WO2013014830A1

WO2013014830A1 - Lithium-ion secondary cell

Info

Publication number: WO2013014830A1
Application number: PCT/JP2012/002666
Authority: WO
Inventors: 竜一夏井; 昌洋木下; 名倉　健祐
Original assignee: パナソニック株式会社
Priority date: 2011-07-26
Filing date: 2012-04-18
Publication date: 2013-01-31
Also published as: JP2014194842A

Abstract

A lithium-ion secondary cell provided with: a positive electrode including a positive electrode current collector and a positive electrode active material layer; and a negative electrode (22) including a negative electrode current collector (10) and a negative electrode active material layer (13). The positive electrode active material layer has a lithium-nickel composite oxide including a different element, and the different element is at least one different element selected from the group consisting of W, Nb, B, Zr, and V. The negative electrode active material layer (13) includes a first layer (11) and a second layer (12) sequentially formed on the surface of the negative electrode current collector (10). The first layer (11) includes a carbon material. The second layer (12) includes lithium titanate and an electroconductive material, and has a thickness equal to 1% to 25% of the thickness of the first layer (11). The amount of the electroconductive material is equal to or less than 3 parts by mass with respect to 100 parts by mass of lithium titanate.

Description

Lithium ion secondary battery

The present invention relates to a lithium ion secondary battery provided with a nonaqueous electrolyte.

The lithium ion secondary battery has a high capacity and a high energy density, and can be easily reduced in size and weight. For this reason, lithium ion secondary batteries are used as power sources for portable small electronic devices such as mobile phones, personal digital assistants (PDAs), notebook personal computers, video cameras, and portable game machines. .

A lithium ion secondary battery generally includes a positive electrode containing a lithium cobalt composite oxide as a positive electrode active material, a negative electrode containing a carbon material as a negative electrode active material, and a polyolefin porous membrane (separator). With the downsizing and weight reduction of equipment, there is an increasing demand for further increase in energy density of lithium ion secondary batteries. In the conventional battery design, the capacity is approaching the limit, and research for long life, high output, and high charge voltage of lithium ion secondary batteries has been widely conducted.

For example, Patent Document 1 discloses that the discharge capacity of a battery is improved by using a positive electrode active material containing a different element.

Patent Document 2 discloses a lithium ion secondary battery using lithium titanate as a negative electrode active material for the purpose of suppressing a decrease in battery capacity due to metal elution.

Patent Document 3 uses a negative electrode material in which a carbon material layer is provided on a negative electrode and a lithium titanate layer is provided on the surface for the purpose of obtaining a high-capacity and long-life lithium ion secondary battery. A lithium ion secondary battery is disclosed.

JP 2006-185794 A JP-A-6-275263 Special Table 2010-537389

However, when a positive electrode active material containing a different element is used in a lithium ion secondary battery, when the carbon material is used for the negative electrode, the different element contained in the positive electrode is eluted, and the different element metal is deposited on the negative electrode surface. This causes an internal short circuit and a problem that the cycle characteristics deteriorate.

On the other hand, when lithium titanate is used for the negative electrode, the redox potential (oxidation-reduction potential) of lithium titanate is high, so that a film is not sufficiently formed on the negative electrode surface, and the nonaqueous electrolyte decomposes during the charge / discharge cycle. As a result, gas is generated and cycle characteristics deteriorate. Furthermore, since lithium titanate has a low theoretical capacity, the capacity of the battery itself is reduced.

The present invention has been made in view of the above problems, and an object of the present invention is to improve battery capacity and cycle characteristics in a lithium ion secondary battery cycled at a high charge voltage.

Therefore, as a result of intensive studies, the present inventors have formed a carbon material layer on the negative electrode and a carbon material layer thereon when a lithium ion secondary battery using a positive electrode material containing a different element as the positive electrode is cycled at a high charge voltage. It has been found that the use of a structure in which a plurality of layers including a lithium titanate layer is used can reduce the redox potential of lithium titanate and improve the capacity and cycle characteristics of the lithium ion secondary battery.

Furthermore, by optimizing the thickness of the lithium titanate layer and the amount of conductive material, different elements can be deposited on the lithium titanate layer and decomposition of the nonaqueous electrolyte can be suppressed, resulting in cycle characteristics. Found to improve.

Specifically, a lithium ion secondary battery according to the present invention includes a positive electrode including a positive electrode current collector and a positive electrode active material layer, a negative electrode including a negative electrode current collector and a negative electrode active material layer, and a positive electrode and a negative electrode. A lithium ion secondary battery comprising a provided separator and a nonaqueous electrolyte, wherein the positive electrode active material layer has a lithium nickel composite oxide containing a different element, such as tungsten, niobium, It is at least one different element selected from the group consisting of boron, zirconium, and vanadium, and the negative electrode active material layer includes a first layer formed on the surface of the negative electrode current collector, and the first layer A second layer formed on the surface, the first layer includes a carbon material, the second layer includes lithium titanate and a conductive material, and has a thickness of the first layer. 1% or more and 25% or less in thickness, included in the second layer That the amount of the conductive material is less than 3 parts by mass with respect to lithium titanate of 100 parts by weight.

