JP2008060076A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve float life performance at a using time of a high temperature float in a nonaqueous electrolyte secondary battery using lithium titanate for a negative electrode active material. <P>SOLUTION: The nonaqueous electrolyte secondary battery comprises a negative electrode containing a compound expressed by a general formula: Li<SB>a</SB>Ti<SB>5/3-b</SB>M<SB>b</SB>O<SB>4</SB>(M is one or more kinds of transition metals and an element except Ti, 4/3≤a≤7/3, and 0≤b≤5/6), and a positive electrode containing a positive electrode active material expressed by a general formula: Li<SB>x</SB>Co<SB>y</SB>M'<SB>1-y</SB>O<SB>2</SB>(M' is one or more kinds of metal elements except Co, 0.2≤x≤1.1, and 0≤y≤0.8), and a passivation film is formed on the negative electrode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery including a negative electrode including lithium titanate and a positive electrode including a lithium transition metal composite oxide.

  Non-aqueous electrolyte secondary batteries widely used as power sources for electronic devices such as mobile phones are generally lithium transition metal composite oxides having a layered rock-salt structure on the positive electrode, carbon materials on the negative electrode, and electrolytes. Carbonates containing lithium salts are used. Its features include high operating voltage and high energy density.

  Such non-aqueous electrolyte secondary batteries have recently been used for industrial backup batteries. One particularly required performance when used in this application is good float life performance at high temperatures. Literature known inventions related to the invention that meets such requirements are listed below.

  Patent Document 1 shows that a lithium ion secondary battery using lithium titanate as a negative electrode has very high cycle life performance.

  In Patent Document 2, in a battery using a lithium transition metal composite oxide for a positive electrode and a carbon material for a negative electrode, the battery life performance at a high temperature is improved as the amount of manganese in the oxide used for the positive electrode is increased. There is a description. In addition, it is not specified about the influence which the amount of cobalt has on the battery life performance under high temperature.

  In Patent Document 3, in a battery using a transition metal oxide for a positive electrode and a carbon material for a negative electrode, the total amount of nickel and manganese in the oxide used for the positive electrode is 0.4 or less (that is, the amount of cobalt is 0). .6 or more), it is disclosed that the amount of lithium extracted during high-temperature charging can be suppressed, and good battery life performance can be obtained. That is, from Patent Document 3, it is considered that the battery life during high-temperature float charging is improved as the amount of cobalt is increased.

In Patent Document 4, Li ₄ Ti ₅ O ₁₂ is used as the negative electrode active material, and the general formula LiNi _1-xy Co _x Al / Mn _y O _{2 is used as} the positive electrode active material (where Al / Mn is at least one of Al and Mn). Or by using lithium metal oxide represented by 0 ≦ x ≦ 0.5, 0 ≦ y ≦ 0.5, x + y <0.66), suitable for operation at high temperature (60 ° C. to 180 ° C.) In addition, it is described that a non-aqueous secondary battery can be obtained.

In Patent Document 5, a lithium-titanium composite oxide such as Li _{4 + x} Ti ₅ O ₁₂ (−1 ≦ x ≦ 3) is used as the negative electrode active material, and a general formula Li _a Ni _b Co _c Mn _d O ₂ (0 By using the lithium nickel cobalt manganese composite oxide represented by ≦ a ≦ 1.1 and b + c + d = 1), a regenerative power storage system in which battery swelling is reduced during rapid charging in a high temperature environment is obtained. Technology is disclosed.

Japanese Patent No. 3502118 International Publication No. 2003/081698 Pamphlet JP 2004-111076 A JP 2006-040896 A JP 2006-120616 A

(A) However, the battery described in Patent Document 1 has a problem that the capacity decreases or the internal resistance increases when float charging is performed at a high temperature (particularly 60 ° C. or higher).

  The techniques disclosed in Patent Document 2 and Patent Document 3 improve the float life performance of the battery by increasing the cobalt content in the positive electrode active material, but when the ambient temperature is set to 80 ° C. As a result, the battery capacity decreased or the battery swelled, and the life performance was insufficient.

  In Patent Document 4 and Patent Document 5, excellent cycle life performance can be obtained at a high temperature of 60 ° C. or higher by combining lithium titanate as a negative electrode active material and a lithium transition metal composite oxide as a positive electrode active material. There was no description about the influence of the cobalt content in the lithium transition metal composite oxide on the life performance, and there was no description about the float life performance at a high temperature of 80 ° C.

  The object of the present invention, which has been made in view of the above problems, is a non-aqueous electrolyte secondary battery using a lithium transition metal composite oxide as a positive electrode active material and lithium titanate as a negative electrode active material. Is to improve.

(B) Furthermore, there has been a problem that it is difficult to uniformly provide a “passive film” which is one of the invention-specific matters of the present invention applied to solve the problem (A). If this passive film is not uniformly provided, when the non-aqueous electrolyte secondary battery is mass-produced, the performance of the non-aqueous electrolyte secondary battery varies, and a homogeneous battery is stably manufactured. It becomes difficult. An object of the present invention is to further improve the passive film which is one of the invention-specific matters in the present invention.

