the negative electrode sheet was adjusted to 75 탆, the same materials and the same process as in Example 1 were used to fabricate a lithium ion secondary battery for evaluation (Example 17).

≪ Examples 18 to 21 &

As the conductive material (conductive carbon material), the same materials and the same process as in Example 17 were used, except that any of the furnace blacks adjusted to have the bulk density and BET specific surface area shown in the corresponding column of Table 3 below was used. And lithium ion secondary batteries for evaluation (Examples 18 to 21) were prepared for each conductive material.

≪ Examples 22 to 29 >

As the negative electrode active material, any one of the amorphous carbon-coated graphite carbon materials (referred to as " graphite " in Table 4) adjusted to have the bulk density and the BET specific surface area shown in the corresponding column of Table 4 below, (Conductive carbon material), one of the furnace blacks adjusted so as to have a bulk density and a BET specific surface area respectively shown in the corresponding column of Table 4 below was used, and a 10 mu m-thick copper foil sheet was used as the anode current collector , And each of the evaluation lithium ion secondary batteries (Examples 22 to 29) was made of the same material and the same process as in Example 1 except that the total thickness of the negative electrode sheet was adjusted to 75 占 퐉.

≪ Examples 30 to 37 >

As the negative electrode active material, any one of amorphous carbon-coated graphite carbon materials (referred to as " graphite " in Table 5) adjusted so as to have a bulk density and a BET specific surface area shown in the corresponding columns of Table 5 below, (Conductive carbon material), and one of the furnace blacks adjusted to have a bulk density and a BET specific surface area shown in the corresponding column of Table 5 below was used, and the mixing ratio of the amorphous carbon-coated black carbon material and the furnace black Weight ratio) was changed to the ratio shown in Table 5, and a 10 mu m-thick copper foil sheet was used as the negative electrode collector, and the total thickness of the negative electrode sheet was adjusted to 75 mu m. Thereby preparing respective evaluation lithium ion secondary batteries (Examples 30 to 37).

≪ Evaluation of high rate cycle characteristic (resistance increase rate) >

Each of the evaluation lithium ion secondary cells (Examples 17 to 37) prepared as described above was subjected to relative evaluation of the rate of increase in resistance after the high rate cycle test.

Specifically, the IV resistance value (m?) Of each of the lithium ion secondary batteries for evaluation after 1000 cycles of charging and discharging at high rate was determined by the above-described method. The IV resistance value (m?) After the cycle in the reference lithium ion secondary battery previously set as the target value is set as the reference IV resistance value = 1, and the IV resistance value of each evaluation lithium ion secondary battery is set as a relative value Respectively. The results are shown in the corresponding columns of Tables 3 to 5. The target value (reference IV resistance value) in Table 3 adopts a value 40% lower than the target value (reference IV resistance value) in Tables 1 and 2. In addition, the target value (reference Ⅳ resistance value) in Table 4 adopts a value 40% lower than the target value (reference Ⅳ resistance value) in Tables 1 and 2. In addition, the target value (reference IV resistance value) in Table 5 adopts a value 40% lower than the target value (reference IV resistance value) in Tables 1 and 2.

As shown in Tables 3 to 5, the negative electrode active material (graphite carbon material) having a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less, Among the lithium ion secondary batteries having a negative active material layer formed using a conductive material (conductive carbon material) having a density of 0.4 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less, a conductive carbon material (carbon black) In the lithium ion secondary batteries (Examples 17 to 37) in which the volume density of the furnace black is 0.02 g / cm3 or more and 0.04 g / cm3 or less in the furnace black, the relative value of the IV resistance is particularly low, (Durability). Specifically, it is as follows.

From Examples 17 to 21 shown in Table 3, particularly high high-rate cycle characteristics (durability) can be obtained by using a furnace black having a bulk density of 0.02 g / cm 3 or more and 0.04 g / cm 3 or less and a BET specific surface area of 50 m 2 / Is obtained. As can be seen from the comparison between Examples 17 and 18 and Examples 20 and 21, the lower the bulk density, the higher the high-rate cycle characteristics. This is considered to be because, if the bulk density is small, the public volume of the negative electrode can be sufficiently secured, and the unevenness of the salt concentration can be further suppressed. In addition, from the comparison of Examples 18 to 20, the lower the BET specific surface area, the higher the high-rate cycle characteristics were. This can be attributed to the fact that the side reaction is more suppressed.

(Negative electrode active material) having a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less, It can be seen that particularly high-rate cycle characteristics (durability) are obtained by using a furnace black having a bulk density of 0.02 g / cm 3 or more and 0.04 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less. Also in Table 4, the lower the bulk density of the furnace black, the higher the high-rate cycle characteristics. Further, the lower the BET specific surface area of the furnace black, the higher the high-rate cycle characteristics were.

(Negative electrode active material) having a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less, It can be seen that particularly high-rate cycle characteristics (durability) are obtained by using a furnace black having a bulk density of 0.02 g / cm 3 or more and 0.04 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less. Also in Table 5, it was seen that the high-rate cycle characteristics were more likely to be improved when the volume density of the furnace black was small and the BET specific surface area of the furnace black was small. As the amount of furnace black increased, the high rate cycle characteristics tended to be more improved. It can be considered that the larger the amount of the furnace black is, the larger the vacancy volume is, and the more easily the unevenness of the salt concentration is suppressed.

While the present invention has been described in detail, the foregoing embodiments are merely illustrative, and the invention disclosed herein includes various modifications and variations of the above-described specific examples. The lithium ion secondary battery disclosed herein can be suitably used as a power source for driving a vehicle, for example, in that it exhibits excellent high-rate cycle characteristics as described above.

10: anode
12: anode current collector
14: cathode active material layer
16: Cathode active material layer non-
20: cathode
22: cathode collector
24: anode active material layer
26: Negative electrode active material layer non-
40: Separator
50: Case
52:
54:
70: positive terminal
72: cathode terminal
80: wound electrode body
100: Lithium ion secondary battery

Claims (5)

An electrode body having a positive electrode having a positive electrode active material layer on the positive electrode collector and a negative electrode having a negative electrode active material layer on the negative electrode collector,
A lithium ion secondary battery comprising a non-aqueous electrolyte,
The negative active material layer
A negative electrode active material having at least a part of graphite structure and at least a part of which is coated with amorphous carbon and a conductive carbon material composed of conductive amorphous carbon as a carbon material different from the carbon black carbon material However,
Wherein the negative electrode active material has a bulk density of 0.5 g / cm 3 or more and 0.7 g / cm 3 or less and a BET specific surface area of 2 m 2 / g or more and 6 m 2 / g or less,
Wherein the conductive carbon material has a bulk density of 0.4 g / cm 3 or less and a BET specific surface area of 50 m 2 / g or less. The method according to claim 1,
Wherein a content ratio of the conductive carbon material in the negative electrode active material layer is not less than 2 parts by mass and not more than 10 parts by mass based on 100 parts by mass of the total solid content of the negative electrode active material layer. 3. The method according to claim 1 or 2,
Wherein the conductive carbon material is at least one kind of carbon black. 3. The method according to claim 1 or 2,
Wherein the conductive carbon material is furnace black,
Wherein the furnace black has a bulk density of 0.02 g / cm 3 or more and 0.04 g / cm 3 or less. 3. The method according to claim 1 or 2,
The positive electrode collector and the negative collector are each a long sheet-like positive electrode sheet and a negative electrode sheet,
Wherein the electrode body is a wound electrode chain in which the positive electrode sheet and the negative electrode sheet are superposed and wound with a separator interposed therebetween.

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