According to the lithium ion secondary battery according to the present invention, the dissimilar elements can be deposited in the second layer, and the decomposition of the nonaqueous electrolyte can be suppressed. As a result, even if the battery is charged at a high voltage, the capacity of the battery A lithium ion secondary battery having excellent cycle characteristics can be obtained.

The lithium ion secondary battery according to the present invention can suppress the decomposition of the non-aqueous electrolyte during charging and discharging even when charged at a high voltage, and can improve the capacity and cycle characteristics of the battery.

FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery according to an embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing a negative electrode of a lithium ion secondary battery according to an embodiment of the present invention.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. Moreover, it can change suitably in the range which does not deviate from the range which has the effect of this invention.

First, the lithium ion secondary battery according to this embodiment will be described with reference to FIG. FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery according to this embodiment.

As shown in FIG. 1, a lithium ion secondary battery 20 according to this embodiment includes a positive electrode 21, a negative electrode 22, and a microporous separator 23 interposed between the positive electrode 21 and the negative electrode 22. The electrode group 24 is provided. A positive-side insulating plate 25 is formed at one end of the electrode group 24 in the winding axis direction, and a negative-side insulating plate 26 is formed at the other end. The electrode group 24 is housed together with a nonaqueous electrolyte in a battery case 27 that also serves as a negative electrode terminal. The battery case 27 is sealed with a sealing plate 28. The battery case 27 is electrically connected to the negative electrode 22 via the negative electrode lead 29, and the positive terminal 30 of the sealing plate 28 is electrically connected to the positive electrode 21 via the positive electrode lead 31.

Next, the positive electrode of the lithium ion secondary battery according to one embodiment of the present invention will be described.

The positive electrode used in this embodiment includes a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector, and the positive electrode active material layer includes a different element.

As the positive electrode current collector of this embodiment, a positive electrode current collector that is usually used for a positive electrode of a lithium ion secondary battery can be used. For example, aluminum, an aluminum alloy, or the like can be used in the form of a foil, a film, a film, a sheet, or the like. The thickness of the positive electrode current collector is about 1 μm to 500 μm, and can be appropriately set according to the capacity and size of the lithium ion secondary battery.

The positive electrode active material layer was prepared, for example, by mixing a lithium nickel composite oxide containing different elements, a binder, a dispersant, and an additive such as a conductive material as necessary to prepare a positive electrode mixture slurry. Thereafter, the obtained positive electrode mixture slurry can be formed on the surface of the positive electrode current collector by drying and rolling.

The different element is a compound containing at least one different element selected from the group consisting of tungsten, niobium, boron, zirconium and vanadium, for example.

As the binder, a binder usually used for a lithium ion secondary battery can be used without any particular limitation. For example, polytetrafluoroethylene, polyvinylidene fluoride and modified products thereof, fluorinated polymers such as tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and rubbers such as styrene-butadiene rubber Polyolefin resins such as polyethylene and polypropylene are used. These can be used individually by 1 type and can also be used in combination of 2 or more type.

As the conductive material, a conductive material usually used for a lithium ion secondary battery can be used without any particular limitation. For example, carbon blacks such as graphite, acetylene black, ketjen black, furnace black, lamp black and thermal black, carbon fiber, and metal fiber are used.

Hereinafter, a method for introducing a different element into the positive electrode active material layer will be described.

As the first method, there is a method of attaching a different element to the surface of the lithium nickel composite oxide.

For example, first, an ethoxide compound containing a different element is dissolved in a solvent such as ethanol to prepare a solution containing the different element. Subsequently, the lithium nickel composite oxide is dispersed in the solution containing the different elements, and the solvent is completely removed by heating. Then, the lithium nickel composite oxide containing a different element is obtained by heat-treating the obtained mixture of the different element and the lithium nickel composite oxide. The heat treatment at this time is preferably performed at about 300 ° C. to 700 ° C. for about 12 hours to 24 hours.

As the second method, there is a method in which a raw material mixture of a lithium nickel composite oxide such as a lithium compound and a nickel compound and a heterogeneous compound are mixed and baked to add them internally.

As the raw material mixture of the lithium nickel composite oxide, various raw materials conventionally used in the production of lithium nickel composite oxides can be used without any particular limitation. Specifically, lithium hydroxide, lithium carbonate, or the like is used for the lithium compound, and nickel hydroxide, nickel oxide, or the like is used for the nickel compound. Various oxides, carbonate nitrates, hydroxides, and the like are used as the different element compound.

For the mixing of the raw material mixture and the different element compound, either dry mixing or wet mixing may be used, but dry mixing is preferable in terms of simplicity. As the mixing apparatus, a ball mill or the like can be used.

The resulting mixture of the raw material mixture and the heterogeneous element compound is baked in an air atmosphere to obtain a lithium nickel composite oxide in which the heterogeneous element is internally added. The firing at this time is preferably performed at about 600 ° C. to 900 ° C. for about 12 hours to 24 hours.

Next, the negative electrode of the lithium ion secondary battery according to one embodiment of the present invention will be described with reference to FIG. FIG. 2 is a cross-sectional view schematically showing the negative electrode of the lithium ion secondary battery according to this embodiment.

