Lithium-ion secondary battery

Abstract

A lithium-ion secondary battery (100) comprising a positive electrode (10), a negative electrode (20) and a non-aqueous electrolyte solution, wherein the positive electrode (10) includes a ternary positive electrode active material (14) formed from a lithium transition metal composite oxide comprising at least nickel (Ni), cobalt (Co) and manganese (Mn), and wherein the negative electrode (20) includes a carbon-based negative electrode active material (24) containing carbon soot, which is formed from a carbon material having a graphite structure in at least a part thereof and which has carbon soot (CB) adhering to at least a part of a surface portion, wherein a molar fraction x of nickel (Ni) calculated by setting a total molar amount of nickel (Ni), cobalt (Co) and manganese (Mn) in the ternary positive electrode active material (14) to 100, which satisfies the following condition: 36 ≤ x ≤ 42 , and a mass fraction α of carbon soot (CB) calculated by setting a total mass of the carbon material and carbon soot in the carbon-based negative electrode active material (24) containing carbon soot to 100, which satisfies the following condition: 0.3 ≤ α ≤ 3.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a lithium-ion secondary battery, and more particularly to a positive electrode material and a negative electrode material for constituting a lithium-ion secondary battery. 2. Description of the related art

[0002] So-called lithium-ion secondary batteries, a type of non-aqueous electrolyte secondary battery in which lithium ions act as charge carriers, are widely used as portable power sources for personal computers, portable devices, and the like, as well as as drive power sources for vehicles. Because lithium-ion secondary batteries are lightweight and offer high energy density, their popularity is expected to increase in the future, particularly as high-output drive power sources for vehicle engines.

[0003] Low-temperature characteristics are one of the properties required for lithium-ion secondary batteries intended for use as vehicle power sources. Specifically, the batteries must be durable, inhibiting battery capacity deterioration and maintaining the desired battery capacity even during repeated charging and discharging in a sub-freezing temperature range (e.g., -10°C or lower). Furthermore, unlike lithium-ion secondary batteries intended for use in consumer applications, lithium-ion secondary batteries intended for use as vehicle power sources must exhibit excellent high-current characteristics (rapid charge / discharge characteristics), enabling high-current charging and discharging over a short period of time.Therefore, improving the low-temperature characteristics and high-current characteristics will also be an important research topic in the field of lithium-ion secondary batteries used as power sources for vehicles.

[0004] The low-temperature characteristics and high-current characteristics of lithium-ion secondary batteries can be improved by modifying the properties, structure, and composition of a positive electrode active material and / or negative electrode material used in the batteries.

[0005] For example, Japanese Patent Application Publication JP 2013-258392 A discloses “an electrode active material formed of a carbon material, characterized in that a diameter of fine pores is 50 nm to 400 nm and a micropore volume is 0.05 cc / g to 0.40 cc / g,” wherein the electrode active material has a function of improving low-temperature characteristics.Furthermore, Japanese Patent Application Publication JP 2008-027664 A discloses “a negative electrode active material for a lithium-ion secondary battery formed of substantially spherical graphite particles having fine protrusions on the surface, the active material being prepared by impregnating and coating a base material of spherical natural graphite with a mixture of pitch and carbon black and then calcining at 900°C to 1500°C” as a negative electrode active material characterized by excellent cycling characteristics (endurance) and high current characteristics.

[0006] Furthermore, US 2014 / 0 134 492 A1 discloses a secondary cell with non-aqueous electrolyte and a manufacturing method thereof according to the prior art. SUMMARY OF THE INVENTION

[0007] By using the active materials disclosed in the above-mentioned patent literature, it is apparently possible to achieve some improvement in battery properties such as a low-temperature characteristic of a lithium-ion secondary battery, but there is still room for improvement.

[0008] The present invention was made to further improve battery characteristics in lithium-ion secondary batteries to be used primarily as driving power sources for vehicles based on different contents and a different approach than those disclosed in the above-mentioned patent literature, and an object of the present invention is to provide a lithium-ion secondary battery in which, in particular, a low-temperature characteristic is improved.

[0009] To achieve the object, the lithium-ion secondary battery provided by the present invention includes a positive electrode, a negative electrode and a non-aqueous electrolyte solution, wherein the positive electrode includes a ternary positive electrode active material formed from a lithium transition metal composite oxide containing at least nickel (Ni), cobalt (Co), and manganese (Mn); and the negative electrode includes a carbon-based negative electrode active material containing carbon soot, which is formed of a carbon material having a graphite structure in at least a part thereof and has carbon soot (CB) adhered to at least a part of a surface portion.

