JP2014127256A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery small in thickness change and long in battery life.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode; a negative electrode; a nonaqueous electrolyte; and a separator. In the nonaqueous electrolyte secondary battery, the nonaqueous electrolyte uses a composition containing a fluorine-containing electrolyte salt and a phosphono acetates compound represented by chemical formula 1; the positive electrode has a positive electrode active material; the positive electrode active material contains a zirconium-containing lithium cobalt composite oxide (A) in which a zirconium-containing oxide is present on the crystal surface of a lithium cobalt composite oxide represented by chemical formula 2; and the content of zirconium (Zr) of the zirconium-containing lithium cobalt composite oxide (A) is in a range of 0.0001 or more and 0.01 or less in a molar ratio (Zr/Co) relative to cobalt (Co) of the lithium cobalt composite oxide represented by chemical formula 2.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery having good cycle characteristics.

  In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, non-aqueous electrolyte secondary batteries with small size, light weight and high capacity have become necessary. It was.

  In addition, non-aqueous electrolyte secondary batteries are required to improve various battery characteristics as well as to increase capacity with the spread of applied devices.

  As one means for achieving improvement in battery characteristics of such a non-aqueous electrolyte secondary battery, it is known to apply various additives to the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery. For example, Patent Document 1 describes that two types of positive electrode active materials having different specific compositions can be mixed to improve cycle characteristics and low temperature heavy load characteristics.

  Patent Document 2 describes that electrochemical characteristics at high temperatures can be enhanced by using a non-aqueous electrolyte to which an organophosphorus compound having a specific structure is added.

JP 2007-2104090 A International Publication No. 2012/14270

By the way, since lithium reacts violently with moisture, in a lithium secondary battery, it is ideal to remove moisture from the battery as much as possible in order to ensure its excellent characteristics and reliability. However, it is difficult to completely reduce the moisture in the battery. If moisture is inevitably mixed, the fluorine-containing electrolyte salt (for example, LiPF ₆ or the like) reacts with moisture to generate hydrogen fluoride (HF ) Is known to occur.

  Hydrogen fluoride may elute metal elements such as cobalt from the positive electrode active material, and may cause deterioration of charge / discharge cycle characteristics of the battery. Furthermore, hydrogen fluoride has been found to cause battery swelling when the battery is stored in a high temperature environment.

  On the other hand, when the battery is stored in a high-temperature environment, the negative electrode active material and the electrolyte react to decompose the negative electrode active material, and at the same time, gas is generated, and the negative electrode deterioration and the battery swelling may occur simultaneously.

A positive electrode, a negative electrode, a non-aqueous electrolyte and a separator;
The non-aqueous electrolyte uses a fluorine-containing electrolyte salt and a phosphonoacetate compound represented by the general formula (1),
The positive electrode has a positive electrode active material,
The positive electrode active material is
The lithium cobalt composite oxide represented by the general formula (2) includes a zirconium-containing lithium cobalt composite oxide (A) in which an oxide containing zirconium is present on the crystal surface,
The zirconium (Zr) content of the zirconium-containing lithium cobalt composite oxide (A) is 0. 0 by molar ratio (Zr / Co) to cobalt (Co) of the lithium cobalt composite oxide represented by the general formula (2). Provided is a non-aqueous electrolyte secondary battery characterized by being in the range of 0001 or more and 0.01 or less.

In the general formula (1), R ¹ , R ² and R ³ each independently represent an alkyl group having 1 to 12 carbon atoms, an alkenyl group or an alkynyl group which may be substituted with a halogen atom, and n is An integer of 0 to 6 is shown.
Li _t Co _1-s M _s O ₂ (2)

(In the general formula (2), M is at least one element selected from Fe, V, Cr, Ti, Mg, Al, B, Ca, Ba. S and t are 0 ≦ s ≦ 0. 03, 0.05 ≦ t ≦ 1.15.)

  According to the present invention, it is possible to provide a battery having a small thickness change and a long battery life.

It is a figure which shows typically an example of the nonaqueous electrolyte secondary battery of this invention, (a) is the top view, (b) is the fragmentary longitudinal cross-sectional view. It is a perspective view of the nonaqueous electrolyte secondary battery shown in FIG.