As shown in FIG. 2, the negative electrode used in the present embodiment includes a negative electrode current collector 10, a first layer 11 having a carbon material formed on the surface of the negative electrode current collector 10 as an active material, and And a negative electrode active material layer 13 formed of a second layer 12 using lithium titanate formed as an active material on the surface of the first layer 11.

A method for manufacturing such a negative electrode will be described below.

As a first method, first, a material mixture slurry is prepared by mixing a carbon material, a binder, a dispersion medium and, if necessary, an additive such as a conductive material to prepare a material mixture slurry. Is applied to the surface of the negative electrode current collector 10, dried and rolled. Thereby, the first layer 11 is formed. Thereafter, a lithium titanate slurry prepared by mixing lithium titanate, a binder, a dispersion medium, and an additive such as a conductive material is applied to the surface of the first layer 11, dried and rolled. Thereby, the second layer 12 is formed, and the negative electrode active material layer 13 composed of the first layer 11 and the second layer 12 is formed.

As the second method, first, a material mixture slurry obtained in the same manner as in the first method is applied to the surface of the negative electrode current collector 10. Thereafter, a lithium titanate slurry prepared in the same manner as in the first method is applied on the applied material mixture slurry, and two layers are simultaneously dried and rolled. As a result, the first layer 11 and the second layer 12 are formed on the surface of the negative electrode current collector 10, thereby forming the negative electrode active material layer 13 made of them.

As the negative electrode current collector 10, a current collector usually used for a negative electrode of a lithium ion secondary battery can be used without any particular limitation. Specifically, stainless steel, nickel, copper, or the like can be used in the form of a foil, a film, a film, a sheet, or the like. The thickness of the negative electrode current collector 10 is in the range of about 1 μm to 500 μm, and can be appropriately set according to the capacity and size of the lithium ion secondary battery.

As the carbon material, a carbon material capable of occluding and releasing lithium, which is usually used in a lithium ion secondary battery, can be used without any particular limitation. Specifically, carbon materials such as graphite and amorphous carbon can be used.

As the binder, those usually used for the negative electrode of a lithium ion secondary battery can be used without any particular limitation. Specifically, polyolefins such as polyethylene and polypropylene, styrene-butadiene rubber (SBR) and modified products thereof are used. These can be used individually by 1 type and can also be used in combination of 2 or more type.

As the conductive material, the same materials as those exemplified as the conductive material used for the positive electrode active material layer can be used.

Lithium titanate is obtained by mixing a lithium compound and a titanium compound and then firing, for example, lithium titanate having a chemical formula of Li _{3 + 3x} Ti _6-3x-y M _y O ₁₂ , where x And y represent a molar ratio, and 0 ≦ x ≦ 1/3 and 0 ≦ y ≦ 0.25. M includes at least one selected from the group consisting of Fe, Al, Ca, Co, B, Cr, Ni, Mg, Zr, Ga, V, Mn, and Zn. As the lithium compound, lithium hydroxide, lithium carbonate or the like can be used, and as the titanium compound, titanium oxide or the like can be used.

When the amount of the conductive material contained in the second layer 12 exceeds 3 parts by mass with respect to 100 parts by mass of the lithium titanate, the electrolytic solution cannot be sufficiently suppressed and the cycle characteristics tend to be deteriorated. Therefore, it is preferably 3 parts by mass or less, and more preferably 3 parts by mass.

When the thickness of the second layer 12 is less than 1% with respect to the thickness of the first layer 11, the total amount of the dissimilar elements contained in the positive electrode active material eluted and deposited on the negative electrode is lithium titanate. More than the amount that can be deposited on the second layer 12 it contains. For this reason, there exists a tendency for cycling characteristics to fall, without suppressing decomposition | disassembly of nonaqueous electrolyte. On the other hand, when the thickness of the second layer 12 exceeds 25% with respect to the thickness of the first layer 11, the influence of lithium titanate having a low theoretical capacity becomes remarkable, and the capacity of the battery decreases. Tend. Therefore, the thickness of the second layer 12 is preferably 1% or more and 25% or less with respect to the thickness of the first layer 11.

When the mixture density of the first layer 11 is less than 1.4 g / ml, the energy density per unit area tends to decrease. On the other hand, when the mixture density of the first layer 11 exceeds 2.8 g / ml, the penetration of the nonaqueous electrolyte into the first layer 11 becomes insufficient, and the battery characteristics tend to be deteriorated. For this reason, it is preferable that the mixture density of the 1st layer 11 is 1.4 g / ml or more and 2.8 g / ml or less.

Moreover, when the mixture density of the second layer 12 is less than 1.3 g / ml, the energy density per unit area tends to decrease. On the other hand, when the mixture density of the second layer 12 exceeds 1.8 g / ml, the penetration of the nonaqueous electrolyte into the second layer 12 becomes insufficient, and the battery characteristics tend to deteriorate. For this reason, it is preferable that the mixture density of the 2nd layer 12 is 1.3 g / ml or more and 1.8 g / ml or less.

When the BET specific surface area of lithium titanate is less than 2 m ² / g, the rate characteristics tend to be reduced. On the other hand, when the BET specific surface area exceeds 8 m ² / g, it is necessary to increase the addition amount of the conductive material and the binder. Yes, the energy density per unit area tends to decrease. Therefore, BET specific surface area of the lithium titanate is preferably less than ^2m 2 / g or more ^8m 2 / g.