(1) The present invention relates to a non-aqueous electrolyte secondary battery having a general formula Li _a Ti _{5 / 3-b} M _b O ₄ (M is one or more transition metals and an element other than Ti, 4/3 ≦ a ≦ 7/3, 0 ≦ b ≦ 5/6) and a general formula Li _x Co _y M ′ _1-y O ₂ (M ′ is one or more metal elements other than Co, 0 2 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.8), and a passive film is formed on the negative electrode.

(2) The invention according to the present application is characterized in that, in addition to the configuration of (1), the negative electrode further includes a graphitic carbon material.

(3) The invention of the present application is characterized in that, in addition to the configuration of (2), the mass of the graphitic carbon material is 2% or more with respect to the total mass of the compound and the graphitic carbon material. To do.

(4) The present invention is characterized in that, in addition to the configuration of (3), d (002) by X-ray diffraction measurement of the graphitic carbon material is 3.4 mm or less.

(5) The present application is a non-aqueous electrolyte secondary battery including a negative electrode, and the negative electrode has a general formula Li _a Ti _{5 / 3-b} M _b O ₄ (M is one or more transition metals other than Ti. Element, 4/3 ≦ a ≦ 7/3, 0 ≦ b ≦ 5/6) and a graphitic carbon material, wherein the mass of the graphitic carbon material includes the compound and the graphitic carbon material. The negative electrode potential is set to 0.05 to 0.5 V (vs. Li / Li) by controlling the terminal voltage of the nonaqueous electrolyte secondary battery and charging / discharging the non-aqueous electrolyte secondary battery. ⁺ ) Relates to an invention of a method for producing a nonaqueous electrolyte secondary battery in which a passive film is formed on the negative electrode.

(6) The present application is a negative electrode having a passive film formed on the surface thereof, and has a general formula Li _a Ti _{5 / 3-b} M _b O ₄ (M is one or more transition metals and an element other than Ti, 4/3 ≦ a ≦ 7/3, 0 ≦ b ≦ 5/6) and a graphitic carbon material, and a general formula Li _x Co _y M ′ _1-y O ₂ (M ′ is One or more metal elements other than Co, 0.2 ≦ x ≦ 1.1, 0 ≦ y ≦ 0.8), and a method of using a nonaqueous electrolyte secondary battery including a positive electrode including a compound represented by Relates to the invention. Here, it is preferable that the mass of the graphitic carbon material is 2% or more with respect to the total mass of the compound and the graphitic carbon material.

  According to the present invention, it is possible to obtain a nonaqueous electrolyte secondary battery that exhibits good float life performance in a high temperature environment of 60 ° C. or higher.

  In addition, according to the present invention, since the passive film can be stabilized, non-aqueous electrolyte secondary batteries exhibiting good float life performance can be mass-produced uniformly.

Furthermore, according to the present invention, a homogeneous non-aqueous electrolyte secondary battery can be obtained. Therefore, even when the non-aqueous electrolyte secondary battery is a so-called assembled battery, stable use is possible when the assembled battery is used. The effect of becoming.

(A) About the negative electrode In the present invention, the negative electrode active material is represented by the general formula Li _a Ti _{5 / 3-b} M _b O ₄ (M is one or more transition metals and elements other than Ti, 4/3 ≦ a ≦ 7 / 3, 0 ≦ b ≦ 5/6) (in the present specification, this compound is hereinafter simply referred to as “lithium titanate”). The negative electrode potential is approximately 1.55 V vs. This is because Li / Li ⁺ causes almost no reductive decomposition of the electrolyte accompanying charging of the negative electrode, and the cycle life performance at a high temperature of 60 ° C. or higher is particularly good.

On the other hand, conventionally used carbon materials and alloy materials are 1.55 Vvs. Since the charge / discharge reaction occurs at a lower potential than Li / Li ^+, the decomposition reaction of the electrolytic solution is remarkable particularly at high temperatures. Therefore, the battery of the conventional type has an increased battery resistance and a depletion of the electrolyte solution at a high temperature, so that its cycle life performance is low.

In lithium titanate, the reason why the range of a is 4/3 ≦ a ≦ 7/3 is that in the case of electrochemically inserting and extracting Li ⁺ ions in this negative electrode active material, 4/3 ≦ a ≦ 7 / This is because, within the range of 3, the crystal lattice volume hardly changes, so that good cycle life performance can be expected. Also, the reason for b ≦ 5/6 is that when b> 5/6, a stable spinel structure cannot be obtained, so that the change in crystal lattice volume becomes large, and good cycle life performance cannot be expected. Because.

  As M in the general formula representing the negative electrode active material, any metal element can be used. From the viewpoint of cycle life performance and reversible capacity, Al, Mn, Fe, Ni, Co, Zr, Nb, Mo, La, W and the like are preferable.