[0010] A molar fraction x of nickel (Ni), which is calculated by setting a total molar amount of nickel (Ni), cobalt (Co), and manganese (Mn) in the ternary positive electrode active material as 100 (that is, x can also be represented in mol% as (Ni / (Ni + Co + Mn) x 100)), satisfies the following condition: 36≤x≤42.

[0011] A mass fraction α of carbon soot (CB), which is calculated by setting a total mass of the carbon material and carbon soot in the carbon-based negative electrode active material containing carbon soot as 100 (that is, α can also be expressed in mass% as (CB / (carbon material + CB) x 100)), satisfies the following condition: 0.3≤α≤3.

[0012] The inventor has found that when a ternary positive electrode active material formed from the lithium-transition metal composite oxide (hereinafter also referred to as "NCM-lithium composite oxide"), which is known to have the so-called layered rock salt type crystal structure, is used as a positive electrode active material, and a carbon-based negative electrode active material formed from a carbon material having a graphite structure in at least a part thereof (hereinafter also referred to as "graphite-based carbon material") and having carbon soot (CB) adhered to at least a part of a surface portion is used as a negative electrode active material, the low-temperature characteristic of a lithium-ion secondary battery can be advantageously improved by setting x and α within predetermined ranges.This finding led to the creation of the present invention.

[0013] Thus, with the configuration of the lithium-ion secondary battery disclosed herein, it is possible to improve the low-temperature characteristic, that is, to improve a capacity retention ratio when the battery is used in a low-temperature environment such that charging and discharging are repeated at a temperature equal to or lower than 0°C (for example, within a low-temperature range of -10°C to -20°C).

[0014] With such features, in addition to improving the low-temperature characteristic, it is also possible to improve the high-current characteristic, for example, to reduce the increase ratio of the internal resistance during repeated high-current charging - for example, at a value of (inclusive) 10C to (inclusive) 30C in a normal temperature range (10°C to 35°C, for example, about 25°C).

[0015] In another preferred aspect of the lithium-ion secondary battery disclosed herein, the NCM lithium composite oxide is a compound represented by the following formula: Li 1+a (No x Co y Mn z ) 1-γ M γ O2 (where 0 ≤ a ≤ 0.14, x + y + z = 1, 0.36 ≤ x ≤ 0.42, 0.99 ≤ y / z ≤ 1.01, 0 ≤ γ ≤ 0.05, and M is at least one element selected from the group consisting of Zr, W, Nb, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F).

[0016] As a result of using the NCM-lithium composite oxide having a composition in which the Ni content is larger than that of Co and Mn (Ni-rich composition) and the content of Co and the content of Mn are substantially equal to each other as indicated by the above formula, the low-temperature characteristic and the high-current characteristic can be improved more advantageously. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 schematically illustrates the internal configuration of the lithium-ion secondary battery according to an embodiment; Fig. 2 is a graph illustrating the results obtained when comparing the capacity retention ratio after a charge-discharge cycle test (300 cycles) in a low-temperature environment at -10°C between a plurality of sample batteries differing in the amount of Ni in the positive electrode active material and the amount of acetylene black adhered in the negative electrode active material; and Fig.3 is a graph illustrating the results obtained when comparing the resistance increase ratio after a charge-discharge cycle test (4000 cycles) involving high-current charging in a normal temperature environment at 25°C between a plurality of sample batteries differing in the amount of Ni in the positive electrode active material and the adhering amount of acetylene black in the negative electrode active material. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The preferred embodiment of the lithium-ion secondary battery disclosed herein will be explained below. It should be noted that matters necessary for carrying out the present invention, except those specifically referred to in the description, are understood by those skilled in the art as design matters based on the related art in the relevant field. The present invention can be implemented based on the contents disclosed in the present specification and common general knowledge in the relevant field. The preferred embodiment of the present invention will be explained below by considering a lithium-ion secondary battery in which a flat wound electrode body and a non-aqueous electrolyte solution are housed in a casing of a corresponding flat shape (box-like shape).

[0018] As in Fig.As shown in FIG. 1, the lithium-ion secondary battery 100 according to the present embodiment includes a casing 50 made of a metal (a resin or composite film may also be advantageously used). The casing (outer casing) 50 includes a casing main body 52 of a flat rectangular parallelepiped shape with an open top end, and a lid 54 that closes the opening.

[0019] A positive electrode terminal 70 electrically connected to a positive electrode 10 of a wound electrode body 80 and a negative electrode terminal 72 electrically connected to a negative electrode 20 are provided on the upper surface (i.e., the lid 54) of the case 50. A flat-shaped wound electrode body 80 obtained by laminating the positive electrode (positive electrode sheet) 10 of an elongated sheet shape, the negative electrode (negative electrode sheet) 20 of an elongated sheet shape, and a total of two separators (separator sheets) 40 of an elongated sheet shape and winding the laminate, as well as the non-aqueous electrolyte solution, are housed within the case 50.