In addition, when the average particle size of lithium titanate is less than 0.5 μm, the surface area of the active material is increased, so it is necessary to increase the addition amount of the conductive material and the binder, and the energy density tends to decrease, On the other hand, when the average particle size exceeds 10 μm, the rate characteristics tend to be lowered. For this reason, it is preferable that the average particle diameter of lithium titanate is 0.5 micrometer or more and 10 micrometers or less.

In addition, the cycle characteristics are improved by setting the oil absorption of lithium titanate to 20 g / 100 g or less. However, the rate characteristics are improved when the oil absorption is 50 g / 100 g or more. Therefore, the oil absorption of lithium titanate is 20 g / 100 g or more. It is preferable that it is 50 g / 100 g or less.

In this embodiment, the nonaqueous electrolyte includes a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent.

As the non-aqueous solvent, various non-aqueous solvents that are usually used for non-aqueous electrolytes of lithium ion secondary batteries can be used. Specifically, cyclic carbonate solvents such as ethylene carbonate, propylene carbonate and butylene carbonate, chain carbonate solvents such as dimethyl carbonate, ethylmethyl carbonate and diethyl carbonate, tetrahydrofuran, 1,4-dioxane and 1,3-dioxolane, etc. Cyclic ether solvents, chain ether solvents such as 1,2-dimethoxyethane and 1,2-diethoxyethane, cyclic ester solvents such as γ-butyrolactone, chain ester solvents such as methyl acetate, and fluoroethylene carbonate, fluoro A fluorinated solvent that is at least one selected from the group consisting of methyl propionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate can be used.

In particular, the nonaqueous electrolyte according to the present embodiment includes a fluorinated solvent that is at least one selected from the group consisting of fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate. Is preferred. In this way, a good film can be formed on the negative electrode surface.

In the present embodiment, the constituent elements such as the positive electrode 21, the negative electrode 22, and the separator other than the nonaqueous electrolyte described above are not particularly limited, and a configuration that is normally used as a lithium ion secondary battery can be used as appropriate. .

In the above description, a wound-type cylindrical battery is illustrated as a specific embodiment of the lithium ion secondary battery, but the shape of the lithium ion secondary battery is not limited to this. The lithium ion secondary battery can be appropriately selected not only in a cylindrical shape but also in a coin shape, a square shape, a sheet shape, a button shape, a flat shape, a stacked shape, and the like in accordance with its application.

In this example, a prismatic lithium ion secondary battery was manufactured according to the method described below, and an initial discharge capacity and cycle characteristic evaluation test was performed on the lithium ion secondary battery.

Example 1
(1) Production of Lithium Nickel Composite Oxide (Positive Electrode Active Material) Containing Different Elements First, tungsten ethoxide was dissolved in dehydrated ethanol. Lithium nickel composite oxide (LiNi _0.8 Co _0.16 Al _0.04 O ₂ : NCA) was dispersed in the obtained solution. Next, the solvent was completely removed by heating with stirring. The obtained mixture was baked at 300 ° C. for 12 hours in an air atmosphere to obtain a lithium nickel composite oxide (W—NCA) containing a tungsten compound as a positive electrode active material. The tungsten compound contained 0.5 mol% with respect to the lithium nickel composite oxide.

(2) Production of positive electrode 90 parts by mass of positive electrode active material particles obtained as described above, 3 parts by mass of a conductive material, and 87.5 parts by mass of a 2-methylpyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) ( A positive electrode mixture slurry was obtained by mixing a PVDF concentration of 8% by mass and a solid content of PVDF of 7 parts by mass. The obtained positive electrode mixture slurry was applied to both sides of a positive electrode current collector made of an aluminum foil having a thickness of 10 μm, dried and rolled to obtain a positive electrode having a positive electrode active material layer and a total thickness of 140 μm.

(3) Production of negative electrode 93.5 parts by mass of artificial graphite (C) powder, 12.5 parts by mass of modified styrene butadiene rubber (solid content is 40 parts by mass), and 1.5 parts by mass of sodium carboxymethylcellulose are mixed. As a result, an artificial graphite mixture slurry was obtained. Next, 90 parts by mass of lithium titanate (Li ₄ Ti ₅ O ₁₂ : LTO), 3 parts by mass of a conductive material, 50 parts by mass of an NMP solution of PVDF (the concentration of PVDF is 8 parts by mass, the solid content of PVDF is 4 parts by mass) was mixed to obtain a lithium titanate mixture slurry. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer and a total thickness of 180 μm.

(4) Preparation of nonaqueous electrolyte Fluoroethylene carbonate (FEC), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 1: 1: 6 to obtain a nonaqueous solvent. LiPF ₆ was dissolved in the non-aqueous solvent thus obtained so as to have a concentration of 1.0 mol / l to obtain a non-aqueous electrolyte (F / E / E).