(B) Passive film It is particularly important in the present invention that a passive film is formed on the negative electrode. The thickness may be any amount, but is preferably 20 nm or more. This is because when the thickness of the film is smaller than 20 nm, the protection of the negative electrode is insufficient, and as a result, the electrolytic solution is reduced and decomposed on the negative electrode to generate gas, and the battery swells and becomes defective. When the film is not formed, the amount of side reaction of the reductive decomposition is increased, the coulomb efficiency of the negative electrode is remarkably lowered, and the battery is remarkably deteriorated.

  Here, the “passive film” referred to in the present invention is a film that is not etched (outermost surface) of the film formed on the negative electrode surface, and in which XPS measurement shows no Ti element peak. means. In the present application, the portion where the Ti peak on the negative electrode surface is not visible is defined as the film thickness.

  As is well known, in a non-aqueous electrolyte secondary battery using lithium titanate as the negative electrode active material, an SEI film is formed on the negative electrode surface even when initially charged at a normal constant current / constant voltage. is there. For this reason, at first glance, it seems as if the passive film referred to in the present application also exists in the existing “nonaqueous electrolyte secondary battery using lithium titanate as the negative electrode active material”. However, the SEI film formed by such initial charge has a peak of Ti element in an unetched state (outermost surface) in XPS measurement. For this reason, the SEI film formed by such normal initial charging cannot be called the “passive film” formed on the negative electrode surface in the present invention, and is different. That is, the conventional “non-aqueous electrolyte secondary battery using lithium titanate as the negative electrode active material” does not include the “passive film” in the present invention.

  Although not related to the passive film of the present invention, analysis of the peak of an element containing C or O on the outermost surface seems to belong to decomposition products of the electrolyte other than the electrolyte components, that is, oligomers. Since the formation of new substances was recognized, it is added for reference. The SEI coating is so thin that it appears to be inadequate to suppress lifetime degradation at high temperatures.

The passive film on the surface of the negative electrode can be formed by various methods, for example, a method of overcharging until the negative electrode potential becomes 0.2 V (vs. Li ^/ Li ⁺ ) at the first charge after the battery is formed. It is done.

  In this case, the potential when the passive film is formed on the negative electrode surface is preferably 0.5 V or less and 0.05 V or more, and particularly preferably 0.1 to 0.3 V. When a film is formed at a potential nobler than 0.5 V, gas generation at room temperature can be suppressed, but the amount of gas generation increases when used for a long time at high temperatures such as 60 ° C. and 80 ° C., which is not preferable. This is probably because the passive film formed in a potential range nobler than 0.5 V cannot exist stably at a high temperature, and the negative electrode is not sufficiently protected. In addition, when the film forming potential is less than 0.05 V, metal Li electrodeposition occurs during charging, and battery deterioration such as internal short circuit is likely to occur, which is not desirable. Note that the potential range of the negative electrode when charging and discharging the battery during normal use is 1.0 V or more.

A method for measuring the thickness of the passive film will be described. The method is that the battery is disassembled in a glove box in an argon atmosphere, the negative electrode is taken out, washed with dimethyl carbonate (DMC), and then Ar ⁺ ion etching and XPS measurement on the negative electrode surface are the negative electrode active material. It repeats until the peak of Ti element is seen. The thickness of the passive film described in the present application is a value in terms of SiO ₂ . For the conversion, a calibration curve obtained by subjecting a SiO ₂ substrate to Ar ⁺ ion etching, and dividing the weight of SiO ₂ reduced from the weight change before and after that by the density (2.2 g / cm ³ ) and the etching spot diameter. Shall be used.

(C) Graphite carbon material An invention in which a graphite carbon material is mixed with the negative electrode will be described.
When the negative electrode further contains a graphitic carbon material in addition to lithium titanate, a more preferable effect can be obtained. The effect is that a nonaqueous electrolyte secondary battery having reliability at high temperatures can be mass-produced uniformly and stably. Such an effect cannot be expected by those skilled in the art related to the present invention and can be said to be a remarkable effect. Moreover, the phenomenon that variation is reduced among non-aqueous electrolyte secondary batteries manufactured in large quantities by such a technical solution is a foreign effect and can be said to have a very high industrial value. In addition, the remarkableness of an effect is disclosed by Table 2 of this-application specification mentioned later.

The mechanism for obtaining such an effect is assumed as follows.
That is, the negative electrode using lithium titanate has a small charge / discharge reversible capacity at 0.1 V to 0.3 V which is a potential band suitable for formation of a passive film. Usually, the passive film is formed by controlling the voltage between the battery terminals. However, the potential at the time of forming the passive film is not constant due to the variation in the normal coating weight that occurs in the manufacturing process of the positive electrode and the negative electrode. As a result, the formation potential of the passive film varies from battery to battery, making it difficult to stably manufacture a large number of batteries having a certain quality and reliability at high temperatures. On the other hand, the graphitic carbon material has a very large charge / discharge reversible capacity in a potential region of 0.1V to 0.3V. Therefore, if a predetermined amount of graphitic carbon material is mixed with lithium titanate and used for the negative electrode, a battery having a relatively large charge / discharge reversible capacity in a potential region of 0.1 V to 0.3 V can be manufactured. Even if there is a variation in weight between the positive electrode and the negative electrode, it is possible to stably manufacture a large quantity of batteries having a certain quality and reliability at high temperatures.