[0020] A gas release mechanism such as a safety valve for releasing the gas generated inside the casing 50 from the casing 50 is provided on the lid 54 in the same manner as in the conventional lithium-ion secondary batteries of this type, but since this mechanism is not a concrete feature of the present invention, its explanation and illustration in the drawings are omitted herein.

[0021] In the positive electrode sheet 10, a positive electrode active material layer 14 including a positive electrode active material (NCM-lithium composite oxide) as the main component is provided on both surfaces of a positive electrode collector 12 of an elongated sheet shape. However, the positive electrode active material layer 14 is not provided on a side edge (end portion on one side in the winding axis direction) in the width direction perpendicular to the longitudinal direction of the positive electrode sheet 10, and thereby a positive electrode active material layer non-formation portion 16 is provided in which the positive electrode collector 12 is exposed over a predetermined width.

[0022] In the lithium-ion secondary battery disclosed herein, the positive electrode active material is formed of the above-described ternary positive electrode active material, that is, an NCM-lithium composite oxide.

[0023] Specifically, an NCM-lithium composite oxide prepared such that a molar fraction x of Ni calculated by taking a total molar amount of Ni, Co, and Mn as 100 is 36 ≤ x ≤ 42 (that is, the molar fraction of Ni is 36 mol% (inclusive) to 42 mol% (inclusive) when the total amount of Ni, Co, and Mn is taken as 100 mol%) is used. By using the positive electrode active material formed from such a Ni-rich NCM-lithium composite oxide, it is possible to improve the low-temperature characteristics of the lithium-ion secondary battery.

[0024] By using the positive electrode active material formed of the NCM-lithium composite oxide prepared such that the Ni molar content is x 36 ≤ x ≤ 42 (that is, the Ni molar content is 36 mol% (inclusive) to 42 mol% (inclusive)), it is possible to improve not only the low-temperature characteristic but also the high-current characteristic.

[0025] The preferred example of such a Ni-rich NCM-lithium composite oxide is a compound represented by the following formula: Li 1+a (No x Co y Mn z ) 1-γ M γ O2

[0026] Where a, x, y, z and γ in the formula are numerical values that satisfy the following conditions. 0≤a≤0.14, x+y+z=1, 0.36≤x≤0.42, 0.99≤y / z≤1.01 (more preferred y=z, i.e. y / z=1), and 0≤γ≤0.05.

[0027] The element [M] in the formula is 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. For example, it is preferable that W and / or Zr are included. The content of M is not particularly limited provided that the functions of the positive electrode active material of the lithium-ion secondary battery are not hindered, and the content of M is suitably 5 mol% or less, typically 2 mol% or less, for example, 0.01 mol% (inclusive) to 2 mol% (inclusive), and preferably 0.05 mol% (inclusive) to 1 mol% (inclusive) when the total amount of Ni, Co, Mn, and M is set at 100 mol%.

[0028] By including Zr in such an amount, the reduction in peel strength between the positive electrode active material layer and the positive electrode collector caused by migration of a binder in the positive electrode active material layer can be avoided.

[0029] Furthermore, W is preferably included in such an amount because the reaction resistance of the battery can be further reduced.

[0030] As a result of using the Ni-rich NCM-lithium composite oxide in which y and z in the formula satisfy the condition of 0.99 ≤ y / z ≤ 1.01, that is, the molar fraction of Co and Mn is substantially the same, the low-temperature characteristic and the high-current characteristic can be further advantageously improved.

[0031] A positive electrode active material formed from such a Ni-rich NCM-lithium composite oxide can be manufactured by the conventional method. For example, the manufacturing method may include the steps of preparing an aqueous solution containing a nickel salt, a cobalt salt, and a manganese salt (examples of salts of such transition metals include sulfates, nitrates, and chlorides) in a predetermined molar ratio; neutralizing the aqueous solution by adding an aqueous basic solution (ammonia water or the like) while regulating the pH to precipitate an NCM composite hydroxide; mixing the NCM composite hydroxide with a lithium salt (e.g., lithium carbonate and lithium hydroxide); adding a compound of the desired element M (e.g., zirconium oxide and tungsten oxide), mixing, and calcining.

[0032] A positive electrode active material with a hollow structure (hollow particles) with a shell and a hollow portion formed therein, or a positive electrode active material with a solid structure (solid particles) without a hollow portion, is suitable as the positive electrode active material (particles) to be used. Positive electrode active material particles with a hollow structure are preferred because mass transfer with the non-aqueous electrolyte solution (e.g., lithium ion migration) can occur more effectively than with positive electrode active material particles with a solid structure.

[0033] By pulverizing, grinding, sieving, and classifying the ternary positive electrode active material formed from the Ni-rich NCM-lithium composite oxide obtained in the above-described manner, its particle size can be adjusted to the desired particle size if necessary.