(5) Production of lithium ion secondary battery Positive electrode, negative electrode, microporous separator interposed between positive electrode and negative electrode (composite film of polyethylene and polypropylene, product number “2300” manufactured by Celgard Co., Ltd., thickness An electrode group was obtained by winding a laminate composed of 25 μm). Further, a positive electrode lead made of aluminum was welded to a part of the positive electrode current collector, and a negative electrode lead made of nickel was welded to a part of the negative electrode current collector. Such an electrode group was accommodated in a battery case having a bottomed substantially cylindrical shape having a diameter of 18 mm and a height of 65 mm. Thereafter, 5.2 ml of nonaqueous electrolyte was injected into the battery case.

(Example 2)
In the same manner as in Example 1, an artificial graphite mixture slurry and a lithium titanate mixture slurry were obtained. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer and a total thickness of 190 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Example 3)
In the same manner as in Example 1, an artificial graphite mixture slurry and a lithium titanate mixture slurry were obtained. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer and a total thickness of 162 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Example 4)
In the same manner as in Example 1, an artificial graphite mixture slurry was obtained. Next, 90 parts by mass of lithium titanate, 2 parts by mass of a conductive material, and 100 parts by mass of PVDF NMP solution (the concentration of PVDF is 8% by mass, the solid content of PVDF is 8 parts by mass) are mixed. Thus, a lithium titanate mixture slurry was obtained. The artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer with a total thickness of 180 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Example 5)
In the same manner as in Example 1, an artificial graphite mixture slurry was obtained. Next, 90 parts by mass of lithium titanate, 1 part by mass of conductive material, and 112.5 parts by mass of PVDF NMP solution (the concentration of PVDF is 8% by mass and the solid content of PVDF is 9 parts by mass) are mixed. As a result, a lithium titanate mixture slurry was obtained. The artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer with a total thickness of 180 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Example 6)
First, niobium ethoxide was dissolved in dehydrated ethanol. Lithium nickel composite oxide (LiNi _0.8 Co _0.16 Al _0.04 O ₂ : NCA) was dispersed in the obtained solution. Next, the solvent was completely removed by heating the solution with stirring. The obtained mixture was baked at 300 ° C. for 12 hours in an air atmosphere, whereby a lithium nickel composite oxide (Nb—) which is a positive electrode active material containing 0.5 mol% of a niobium compound with respect to the lithium nickel composite oxide. NCA). A lithium ion secondary battery was produced in the same manner as in Example 1 except that the obtained positive electrode active material was used.

(Example 7)
First, boron ethoxide was dissolved in dehydrated ethanol. Lithium nickel composite oxide (LiNi _0.8 Co _0.16 Al _0.04 O ₂ : NCA) was dispersed in the obtained solution. Next, the solvent was completely removed by heating the solution with stirring. The obtained mixture was baked at 300 ° C. for 12 hours in an air atmosphere, whereby a lithium nickel composite oxide (B--) which is a positive electrode active material containing 0.5 mol% of a boron compound with respect to the lithium nickel composite oxide. NCA). A lithium ion secondary battery was produced in the same manner as in Example 1 except that the obtained positive electrode active material was used.

(Example 8)
First, zirconium ethoxide was dissolved in dehydrated ethanol. Lithium nickel composite oxide (LiNi _0.8 Co _0.16 Al _0.04 O ₂ : NCA) was dispersed in the obtained solution. Next, the solvent was completely removed by heating the solution with stirring. The obtained mixture was baked at 300 ° C. for 12 hours in an air atmosphere, whereby a lithium nickel composite oxide (Zr—), which is a positive electrode active material containing 0.5 mol% of a zirconium compound with respect to the lithium nickel composite oxide. NCA). A lithium ion secondary battery was produced in the same manner as in Example 1 except that the obtained positive electrode active material was used.

Example 9
First, vanadium ethoxide was dissolved in dehydrated ethanol. Lithium nickel composite oxide (LiNi _0.8 Co _0.16 Al _0.04 O ₂ : NCA) was dispersed in the obtained solution. Next, the solvent was completely removed by heating the solution with stirring. The obtained mixture was baked at 300 ° C. for 12 hours in an air atmosphere to obtain a lithium nickel composite oxide (V−) that is a positive electrode active material containing 0.5 mol% of a vanadium compound with respect to the lithium nickel composite oxide. NCA). A lithium ion secondary battery was produced in the same manner as in Example 1 except that the obtained positive electrode active material was used.

(Example 10)
First, EC and EMC were mixed at a volume ratio of 1: 3 to obtain a non-aqueous solvent. A nonaqueous electrolyte (E / E) was obtained by dissolving LiPF ₆ in the obtained nonaqueous solvent so as to have a concentration of 1.0 mol / l. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the obtained nonaqueous electrolyte was used.