  Here, the mixing amount of the graphitic carbon material will be described. The mass of the graphitic carbon material relative to the total mass of the lithium titanate and the graphitic carbon material is desirably 2% or more. When the graphitic carbon material is less than 2%, the charge / discharge reversible capacity in the potential region of 0.1 V to 0.3 V becomes small, and thus the effect of stably producing a battery having a certain quality and reliability at a high temperature. However, it cannot be said that it is remarkable. This is because, in the case of 2% or more, a difference in effect that cannot be expected by those skilled in the art appears compared to the case of less than 2%.

On the other hand, when the graphitic carbon material is larger than 10%, the energy density of the nonaqueous electrolyte secondary battery is naturally reduced. This is because the graphitic carbon material has almost no charge / discharge reversible capacity at 2.0 to 1.0 V (vs. Li / Li ⁺ ) which is a normal charge / discharge potential range of a negative electrode using lithium titanate. .

D (002) by X-ray diffraction measurement of the graphitic carbon material applied in the present application is 3.4 mm or less. When d (002) is 3.4 mm or less, the battery has a large charge-discharge reversible capacity in the potential region of 0.1 V to 0.3 V, and has a certain quality and high temperature reliability by adding a graphitic carbon material. Can be manufactured stably. Here, d (002) of the graphitic carbon material is calculated from a peak position appearing in the vicinity of 25 ^° in the X-ray diffraction pattern by the Cukα ₁ line.

In addition, since the solvent decomposition reaction amount increases as the specific surface area of the graphitic carbon material increases, the specific surface area is desirably 9.0 m ² / g or less, and from the viewpoint of ensuring conductivity, the specific surface area is It has been found desirable to be 0.2 m ² / g or more.

(D) Positive Electrode As the positive electrode active material used in the present invention, the general formula Li _x Co _y M ′ _1-y O ₂ (M ′ represents one or more metal elements other than Co, 0.2 ≦ x ≦ 1. 1, 0 ≦ y ≦ 0.8). It has a layered rock salt structure. A compound having a layered rock salt structure (R3m structure) has a relatively high capacity and is suitable for obtaining a battery having a high energy density. On the other hand, a compound having a structure other than that (for example, LiMn ₂ O ₄ having a spinel structure) is generally appropriate because it has a smaller charge / discharge capacity than a compound having a layered rock salt structure. That's not true.

  In the general formula of the positive electrode active material which is one of the invention-specifying matters of the present invention, when x is less than 0.2, the crystal structure becomes unstable, resulting in thermal stress during high-temperature storage. When exposed to, the crystals break down and lithium occlusion and release are not performed normally. On the other hand, when x exceeds 1.1, the amount of remaining lithium not involved in the battery reaction increases, so the charge / discharge capacity decreases. Furthermore, when y exceeds 0.8, surprisingly, unlike the prior examples described in Patent Document 2 and Patent Document 3, the capacity retention rate after float charging is significantly reduced as the Co content increases.

  Here, the cause is presumed as follows. After dissolving the negative electrode after the float life test in a mixed acid, ICP analysis was performed and the amount of Co was investigated. As a result, it was found that the amount of Co increased as the value of y increased. From this result, a substance containing Co is deposited on the negative electrode, which acts catalytically to form a high resistance film different from the conventional passive film at a high temperature. As a result, the battery resistance increases, It is presumed that the capacity maintenance rate has decreased.

  In the general formula of the positive electrode active material, specific examples of M ′ include, for example, at least one element selected from the group consisting of Mn, Co, Al, Fe, Ni, Cr, Ti, and Zn, or P , B and the like can be used.

(E) Separator The separator used in the nonaqueous electrolyte battery of the present invention may be any lithium-ion conductive and non-electroconductive material. Preferred materials include polyolefin and paper. can give. Among these, polyolefin is preferable, and specific examples include polyethylene, polypropylene, and polybutene. Among them, polyethylene, polypropylene, and copolymers thereof are preferable in terms of film strength.

  Moreover, as a form of a separator, a microporous film, a woven fabric, a nonwoven fabric etc. are mentioned, for example. Among these, a polyolefin microporous membrane is preferable because it has shutdown performance and is excellent in safety. In addition, materials, laminates of multiple microporous membranes with different weight average molecular weights and porosity, and appropriate additives such as various plasticizers, antioxidants, and flame retardants are contained in these microporous membranes You can use what you are doing.

(F) Binder, Conductive Agent, Electrolytic Solution, and Other Materials The binder used for the positive electrode is not particularly limited, and various materials can be used as appropriate. For example, polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene copolymer (HFP), polytetrafluoroethylene (PTFE), fluorinated polyvinylidene fluoride, ethylene-propylene-diene terpolymer, styrene- At least one selected from the group consisting of butadiene rubber, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, and derivatives thereof can be used.