[0034] The preferred average particle size of the positive electrode active material particles (secondary particles) disclosed herein is generally from 1 µm (inclusive) to 25 µm (inclusive). With the positive electrode active material particles having such an average particle size, good battery performance with improved durability can be achieved. In the preferred embodiment, the average particle size of the positive electrode active material particles is approximately from 3 µm (inclusive) to 10 µm (inclusive). The average particle size of the positive electrode active material particles can be determined by a method well known in the relevant field, for example, by measurements based on a laser diffraction and scattering method. The aforementioned average particle size is based on the measurements performed by the laser diffraction and scattering method.

[0035] The positive electrode active material layer 14 can be formed by mixing the above-described positive electrode active material (NCM-lithium composite oxide) with a variety of additives to prepare a composition, and applying the prepared composition (for example, a slurry-like composition prepared by adding a non-aqueous solvent or a granulated material obtained by granulating the positive electrode active material together with the additives) to the positive electrode collector 12 to obtain a predetermined thickness.

[0036] An electrically conductive material is one example of the additive. A carbon material, such as carbon powder and carbon fiber, is preferably used as the electrically conductive material. Examples of other additives include various polymer materials capable of acting as binders. For example, polymers such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC) can be advantageously used. Alternatively, styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyethylene (PE), and polyacrylic acid (PAA) can be used.

[0037] Similar to the positive electrode sheet 10, the negative electrode sheet 20 also has the configuration in which the negative electrode active material layer 24, which includes a negative electrode active material (carbon-based negative electrode active material coated with carbon black) as the main component, is provided on both surfaces of an elongated negative electrode collector. However, the negative electrode active material 24 is not provided on a side edge in the width direction of the negative electrode sheet 20 (that is, the end on one side in the direction of the winding axis, this end being opposite to that of the positive electrode active material layer non-formation portion 16), and thereby a negative electrode active material layer non-formation portion 26 is formed in which the negative electrode collector 22 is exposed over a predetermined width.

[0038] In the lithium-ion secondary battery disclosed herein, the above-described carbon-based negative electrode active material containing carbon soot is used as the negative electrode active material, that is, the carbon-based negative electrode active material containing carbon soot in which carbon soot (CB) is adhered to at least a part of the surface portion of the graphite-based carbon material having a graphite structure in at least a part thereof.

[0039] More specifically, the carbon-based negative electrode active material containing carbon soot is used, which is prepared such that the mass fraction α of CB is 0.3 ≤ α ≤ 3 when the total mass of the graphite-based carbon material and CB is taken as 100 (that is, the CB content is 0.3 mass% (inclusive) to 3 mass% (inclusive) when the total amount of the graphite-based carbon material and carbon soot (CB) is taken as 100 mass%).

[0040] As a result of using the carbon-based carbon black-coated negative electrode active material having such a CB content in combination with the positive electrode active material formed from the Ni-rich NCM-lithium composite oxide described above, the low-temperature characteristic of the lithium-ion secondary battery can be further improved.

[0041] As a result of the use of the carbon-based carbon black-containing negative electrode active material prepared such that the mass fraction α of CB is 0.3 ≤ α ≤ 3 (that is, the CB content is 0.3 mass% (inclusive) to 3 mass% (inclusive)) in combination with the positive electrode active material formed of the Ni-rich NCM-lithium composite oxide described above, it is possible to improve not only the low-temperature characteristics but also the high-current characteristics.

[0042] Various graphite materials, such as natural graphite and artificial graphite formed into spheres or flakes, can be used as the graphite-based carbon material, which is advantageous for fabricating the carbon-based carbon black-containing negative electrode active material having such a CB content.

[0043] Alternatively, a graphite-based carbon material in which the surface of graphite particles is coated with amorphous carbon can be advantageously used.

[0044] A CB caused to adhere to at least a part of the surface portion (in the graphite-based carbon material having the amorphous carbon coating layer, the surface portion includes the amorphous carbon coating layer present on the surface of the graphite-based carbon material) of such a graphite-based carbon material is not limited to a particular type, and a typical carbon black such as acetylene black (AB), Ketjen black, and furnace black can be used without any limitation.

[0045] A method for causing the CB to adhere to the surface portion of the graphite-based carbon material is not particularly limited. For example, a manufacturing method can be used by kneading particles formed from the graphite-based carbon material, a material (pitch or the like) for forming the amorphous coating layer on the surface of the particles, and CB particles, and then calcining them in a high-temperature range (for example, 500°C (inclusive) to 1500°C (inclusive)).