(Comparative Example 1)
In the same manner as in Example 1, an artificial graphite mixture slurry and a lithium titanate mixture slurry were obtained. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Thereafter, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer and a total thickness of 220 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 2)
In the same manner as in Example 1, an artificial graphite mixture slurry and a lithium titanate mixture slurry were obtained. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer with a total thickness of 161 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 3)
In the same manner as in Example 1, an artificial graphite mixture slurry was obtained. Next, 90 parts by mass of lithium titanate, 5 parts by mass of a conductive material, and 40 parts by mass of PVDF NMP solution (the concentration of PVDF is 8% by mass and the solid content of PVDF is 5 parts by mass) are mixed. Thus, a lithium titanate mixture slurry was obtained. The artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer with a total thickness of 180 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 4)
In the same manner as in Example 1, an artificial graphite mixture slurry was obtained. Next, 90 parts by mass of lithium titanate and 80 parts by mass of PVDF NMP solution (the concentration of PVDF is 8% by mass and the solid content of PVDF is 10 parts by mass) are mixed with the lithium titanate mixture slurry. Got. The artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to a total thickness of 160 μm. Then, the lithium titanate mixture slurry was applied on the artificial graphite layer, dried and rolled to obtain a negative electrode having a negative electrode active material layer with a total thickness of 180 μm. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 5)
A lithium nickel composite oxide containing no tungsten compound was used as the positive electrode active material. Except for this, a lithium ion secondary battery was fabricated in the same manner as in Example 1.

(Comparative Example 6)
In the same manner as in Example 1, an artificial graphite mixture slurry was obtained. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to obtain a negative electrode having a total thickness of 180 μm equipped with a negative electrode active material layer. . A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 7)
In the same manner as in Example 1, a lithium titanate mixture slurry was obtained. The obtained lithium titanate mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to obtain a negative electrode having a total thickness of 180 μm equipped with a negative electrode active material layer. It was. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used.

(Comparative Example 8)
A lithium nickel composite oxide containing no tungsten compound was used as the positive electrode active material. Further, an artificial graphite mixture slurry was obtained in the same manner as in Example 1. The obtained artificial graphite mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to obtain a negative electrode having a total thickness of 180 μm equipped with a negative electrode active material layer. . A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used and the positive electrode active material was used.

(Comparative Example 9)
A lithium nickel composite oxide containing no tungsten compound was used as the positive electrode active material. Moreover, it carried out similarly to Example 1, and obtained the lithium titanate mixture slurry. The obtained lithium titanate mixture slurry was applied to both sides of a negative electrode current collector made of a copper foil having a thickness of 10 μm, dried and rolled to obtain a negative electrode having a total thickness of 180 μm equipped with a negative electrode active material layer. It was. A lithium ion secondary battery was produced in the same manner as in Example 1 except that the negative electrode thus obtained was used and the positive electrode active material was used.

(Battery evaluation)
The initial discharge capacity and cycle characteristics of each lithium ion secondary battery obtained as described above were evaluated according to the following methods.

<Initial discharge capacity>
The lithium ion secondary battery was charged at a charge end voltage of 4.4 V and charged at a rate of 1 C, and then the discharge end voltage was set at 2.5 V and discharged at a rate of 0.2 C. The discharge capacity obtained at this time was defined as the initial discharge capacity.

<Cycle characteristics>
The charge / discharge cycle under the following conditions was repeated for the lithium ion secondary battery. The environmental temperature at the time of charging / discharging was set to 25 degreeC. First, the maximum current value was set to 2.5 A, and constant current charging was performed until the voltage reached 4.4 V, and then constant voltage charging was performed at 4.2 V until the current value decreased to 50 mA. Next, a constant current discharge was performed at a rate of 0.2 C at a discharge end voltage of 2.5V. The pause time between charging and discharging was 30 minutes. This charging / discharging cycle was made into 1 cycle, and charging / discharging was repeated 100 cycles. The cycle capacity was evaluated by regarding the discharge capacity at the first cycle in the charge / discharge cycle as 100% and expressing the discharge capacity as a percentage of capacity retention when 100 cycles passed. The results are shown in Table 1.

As shown in Table 1, the battery of Example 1 had good initial discharge capacity. This is because the surface of the positive electrode active material was modified by using a lithium nickel composite oxide containing a tungsten compound as a different element as the positive electrode active material, and the discharge capacity showed a good value. Furthermore, the thickness of the second layer using lithium titanate of the negative electrode as the active material is 13.3% with respect to the first layer using artificial graphite as the active material, so that lithium titanate having a small theoretical capacity is obtained. This is considered to be because the influence on the decrease in the battery capacity was suppressed.

Furthermore, in Example 1, the capacity retention rate after 100 cycles was good. As described above, the artificial graphite layer as the first layer and the lithium titanate layer as the second layer thinner than the first layer are provided on the negative electrode, thereby reducing the redox potential of the lithium titanate. This is considered to be because gas generation due to decomposition of the nonaqueous electrolyte containing the fluorinated solvent was suppressed by forming a good film of fluoroethylene carbonate on the negative electrode surface. Furthermore, it is considered that the amount of the conductive material contained in the second layer is 3 parts by mass with respect to 100 parts by mass of lithium titanate, so that the decomposition of the nonaqueous electrolyte can be suppressed and the cycle characteristics are improved.

The battery of Example 2 had good initial discharge capacity as in Example 1. This is considered to be the same reason as in the first embodiment. However, the initial discharge capacity is smaller than that of Example 1, and this is considered to be because the influence on the capacity reduction of the battery becomes remarkable because the thickness of the second layer is increased. Further, similarly to Example 1, the capacity retention rate after 100 cycles was good.