  There is no restriction | limiting in particular as a electrically conductive agent used for a positive electrode, A various material can be used suitably. For example, Ni, Ti, Al, Fe, or an alloy of two or more of these, or a carbon material can be used. Among these, it is preferable to use a carbon material. Examples of the carbon material include amorphous carbon such as natural graphite, artificial graphite, vapor-grown carbon fiber, acetylene black, ketjen black, and needle coke.

  There is no restriction | limiting in particular as a binder used for a negative electrode, A various material can be used suitably. For example, styrene-butadiene rubber (SBR) or carboxymethyl cellulose (CMC), PVdF, carboxy-modified polyvinylidene fluoride, PTFE, HFP, vinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-hexafluoropropylene copolymer , Ethylene-propylene-diene copolymer, acrylonitrile-butadiene rubber, fluororubber, polyvinyl acetate, polymethyl methacrylate, nitrocellulose, polyethylene, polypropylene, and derivatives thereof are used. be able to.

  As the solvent used when mixing the active material, the binder and the conductive agent, a non-aqueous solvent or an aqueous solution can be used. Non-aqueous solvents include N-methyl-2-pyrrolidone (generally called NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine. , Ethylene oxide, tetrahydrofuran and the like. Moreover, you may add a dispersing agent, a thickener, etc. to these.

  As the current collector substrate of the electrode used in the present invention, aluminum, copper, iron, nickel, SUS can be used. Of these, aluminum or copper is preferred because of its high thermal conductivity and electronic conductivity. Examples of the shape include a sheet, a foam, a sintered porous body, and an expanded lattice. Further, a current collector having a hole in an arbitrary shape can be used.

  There is no restriction | limiting in particular as an organic solvent of the electrolyte solution used for the nonaqueous electrolyte battery of this invention, A various material can be used suitably. For example, ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, halogenated hydrocarbons, esters, carbonates, nitro compounds, phosphate ester compounds, sulfolane hydrocarbons, etc. Among these, ethers, ketones, esters, lactones, halogenated hydrocarbons, carbonates, and sulfolane hydrocarbons are preferable.

  Further examples of these include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, 1,2-dichloroethane, γ-butyrolactone, γ-valerolactone, dimethoxyethane, diethoxyethane, methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethylene carbonate, propylene carbonate, Vinylene carbonate, butylene carbonate, dimethylformamide, dimethyl sulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfo Examples thereof include run, trimethyl phosphate, triethyl phosphate, and phosphazene derivatives, and mixed solvents thereof.

  Among them, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-BL), dimethyl carbonate (DMC), methyl ethyl carbonate (MEC) and diethyl carbonate (DEC) are used alone or in combination of two or more. It is preferable to use what contains 30 to 80 mass% of PC.

Moreover, there is no restriction | limiting in particular as a solute used for the nonaqueous electrolyte battery of this invention, A various solute can be used suitably. For example, LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiPF (CF ₃ ) ₅ , LiPF ₂ (CF ₃ ) ₄ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₅ (CF ₃ ), LiPF ₃ (C ₂ F ₅ ) ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (C ₂ F ₅ CO) ₂ , LiI, LiAlCl ₄ , LiBC ₄ O ₈ or the like can be used alone or in admixture of two or more. Of these, LiPF ₆ is preferably used because of its good ion conductivity. Furthermore, the lithium salt concentration is preferably 0.5 to 2.0 mol / dm ³ .

Also included in the electrolyte are carbonates such as vinylene carbonate and butylene carbonate, benzenes such as biphenyl and cyclohexylbenzene, sulfurs such as propane sultone and propene sultone, ethylene sulfide, hydrogen fluoride, triazole-based cyclic compounds, and fluorine-containing esters. , A hydrogen fluoride complex of tetraethylammonium fluoride or a derivative thereof, phosphazene and its derivatives, an amide group-containing compound, an imino group-containing compound, or a nitrogen-containing compound. it can. _{Moreover, CO 2, NO 2, CO} , also contain at least one selected from such SO ₂ may be used. From the viewpoint of strengthening the passive film of the negative electrode, it is particularly preferable to use vinylene carbonate and propane sultone.

  In addition, a solid or gel ion conductive electrolyte can be used alone or in combination for the electrolyte. When combined, the non-aqueous electrolyte battery is composed of a combination of a positive electrode, a negative electrode and a separator, an organic or inorganic solid electrolyte and the non-aqueous electrolyte, or an organic or inorganic solid electrolyte membrane as a positive electrode, a negative electrode and a separator. A combination with the non-aqueous electrolyte is mentioned. A porous polymer solid electrolyte membrane can also be used for the ion conductive electrolyte.

Examples of the ion conductive electrolyte include polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyethylene glycol and derivatives thereof, LiI, Li ₃ N, Li _{1 + x} M ″ _x Ti _2-x (PO ₄ ) ₃ (M ″ = Al, Sc, Y, La), Li _0.5-3x R _{0.5 + x} TiO ₃ (R = La, Pr, Nd, Sm), or Li _4-x Ge _1-x P _x S ₄ Can be used. Furthermore, oxide glass such as LiI-Li ₂ O—B ₂ O ₅ system, Li ₂ O—SiO ₂ system, or LiI—Li ₂ S—B ₂ S ₃ system, LiI—Li ₂ S—SiS ₂ system, Sulfide glass such as Li ₂ S—SiS ₂ —Li ₃ PO ₄ can be used.