[0046] Particles of the carbon-based negative electrode active material coated with carbon black obtained by calcination may be cooled and then milled, etc., as needed to adjust their particle size. A suitable binder may be incorporated into the mixture of carbon particles and CB particles to improve the adhesion between the CB particles and the graphite-based carbon particles during the process of supporting the CB particles on the surface of the graphite-based carbon particles.

[0047] The size of the carbon-based carbon-black-bearing negative electrode active material thus obtained is not particularly limited, but it is preferable to use a material having an average particle size of, for example, 1 µm (inclusive) to 50 µm (inclusive) (typically 5 µm (inclusive) to 20 µm (inclusive), preferably 8 µm (inclusive) to 12 µm (inclusive)) based on the laser diffraction and scattering method.

[0048] The negative electrode active material layer 24 can be formed by mixing the above-described negative electrode active material (carbon-based negative electrode active material containing carbon black) with a variety of additives to prepare a composition, and applying the prepared composition (for example, a slurry composition prepared by adding an aqueous solvent or a non-aqueous solvent, or a granulated material obtained by granulating the positive electrode active material together with the additives) to the negative electrode collector to obtain a predetermined thickness.

[0049] A binder is one example of the additive. For example, the binder of the same type as that contained in the above-described positive electrode active material layer 14 can be used. A thickener and a dispersant can be used as further additives if desired. For example, carboxymethylcellulose (CMC) or methylcellulose (MC) can be advantageously used as the thickener.

[0050] The separator 40, which is laminated together with the positive electrode sheet 10 on which the positive electrode active material layer 14 has been formed and with the negative electrode sheet 20 on which the negative electrode active material layer 24 has been formed, is a member that separates the positive electrode sheet 10 and the negative electrode sheet 20 from each other.

[0051] The separator 40 is typically formed from a strip-shaped sheet material of a predetermined width having a plurality of fine holes. For example, a separator of a single-layer structure or a separator of a multi-layer structure formed from a porous polyolefin resin may be used as the separator 40. A layer of electrically insulating particles may be further formed on the surface of the sheet material formed from such a resin. The electrically insulating particles may be in the form of an electrically insulating inorganic filler (for example, a filler formed from a metal oxide or metal hydroxide) or electrically insulating resin particles (for example, polyethylene or polypropylene particles).

[0052] During lamination, the positive electrode sheet 10 and the negative electrode sheet 20 are stacked with a slight offset in the width direction, so that the positive electrode active material layer non-formation portion 16 of the positive electrode sheet 10 and the negative electrode active material layer non-formation portion 26 of the negative electrode 20 protrude from both sides in the width direction of the separator 40. As a result, the active material layer non-formation portions 16, 26 of the positive electrode sheet 10 and the negative electrode sheet 20 protrude outward in the transverse direction relative to the winding direction of the wound electrode body 80 from the respective winding core portions (that is, portions where the positive electrode active material layer formation portion of the positive electrode sheet 10, the negative electrode active material layer formation portion of the negative electrode sheet 20, and the two separator sheets 40 are tightly wound).A positive electrode lead terminal 74 and a negative electrode lead terminal 76 are provided at the positive electrode side protruding portion (i.e., the positive electrode active material layer non-formation portion) 16 and the negative electrode side protruding portion (i.e., the negative electrode active material non-formation portion) 26, respectively, and are electrically connected to the positive electrode terminal 70 and the negative electrode terminal 72, respectively.

[0053] The same solutions as the non-aqueous electrolyte solutions conventionally used in lithium-ion secondary batteries can be used as the electrolyte solution (non-aqueous electrolyte solution) without any restrictions. Such a non-aqueous electrolyte solution typically has a composition including a carrier salt in a suitable non-aqueous solvent. Examples of the non-aqueous solvents include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 1,3-dioxolane. Such non-aqueous solvents can be used singly or in combinations of two or more thereof. Examples of suitable carrier salts include lithium salts such as LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiC4F9AO3, LiN(CF3SO2)2 and LiC(CF3SO2)3.For example, a non-aqueous electrolyte solution can be used in which LiPF6 is contained at a concentration of about 1 mol / l in a solvent mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (for example, in a volume ratio of 3:4:3).

[0054] During assembly of the lithium-ion secondary battery, the wound electrode body 80 is received into the interior of the case main body 52 through the opening at the top end of the main body 52, and the appropriate non-aqueous electrolytic solution is also arranged (poured) into the case main body 52. Then, the opening is sealed with the lid 54 by welding, etc., to complete the assembly of the lithium-ion secondary battery 100 of the present embodiment. The process of sealing the case 50 and the process of arranging (pouring) the electrolytic solution can be performed according to the conventional method for manufacturing a lithium-ion secondary battery and do not characterize the present invention. The construction of the lithium-ion secondary battery 100 according to the present embodiment is thus completed.