As with Example 1, the battery of Example 3 had good initial discharge capacity and capacity retention after 100 cycles. This is considered to be the same reason as in the first embodiment. However, compared with Example 1, the capacity retention rate after 100 cycles is a low value. This is because the thickness of the second layer is relatively small, so that the different element contained in the positive electrode active material is eluted and the total amount of the different element metal deposited on the negative electrode can be deposited on the lithium titanate. This is considered to be because the decomposition of the nonaqueous electrolyte could not be suppressed and the cycle characteristics deteriorated.

As with Example 1, the battery of Example 4 had good initial discharge capacity and capacity retention after 100 cycles. This is considered to be the same reason as in the first embodiment. However, compared with Example 1, the initial discharge capacity was a low value. This is probably because the amount of the conductive material contained in the second layer was smaller than that in Example 1, and therefore sufficient battery characteristics could not be expressed as compared with Example 1.

As in Example 1, the battery of Example 5 had good initial discharge capacity and capacity retention after 100 cycles. This is considered to be the same reason as in the first embodiment. However, the initial discharge capacity was lower than in Examples 1 and 4. This is probably because the amount of the conductive material contained in the second layer was smaller than those in Examples 1 and 4, and therefore, sufficient battery characteristics could not be expressed as compared with Examples 1 and 4.

The battery of Example 6 showed a good initial discharge capacity equivalent to that of Example 1. This is because the surface of the positive electrode active material was modified by using a lithium nickel composite oxide containing a niobium compound as a different element as the positive electrode active material, and the discharge capacity showed a good value. Furthermore, the thickness of the second layer using lithium titanate of the negative electrode as the active material is 13.3% with respect to the first layer using artificial graphite as the active material, so that lithium titanate having a small theoretical capacity is obtained. This is considered to be because the influence on the decrease in the battery capacity was suppressed. Further, the battery of Example 6 had a good capacity retention rate after 100 cycles as in Example 1. This is considered to be the same reason as in the first embodiment.

The battery of Example 7 showed a good initial discharge capacity equivalent to that of Example 1. This is because the surface of the positive electrode active material was modified by using a lithium nickel composite oxide containing a boron compound as a different element as the positive electrode active material, and the discharge capacity showed a good value. Furthermore, the thickness of the second layer using lithium titanate of the negative electrode as the active material is 13.3% with respect to the first layer using artificial graphite as the active material, so that lithium titanate having a small theoretical capacity is obtained. This is considered to be because the influence on the decrease in the battery capacity was suppressed. Further, the battery of Example 7 had a good capacity retention rate after 100 cycles as in Example 1. This is considered to be the same reason as in the first embodiment.

The battery of Example 8 exhibited a good initial discharge capacity equivalent to that of Example 1. This is because the surface of the positive electrode active material was modified by using a lithium nickel composite oxide containing a zirconium compound as a different element as the positive electrode active material, and the discharge capacity showed a good value. Furthermore, the thickness of the second layer using lithium titanate of the negative electrode as the active material is 13.3% with respect to the first layer using artificial graphite as the active material, so that lithium titanate having a small theoretical capacity is obtained. This is considered to be because the influence on the decrease in the battery capacity was suppressed. Further, the battery of Example 8 had a good capacity retention rate after 100 cycles as in Example 1. This is considered to be the same reason as in the first embodiment.

The battery of Example 9 showed a good initial discharge capacity equivalent to that of Example 1. This is because the surface of the positive electrode active material was modified by using a lithium nickel composite oxide containing a vanadium compound as a different element for the positive electrode active material, and the discharge capacity showed a good value. Furthermore, the thickness of the second layer using lithium titanate of the negative electrode as the active material is 13.3% with respect to the first layer using artificial graphite as the active material, so that lithium titanate having a small theoretical capacity is obtained. This is considered to be because the influence on the decrease in the battery capacity was suppressed. Further, the battery of Example 9 had a good capacity retention rate after 100 cycles as in Example 1. This is considered to be the same reason as in the first embodiment.

Although the battery of Example 10 had good battery characteristics, the initial discharge capacity and the capacity retention rate after 100 cycles were low as compared with the battery of Example 1. From this result, it is considered that the inclusion of FEC in the non-aqueous electrolyte facilitates the formation of a good film on the negative electrode surface and improves battery characteristics.

The battery of Comparative Example 1 had a lower initial discharge capacity than the batteries of Examples 1-9. This is because the lithium titanate battery having a low theoretical capacity is obtained because the thickness of the second layer using lithium titanate as the active material is greater than 25% of the thickness of the first layer using artificial graphite as the active material. This is thought to be due to the remarkable impact on the capacity reduction.

On the other hand, the battery of Comparative Example 2 showed a lower capacity retention rate after 100 cycles than the batteries of Examples 1 to 9. This is because when the thickness of the second layer is less than 1% with respect to the first layer, the different elements contained in the positive electrode active material are eluted and the total amount deposited on the negative electrode is deposited on lithium titanate. This is considered to be due to the fact that the decomposition of the non-aqueous electrolyte could not be suppressed and the cycle characteristics deteriorated.

The battery of Comparative Example 3 had a lower capacity retention rate after 100 cycles than the batteries of Examples 1-9. This is presumably because the amount of the conductive material contained in the second layer was 5 parts by mass with respect to 100 parts by mass of lithium titanate, and the decomposition of the nonaqueous electrolyte could not be suppressed.