In addition, the shape of the battery is not particularly limited. In the present invention, various shapes such as a square shape, an oval shape, a cylindrical shape, a coin shape, a button shape, and a sheet shape battery can be applied to the nonaqueous electrolyte secondary battery. Is possible.

  A preferred embodiment of the present invention will be described. However, the present invention is not limited to the following examples.

(1) Embodiment 1
[Examples 1-4 and Comparative Examples 1-2]
[Example 1] As one of lithium titanates, a compound of 85% by mass of Li _4/3 Ti _5/3 O ₄ , 5% by mass of acetylene black (d (002) = 3.49 表面積, specific surface area: 70 m ² / G), and 10% by weight of PVdF were dispersed in NMP to prepare a paste. This paste was applied onto a copper foil having a thickness of 20 μm and then dried at 150 ° C. to evaporate NMP. The above operation was performed on both sides of the copper foil. Then, this was compression molded with a roll press. Thus, the negative electrode provided with the negative mix layer on both surfaces was manufactured.

Next, a paste is prepared by dispersing 90% by mass of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ compound, 5% by mass of acetylene black, and 5% by mass of PVdF in NMP. did. This paste was applied onto an aluminum foil having a thickness of 20 μm and then dried at 150 ° C. to evaporate NMP. The above operation was performed on both sides of the aluminum foil, and compression molding was performed with a roll press. Thus, the positive electrode provided with the positive mix layer on both surfaces was manufactured. _{Incidentally, LiCo 1/3 Ni 1/3 Mn 1/3 O} 2 used here was confirmed to be attributed to a layered rock salt type structure (R3m) by XRD measurement.

  After winding the positive electrode and the negative electrode with a separator having a thickness of 25 μm and an air permeability of 90 seconds / 100 cc so as to be positioned between the two electrodes, this is 48 mm in height, 30 mm in width, and 5 mm in thickness. Inserted into a 2 mm container.

  Furthermore, after injecting a non-aqueous liquid electrolyte (electrolyte) into the container, constant current-constant voltage charging at 90 mA, 4.1 V is performed for 10 hours, and constant current discharging up to 90 mA, 1.0 V is performed three times. By repeating, a film was formed on the negative electrode. Thus, a battery A1 of Example 1 having a rated capacity of 450 mAh was obtained.

Incidentally, the electrolyte, the volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) 1: 1: is obtained by dissolving a LiPF ₆ of 1 mol / l in a mixed solvent of 1.

The battery was disassembled in an Ar atmosphere, the negative electrode was taken out, and the thickness of the film was calculated by repeating the above XPS measurement and Ar ⁺ ion etching. As a result of XPS measurement, it was found that a film having no Ti peak was formed, and the thickness of the film was 30 nm or more. It means that the passive film said by this application is provided 30 nm or more.

Example 2 Example 1 except that LiCo _2/3 Ni _1/6 Mn _1/6 O ₂ was used instead of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as the positive electrode active material. Similarly, a battery A2 of Example 2 was obtained. The LiCo _2/3 Ni _1/6 Mn _1/6 O ₂ used here was confirmed to be attributed to a layered rock salt structure by XRD measurement.

[Example 3] In the same manner as in Example 1 except that LiCo _3/20 Ni _17/20 O ₂ was used instead of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as the positive electrode active material, A battery A3 of Example 3 was obtained. Incidentally, _LiCo 3/20 _{Ni 17/20} O ₂ used here was confirmed to be attributable to layered rock salt type structure by XRD measurement.

[Example 4] In the same manner as in Example 1 except that LiCo _4/5 Ni _1/5 O ₂ was used instead of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as the positive electrode active material. A battery A4 of Example 4 was obtained. In addition, it was confirmed that LiCo _4/5 Ni _1/5 O ₂ used here was attributed to a layered rock salt structure by XRD measurement.

Comparative Example 1 A battery B1 of Comparative Example 1 was obtained in the same manner as in Example 1 except that LiCoO ₂ was used instead of LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as the positive electrode active material. It was. The LiCoO ₂ used here was confirmed to be attributed to a layered rock salt structure by XRD measurement.

  [Comparative Example 2] Constant current at 90 mA, 4.1 V—Constant voltage charging was performed for 10 hours and 90 mA, constant current discharge to 1.0 V was not performed, and a film was not formed on the negative electrode. In the same manner as in Example 1, a battery B2 of Comparative Example 2 was obtained.

[Evaluation methods]
A capacity confirmation test was performed using A1 to A4 which are the batteries of Examples 1 to 4 and B1 and B2 which are the batteries of Comparative Examples 1 and 2. Specifically, after 10 hours of constant current and constant voltage charging at 90 mA and 2.60 V, constant current discharging up to 1.0 V at 90 mA is performed, and the obtained discharge capacity is referred to as initial capacity C1 (mAh). did.