[0055] Several experimental examples relating to the present invention are explained below, but the present invention should not be limited by the experimental examples. <Fertigung von Lithium-Ionen-Sekundärbatterien (Probebatterien zur Evaluierung)>

[0056] First, ternary positive electrode active materials (NCM-lithium composite oxides) were fabricated that differed from each other in terms of Ni content. Thus, a total of 11 types of NCM-lithium composite oxides were fabricated, in which the Ni content (mol%) relative to the total amount of Ni, Co, and Mn was varied within a range of 30 mol% to 50 mol%. The specific fabrication process is described below.

[0057] The inside of a reaction vessel containing water heated to 40 °C was purged with nitrogen, and then an aqueous basic solution was prepared by adding the appropriate amounts of a 3.25% aqueous sodium hydroxide solution and 25% ammonia water under a nitrogen stream and adjusting the pH to 12.0 at a liquid temperature of 25 °C and a liquid phase ammonia concentration of 20 g / L.

[0058] Then, an aqueous NCM solution was prepared by adjusting the mixing ratio of a nickel salt (in this case, NiSO4), a cobalt salt (in this case, CoSO4), and a manganese salt (in this case, MnSO4) to obtain the predetermined molar proportions of Ni, Co, and Mn (thus, such that the Ni content was within a range of 30 mol% to 50 mol%, and the molar content of Co and the molar content of Mn were equal to each other), and the mixture was then dissolved in water. Then, an NCM composite hydroxide was precipitated by adding the aqueous NCM solution to the aqueous basic solution while maintaining the pH at 12 and mixing. The desired NCM composite hydroxide was obtained by filtering off the precipitate, washing the alkali component, and drying.

[0059] Then, lithium carbonate (Li2CO3) was weighed such that the molar ratio (Li / T) of lithium to the total number of moles T of all transition metal elements (Ni, Co, Mn) in the NCM composite hydroxide was 1. Tungsten oxide was then weighed such that the amount of tungsten (W) in the positive electrode active material was 0.8 mass%, and the weighed components were evenly mixed with the heated hydroxide particles. Eleven types of W-containing NCM-lithium composite oxides (see Tables 1 and 2), which differed from each other in terms of Ni content and had an average particle size of about 10 µm, were then prepared by calcining the resulting mixture for 4 h at 760 °C in air and then calcining for 10 h at 950 °C, grinding, and classification.

[0060] The mass fractions of the positive electrode active material formed from the W-containing WCM-lithium composite oxide obtained in the above-described manner, an electrically conductive material (carbon black), and a binder (PVDF) were adjusted to positive electrode active material: electrically conductive material: binder = 90:8:2. A composition (positive electrode mixture) for forming the positive electrode active material layer was prepared by mixing the positive electrode active material, electrically conductive material, and binder with N-methylpyrrolidone (NMP). The positive electrode mixture was then applied to both surfaces of a positive electrode collector (aluminum foil with a thickness of 15 μm), dried, and pressed to prepare a positive electrode (positive electrode sheet) in which the positive electrode active material layer was coated with approximately 25 mg / cm 2per unit surface area on both surfaces of the positive electrode collector.

[0061] Meanwhile, a graphite material and pitch were prepared, and acetylene black (AB) with an average particle size of 100 nm or less was prepared as a carbon black (CB).

[0062] Then, a precalculated amount of AB and an appropriate amount of pitch were added to the prepared graphite material to obtain a total AB content of 0 wt% to 10 wt% per 100 g of the prepared graphite material, and the mixture was mixed to obtain samples (or a sample without the addition of AB). The samples were calcined at a high temperature range of 500 °C or higher (500 °C (inclusive) to 800 °C (inclusive)), ground, and classified to prepare 10 types of carbon-based negative electrode active materials with or without carbon black (see Tables 1 and 2) with an average particle size of approximately 10 µm, which differed from each other in terms of the CB (in this case, AB) content.

[0063] The mass fractions of the carbon-based negative electrode active material obtained in the above-described manner, a styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were adjusted to negative electrode active material:binder:thickener = 98:1:1. The components were dispersed in water to prepare a composition (negative electrode mixture) for forming a negative electrode active material layer. The negative electrode mixture was then applied to both surfaces of a negative electrode collector (copper foil with a thickness of 10 μm), dried, and pressed to prepare a negative electrode (negative electrode sheet) in which the negative electrode active material layer was coated with about 17 mg / cm 2 per unit surface area on both surfaces of a negative electrode collector.

[0064] A flat wound electrode body was manufactured by laminating the obtained positive electrode sheet and negative electrode sheet with two separator sheets (a sheet of a three-layer structure including polypropylene (PP) / polyethylene (PE) / polypropylene (PP) and having a thickness of 20 µm and a pore size of 0.1 µm was used), winding the laminate, and pressing and upsetting the wound body from the side surface direction.