The battery of Comparative Example 4 had a significantly lower initial discharge capacity than the batteries of Examples 1-9. This is presumably because the conductivity of the second layer was insufficient because the conductive material was not added to the second layer, and electrode activity did not appear.

The battery of Comparative Example 5 had lower initial discharge capacity and capacity retention after 100 cycles than the batteries of Examples 1-9. This is considered to be due to the use of a positive electrode active material that does not contain different elements.

The battery of Comparative Example 6 had a lower capacity retention rate after 100 cycles than the batteries of Examples 1-9. This is thought to be due to the fact that, since the negative electrode did not contain lithium titanate, the dissimilar elements contained in the eluted positive electrode were deposited on the negative electrode surface, so that the decomposition of the nonaqueous electrolyte was not suppressed and the cycle characteristics were reduced. It is done.

The battery of Comparative Example 7 had lower initial discharge capacity and capacity retention after 100 cycles than the batteries of Examples 1-9. This is because the capacity of the battery was reduced because a single layer of lithium titanate having a low theoretical capacity was used for the negative electrode. Furthermore, since the redox potential of lithium titanate is high, the formation of a coating on the surface of lithium titanate is insufficient, and the generation of gas due to the decomposition of the nonaqueous electrolyte containing the fluorinated solvent cannot be suppressed, resulting in reduced cycle characteristics. It is thought that.

The battery of Comparative Example 8 had lower initial discharge capacity and capacity retention after 100 cycles than the batteries of Examples 1-9. This is because the use of a positive electrode active material not containing a different element and the absence of lithium titanate in the negative electrode did not cause the precipitation of a different element on the negative electrode surface, and the decomposition of the non-aqueous electrolyte was not suppressed. This is thought to be due to the deterioration of the characteristics.

The battery of Comparative Example 9 had lower initial discharge capacity and capacity retention after 100 cycles than the batteries of Examples 1-9. This is due to the use of a positive electrode active material that does not contain foreign elements, and because the redox potential of lithium titanate is high, the formation of a film on the surface of lithium titanate is insufficient, and a non-fluorinated solvent is contained. It is considered that gas generation due to decomposition of the water electrolyte could not be suppressed and cycle characteristics were deteriorated.

The lithium ion secondary battery of the present invention has a large discharge capacity even when charged at a high voltage, can suppress the decomposition of the nonaqueous electrolyte during the charge / discharge cycle, and has good cycle characteristics. Therefore, the lithium ion secondary battery of the present invention is useful as a power source for various portable electronic devices such as mobile phones, PDAs, notebook personal computers, digital cameras, and portable game machines. Further, it can be applied to the use of a driving power source for an electric vehicle, a hybrid vehicle and the like.

DESCRIPTION OF SYMBOLS 10 Negative electrode collector 11 1st layer 12 2nd layer 13 Negative electrode active material layer 20 Lithium ion secondary battery 21 Positive electrode 22 Negative electrode 23 Separator 24 Electrode group 25 Positive electrode side insulating plate 26 Negative electrode side insulating plate 27 Battery case 28 Sealing Plate 29 Negative electrode lead 30 Positive electrode terminal 31 Positive electrode lead

Claims

A positive electrode including a positive electrode current collector and a positive electrode active material layer; a negative electrode including a negative electrode current collector and a negative electrode active material layer; a separator provided between the positive electrode and the negative electrode; and a nonaqueous electrolyte. A lithium ion secondary battery,
The positive electrode active material layer has a lithium nickel composite oxide containing a different element,
The different element is at least one different element selected from the group consisting of tungsten, niobium, boron, zirconium and vanadium,
The negative electrode active material layer includes a first layer formed on the surface of the negative electrode current collector, and a second layer formed on the surface of the first layer;
The first layer includes a carbon material;
The second layer includes lithium titanate and a conductive material, and has a thickness of 1% to 25% of the thickness of the first layer;
The amount of the conductive material contained in the second layer is 3 parts by mass or less with respect to 100 parts by mass of the lithium titanate.
In claim 1,
The lithium ion secondary battery in which the mixture density of the first layer is 1.4 g / ml or more and 2.8 g / ml or less.
In claim 1 or 2,
The lithium ion secondary battery in which the mixture density of the second layer is 1.3 g / ml or more and 1.8 g / ml or less.
In any one of claims 1 to 3,
The lithium titanate has a BET specific surface area of 2 m 2 / g or more and 8 m 2 / g or less.
In any one of claims 1 to 4,
The lithium ion secondary battery has an average particle size of the lithium titanate of 0.5 μm or more and 10 μm or less.
In any one of claims 1 to 5,
The lithium ion secondary battery, wherein the lithium titanate has an oil absorption of 20 g / 100 g or more and 50 g / 100 g or less.
In any one of claims 1 to 6,
The non-aqueous electrolyte is a lithium ion secondary battery including a fluorinated solvent that is at least one selected from the group consisting of fluoroethylene carbonate, methyl fluoropropionate, fluorobenzene, fluoroethyl methyl carbonate, and fluorodimethylene carbonate.