  Thereafter, a float life performance test for 90 days at 2.60 V at 80 ° C. was performed using these batteries. Thereafter, a discharge test was conducted at 90 mA up to 1.0 V, and the post-float capacity C2 (mAh) was obtained. The capacity retention ratio R (%) was determined by the formula “R = (C2 / C1) × 100”.

[Evaluation results]
Table 1 shows the value of y in LiCo _y M _1-y O ₂ , the film thickness on the negative electrode, and the capacity retention rate R after the float life test.

FIG. 1 shows the relationship between the value of y of the positive electrode active material LiCo _y M _1-y O ₂ and the capacity retention ratio R.

  As apparent from FIG. 1, when y exceeds 0.8, the capacity retention rate after the float charging is remarkably lowered, which is not suitable.

  This fact is very different from the conventional knowledge that as the amount of cobalt in the lithium nickel manganese cobalt oxide as shown in Patent Documents 2 and 3 and the like increases, the float life performance at a high temperature becomes better. is there. The reason is presumed as follows.

  The negative electrode after the float life test was dissolved in a mixed acid, and the solution was subjected to ICP analysis to examine the amount of Co. As a result, it was found that the amount of Co increased as the value of y increased. From this result, the elution of Co in the positive electrode occurs at high temperatures, and Co is deposited on the negative electrode, which acts catalytically to form a high resistance film at high temperatures, resulting in increased battery resistance. Therefore, it is estimated that the capacity maintenance rate has decreased.

  Further, in the battery B2 of Comparative Example 2 in which the passive film was not formed on the negative electrode, the capacity retention rate R was significantly reduced as compared with the battery A1 of Example 1 in which the passive film was formed on the negative electrode. It is done. This is considered to be due to the fact that in Battery B2, since no passive film is present on the negative electrode, the Coulomb efficiency of the negative electrode is reduced, and as a result, the discharge capacity that can be taken out as a battery is reduced. Therefore, it can be said that it is an essential requirement to form a passive film on the negative electrode in the present invention.

(2) Embodiment 2
[Examples 5 to 10]

Example 5 5% by mass of the compound represented by Li _4/3 Ti _5/3 O ₄ used for the negative electrode of Example 1 was converted to a graphitic carbon material (SFG15, manufactured by TIMCAL, d (002) = A battery (C1) of Example 5 was manufactured in the same manner as in Example 1 except that the negative electrode was manufactured by replacing with 3.4 mm, specific surface area: 7.7 m ² / g). That is, the mass ratio of lithium titanate and graphitic carbon material in the negative electrode is 95: 5. The rated capacity is 425 mAh.

  [Example 6] A battery (C2) was obtained in the same manner as in Example 5 except that the mass ratio of lithium titanate to the graphitic carbon material was 98: 2. The rated capacity is 440 mAh.

  [Example 7] A battery (C3) was obtained in the same manner as in Example 5 except that the mass ratio of lithium titanate to the graphitic carbon material was 97: 3. The rated capacity is 435 mAh.

  [Example 8] A battery (C4) was obtained in the same manner as in Example 5 except that the mass ratio of lithium titanate to the graphitic carbon material was 75:25. The rated capacity is 330 mAh.

  [Example 9] A battery (C5) was obtained in the same manner as in Example 5 except that the mass ratio of lithium titanate to the graphitic carbon material was 60:40. The rated capacity is 260 mAh.

  [Example 10] A battery (R1) was obtained in the same manner as in Example 5 except that the mass ratio of lithium titanate to the graphitic carbon material was 99: 1. The rated capacity is 445 mAh.

[Evaluation methods]
A charge / discharge test was conducted using 30 batteries (A1), (C1) to (C6), and (R1) to (R3). The charge / discharge test is a constant current charge of 1 CmA up to 2.6 V, followed by a constant voltage charge until the total energization time becomes 3 hours, left in an open circuit state for 10 minutes, and then up to 1.0 V at 1 CmA. The battery was discharged at a constant current of 10 minutes and left in an open circuit state for 10 minutes. This series of charging, leaving, discharging and leaving is defined as one cycle. The discharge capacity at this time is C1. After the test, the battery thickness t1 (mm) was measured.

  Then, after putting this battery in a thermostat kept at 60 ° C. and keeping it for 24 hours, a 90-day float life test at 2.60 V was conducted. After the end of the float life test, the battery thickness t2 (mm) was measured. Here, the value of 60 ° C. battery swelling was calculated from the formula “[60 ° C. battery swelling] (mm) = t2 (mm) −t1 (mm)”.

[Evaluation results]
Table 2 shows the initial discharge capacity C1 of each of the batteries (A1), (C1) to (C5), and (R1), and the average value of the 60 ° C. battery swelling and the value of the 60 ° C. battery swelling. Standard deviation is shown.

  From Table 2, when the graphitic carbon material was 2% or more, the average value of 60 ° C. battery swelling was stable at a small value of 0.42 mm to 0.39 mm. Furthermore, the standard deviation was as very small as 0.10 or less, and the variation among the manufactured nonaqueous electrolyte secondary batteries was small.