[0065] The wound electrode body was housed in a box-shaped battery case along with a non-aqueous electrolyte solution, and the opening in the battery case was hermetically sealed. The non-aqueous electrolyte solution used was one prepared by incorporating LiPF6 as a carrier salt at a concentration of approximately 1 mol / L into a solvent mixture containing EC, DMC, and EMC in a volume ratio of 3:4:3.

[0066] Sample batteries (lithium-ion secondary batteries) for evaluation were obtained by performing the initial charge / discharge treatment (conditioning) of the lithium-ion secondary batteries constructed as described above using the usual method. <Evaluierung der Niedrigtemperaturcharakteristik (Kapazitätsretentionsverhältnis)>

[0067] A predetermined number (in this case, 300 cycles) of predetermined charge / discharge cycles were performed in a temperature environment of -10°C, and the capacity retention ratio of each sample battery after the cycles was measured.

[0068] First, each sample battery was set to a SOC of 60%. Charging for 10 s at a constant current of 25C, a rest interval of 10 min, discharging for 10 s at a constant current of 25C, and a rest interval of 10 min were considered one cycle. A total of 300 such charge / discharge cycles were performed, with the sample battery being set to a SOC of 60% after every 50 cycles. The capacity retention ratio (%) was determined using the following formula. Capacity retention ratio (%) = [(battery capacity after charge / discharge cycle test) / (initial battery capacity) × 100]

[0069] The results are shown in Table 1 and Fig. 2 shown. Table 1 AB quantity (%) Ni amount in positive electrode (%) 30 33 34 36 38 40 42 44 46 48 50 0 62,5 79,8 82,9 81,5 83,3 82,9 82,8 82,8 83,3 72,6 64,4 0,1 63,3 79,9 83,6 85,8 85,2 86,3 85,8 86,2 86,7 73,9 65,5 0,3 62,9 80,5 98,7 98,8 98,3 98,9 99 98,7 99,1 71,2 65,2 0,5 61,5 80,7 98,9 98,5 98,5 98,9 98,8 98,6 98,8 71,8 64,7 1 64,2 80,2 98,6 98,7 98,7 98,9 98,5 98,6 99,2 72,8 62,1 1,5 63,6 81 98,7 98,9 98,8 98,4 98,7 98,9 98,6 73,3 64,2 2 63,7 80,4 98,9 98,3 98,8 98,6 98,9 98,8 99 72,6 64,8 3 62,2 80,1 98,5 98,6 98,4 98,6 98,8 98,9 98,5 70,8 64,8 5 60,8 81,3 98,3 98,1 98,5 98,3 98,6 98,7 98,6 69,8 65,1 10 53,3 65,5 72,4 73,5 75,5 74,2 73,8 74,8 74,3 60 58,5

[0070] The data shown in Table 1 and Fig.The results shown in Figure 2 clearly indicate that the capacity retention ratio of the sample batteries using the ternary positive electrode active material formed from the NCM-lithium composite oxide with the Ni content of 34 mol% (inclusive) to 46 mol% (inclusive) was better than that of the sample batteries using the ternary positive electrode active material with the Ni content below or above this suitable range.

[0071] Furthermore, the sample batteries that used the carbon-based negative electrode active material coated with carbon black (in this case, AB) and acetylene black with an acetylene black content of 0.3 mass% (inclusive) to 5 mass% (inclusive) in addition to the ternary positive electrode active material formed from the NCM-lithium composite oxide with a Ni content of 34 mol% (inclusive) to 46 mol% (inclusive) showed a remarkably high capacity retention ratio. Thus, the capacity retention ratio after the charge / discharge cycle test was 95% or higher.These results indicate that by adjusting the Ni content in the ternary positive electrode active material formed from the NCM-lithium composite oxide and the CB content in the carbon-based carbon black-coated negative electrode active material to the ranges described above, a lithium-ion secondary battery exhibiting very good low-temperature characteristics (e.g., a very high capacity retention ratio) can be provided. <Evaluierung der Hochstromcharakteristik (Widerstandsanstiegsverhältnis)>

[0072] The resistance increase ratio after a high-current charging cycle test was investigated for each sample battery. Specifically, each sample battery was charged at a constant current of 1C to a terminal voltage of 3.75V under a temperature condition of 25°C, and then charged at a constant voltage for a total charging time of 120 minutes to adjust the SOC to 60%. Then, each sample battery was alternately charged and discharged every 10 seconds at current values of 1 / 3C, 1C, 2C, and 3C under the same temperature condition, and a voltage was measured 10 seconds after the start of discharge. The current value (x-axis) and voltage value (y-axis) at this time were subjected to linear regression, and the initial value R1 of the internal resistance (mΩ) of each sample battery was determined from the slope thereof.