  On the other hand, when the mass ratio of the graphitic carbon material was not 2% or more, the standard deviation was not 0.10 or less. In order to investigate this cause, the batteries of the batteries (A1) and (R1) were sampled, disassembled while being charged to 4.1 V again, and the negative electrode potential was examined. There was a portion that did not reach the potential of 0.5 V or less, which seems to occur. In addition, in the battery (A1), since a portion where a substance that seems to be metallic Li was deposited was observed on the negative electrode, it reached a base potential lower than 0.05 V at the end of the film formation process. Presumed to be. From these facts, it is presumed that when the graphitic carbon material was not 2% or more, the potential of the entire negative electrode could not be made constant when the negative electrode film was formed.

  From the above, in order to keep the negative electrode potential constant in the film forming step and to produce a large amount of non-aqueous electrolyte secondary batteries with a constant quality, it is preferable that the negative electrode contains 2% or more of graphitic carbon material.

  In addition, since the capacity | capacitance at the time of normal use will fall remarkably when the ratio of a graphitic carbon material becomes large, it is unpreferable from an energy density viewpoint as above-mentioned. Therefore, the upper limit for mixing the graphitic carbon material is generally 25% or less (although this is a matter that can be appropriately determined from the viewpoint of energy density by those skilled in the art of the nonaqueous electrolyte secondary battery). Is appropriate. As shown in Table 2, the standard deviation of the battery swelling of 60 ° C., which is the effect of the present invention, is small even when it is 25% or larger than 25%.

(3) Embodiment 3
As a carbon material to be mixed with the lithium titanate of the negative electrode, an alternative material for the graphitic carbon material was also examined.

  In addition, hard carbon and soft carbon are known as carbon materials other than the graphitic carbon material. The difference between the graphitic carbon material and these hard carbon or soft carbon is generally distinguished by the value of d (002). That is, when d (002) is 3.4 mm or less, it is a graphitic carbon material.

  The aspect used as the comparative example of this invention is shown below.

  [Comparative Example 3] A battery (R2) was obtained in the same manner as in Example 5 except that hard carbon (d (002) = 3.8cm) was used instead of the graphitic carbon material. The rated capacity is 425 mAh.

  Comparative Example 4 A battery (R3) was obtained in the same manner as in Example 5 except that soft carbon (d (002) = 3.5%) was used instead of the graphitic carbon material. The rated capacity is 425 mAh.

[Evaluation methods]
The evaluation method is the same as in the second embodiment.

[Evaluation results]
The evaluation results are shown in Table 3 in the same format as Table 2.

  From Table 3, as described above, in the case of the graphitic carbon material (that is, d (002) is 3.4 mm) as an example of the present application, the standard deviation of 60 ° C. battery swelling was as small as 0.10 or less. On the other hand, the batteries (R2) and (R3), which are not graphitic carbon materials as comparative examples, both had a large standard deviation of 60 ° C. battery swelling.

  As described above, it was found that the carbon material further mixed with lithium titanate in the negative electrode is a graphitic carbon material in order to obtain the effects of the present invention.

  As a result of taking out a battery having a relatively large battery swelling from the batteries (R2) and (R3), disassembling it in a state of being charged to 4.1 V again, and examining its negative electrode potential, it was found that good film formation was obtained on the negative electrode. It has been found that there is a portion that does not reach the potential of 0.5 V or less, which seems to occur. In addition, in the battery (R3), there was also a portion where a metal suspected to be deposited on the negative electrode, so that it was estimated that the potential reached less than 0.1 V at the end of the film formation process. The

Generally, the smaller the d (002) of the carbon material is, the higher the crystallinity is. Therefore, the charge in the range of 0.05 to 0.2 V, which is generally the potential band for Li _x C ₆ stage formation, is considered. The dischargeable capacity increases, and the chargeable / dischargeable capacity of 0.2 V to 1.0 V decreases. When d (002) exceeds 3.4 Å, the discharge capacity in a potential band suitable for film formation is relatively small, so that d (002) is held at a constant potential compared to that with 3.4 Å or less. It is thought that ability became low.

Diagram showing the relationship between the positive electrode active material LiCo _y M _1-y value of y of _{O 2} and capacity retention rate R.

Claims (2)

Represented by the general formula Li _a Ti _{5 / 3-b} M _b O ₄ (M is one or more transition metals and elements other than Ti, 4/3 ≦ a ≦ 7/3, 0 ≦ b ≦ 5/6) And a general formula Li _x Co _y M ′ _1-y O ₂ (M ′ is one or more metal elements other than Co, 0.2 ≦ x ≦ 1.1, 0 ≦ y ≦ 0. A positive electrode containing a positive electrode active material represented by 8),
A nonaqueous electrolyte secondary battery, wherein a passive film is formed on the negative electrode. The negative electrode further includes a graphitic carbon material,
The nonaqueous electrolyte secondary battery according to claim 1, wherein a mass of the graphitic carbon material is 2% or more based on a total mass of the compound and the graphitic carbon material.