[0073] After each sample battery was adjusted to a SOC of 60%, the batteries were charged for 10 s at a constant current of 30C, rested for 5 s, discharged for 200 s at a constant current of 3C, and rested for 145 s. This was considered one cycle, and 4000 cycles were performed. The process of readjusting the SOC of the sample batteries to 60% during the test was performed every 100 cycles.

[0074] After completing 4000 cycles, the internal resistance value R2 (mΩ) after low-temperature high-current cycling under a temperature condition of 25 °C was determined using the same method as for measuring the initial internal resistance value R1. The R2 / R1 ratio was used as the resistance increase ratio.

[0075] The results are shown in Table 2 and Fig. 3 shown. Table 2 AB quantity (%) Ni amount in positive electrode (%) 30 33 34 36 38 40 42 44 46 48 50 0 1,55 1,25 1,14 1,13 1,15 1,13 1,17 1,13 1,12 1,12 1,11 0,1 1,58 1,22 1,15 1,13 1,13 1,12 1,14 1,14 1,13 1,13 1,13 0,3 1,53 1,26 1,04 1,02 1,02 1,01 1,02 1,04 1,06 1,04 1,04 0,5 1,59 1,24 1,05 1,01 1,01 1,02 1,01 1,04 1,04 1,05 1,05 1 1,58 1,28 1,04 1,02 1,02 1,01 1,02 1,04 1,04 1,05 1,04 1,5 1,59 1,25 1,04 1,01 1,01 1,01 1,01 1,05 1,05 1,04 1,06 2 1,59 1,23 1,04 1,01 1,02 1,02 1,02 1,04 1,04 1,04 1,05 3 1,57 1,26 1,05 1,02 1,01 1,01 1,02 1,04 1,04 1,05 1,05 5 1,56 1,28 1,26 1,24 1,2 1,22 1,19 1,24 1,22 1,22 1,24 10 1,58 1,54 1,57 1,54 1,58 1,54 1,62 1,61 1,64 1,67 1,64

[0076] The data shown in Table 2 and Fig.The results shown in Figure 3 clearly indicate that for sample batteries using the ternary positive electrode active material formed from the NCM-lithium composite oxide with the Ni content of 36 mol% (inclusive) to 42 mol% (inclusive) and also the carbon-based negative electrode active material contaminated with carbon black (in this case, AB) with the acetylene black content of 0.3 mass% (inclusive) to 3 mass% (inclusive), the resistance increase rates after the high-current charging cycle test at 25°C were confirmed to be very low.Therefore, by adjusting the Ni content in the ternary positive electrode active material formed from the NCM-lithium composite oxide and the CB content in the carbon black-based negative electrode active material within the above-mentioned ranges, it is possible to provide a lithium-ion secondary battery exhibiting excellent high current characteristics in addition to the above-described good low-temperature characteristics (for example, capacity retention ratio).

[0077] While the present invention has been described in detail above, the embodiments thereof are merely illustrative. Since the lithium-ion secondary battery disclosed herein exhibits the above-described excellent low-temperature characteristics, it can be advantageously used as a driving power source for a motor (electric motor) mounted in a vehicle such as an automobile.

Claims

[1] A lithium-ion secondary battery (100) comprising a positive electrode (10), a negative electrode (20) and a non-aqueous electrolyte solution, wherein the positive electrode (10) includes a ternary positive electrode active material (14) formed from a lithium transition metal composite oxide comprising at least nickel (Ni), cobalt (Co) and manganese (Mn), and wherein the negative electrode (20) includes a carbon-based negative electrode active material (24) containing carbon soot, which is formed from a carbon material having a graphite structure in at least a part thereof and which has carbon soot (CB) adhering to at least a part of a surface portion, wherein a molar fraction x of nickel (Ni) calculated by setting a total molar amount of nickel (Ni), cobalt (Co) and manganese (Mn) in the ternary positive electrode active material (14) to 100, which satisfies the following condition: 36≤x≤42, and a mass fraction α of carbon soot (CB) calculated by setting a total mass of the carbon material and carbon soot in the carbon-based negative electrode active material (24) containing carbon soot to 100, which satisfies the following condition: 0.3≤α≤3. [2] Lithium-ion secondary battery (100) according to claim 1, wherein the lithium transition metal composite oxide is a compound represented by the following formula: Li 1+a (No x Co y Mn z ) 1-γ M γ O2 (where 0 ≤ a ≤ 0.14, x + y + z = 1, 0.36 < x ≤ 0.42, 0.99 ≤ y / z ≤ 1.01, 0 ≤ γ ≤ 0.05, and M is at least one element selected from the group consisting of Zr, W, Nb, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F).

Images