  In the non-aqueous electrolyte secondary battery of the present invention, the positive electrode active material contains a zirconium-containing lithium cobalt composite oxide (A) in which an oxide containing zirconium is present on the crystal surface in a lithium cobalt composite oxide, and the non-aqueous electrolyte contains A configuration including a phosphonoacetate compound represented by the general formula (1) is adopted.

  When an oxide containing zirconium is present on the crystal surface of the lithium cobalt composite oxide, hydrogen fluoride is adsorbed to the oxide containing zirconium, preventing battery swelling and deterioration of the positive electrode active material derived from hydrogen fluoride, Cycle characteristics can be improved by reducing the change in battery thickness.

  In addition, the phosphonoacetate compound forms a film on the negative electrode surface to prevent the negative electrode and the electrolyte solvent from reacting during high-temperature storage of the battery, prevent blistering due to gas generation and deterioration of the negative electrode active material, and The cycle characteristics can be improved by reducing the change.

  By the way, in order to evaluate the cycle characteristics of a battery, a cycle test in which the cycle is repeated under specific charge / discharge conditions is common. During the cycle test of the battery, the battery capacity gradually decreases as the cycle is repeated. However, there is a phenomenon in which the battery capacity decrease rate increases at a certain time and the decrease rate gradually increases thereafter.

  Although the deterioration of the cycle characteristics of the battery is caused by various factors, it is largely caused by the deterioration of the positive and negative electrodes.

  Even if the stability of the positive electrode active material is increased by paying attention to the deterioration of the positive electrode, a certain degree of cycle characteristic improvement effect can be expected, and the same applies to the negative electrode. However, since the battery is charged / discharged between the positive electrode and the negative electrode as a matter of course, even if the deterioration of the positive electrode is prevented, for example, if the deterioration of the negative electrode progresses, the decrease rate of the battery capacity is large even if the positive electrode is not deteriorated so much. On the contrary, only the deterioration of the negative electrode is prevented, and even if the deterioration of the positive electrode is not so advanced, the rate of decrease in battery capacity is increased.

  In the present invention, since the deterioration of the positive electrode and the negative electrode is prevented at the same time and balanced, the change in the thickness of the battery is small, and the phenomenon that the rate of decrease in battery capacity increases during the cycle can be as slow as possible. The effect which becomes long can be acquired.

[Positive electrode]
For the positive electrode according to the nonaqueous electrolyte secondary battery of the present invention, for example, a positive electrode mixture layer containing a positive electrode active material, a binder, a conductive auxiliary agent and the like can be used on one side or both sides of the current collector. .

<Positive electrode active material>
[Positive electrode active material]
The positive electrode active material used for the positive electrode of the non-aqueous electrolyte secondary battery of the present invention is a zirconium-containing lithium cobalt composite oxide in which an oxide containing zirconium is present on the crystal surface in the lithium cobalt composite oxide represented by the general formula (2) The thing (A) is included.
Li _t Co _1-s M _s O ₂ (2)
(In the general formula (2), M is at least one element selected from Fe, V, Cr, Ti, Mg, Al, B, Ca, Ba. S and t are 0 ≦ s ≦ 0. 03, 0.05 ≦ t ≦ 1.15.)

The zirconium (Zr) content in the zirconium-containing lithium cobalt composite oxide (A) is 0 in terms of the molar ratio (Zr / Co) of the lithium cobalt composite oxide represented by the general formula (2) to cobalt (Co). It is preferable that it is in the range of not less than 0.0001 and not more than 0.01. Note that examples of the oxide containing zirconium include zirconium oxide or lithium zirconate, and an oxide containing zirconium or another metal element. The oxide containing zirconium is present in the form of particles on the crystal surface of the lithium cobalt composite oxide represented by the general formula (2). Part of the oxide containing zirconium may be dissolved in the lithium cobalt composite oxide represented by the general formula (2), and particles of the lithium cobalt composite oxide represented by the general formula (2) It may be present on the surface.

  The particles of the zirconium-containing compound present on the crystal surface have a size of about 0.05 to 3.0 μm, and this can be visually confirmed with an SEM photograph or the like.

  The present invention can prevent elution of cobalt from the positive electrode active material and prevent deterioration of the positive electrode by containing the above zirconium-containing lithium cobalt composite oxide (A).

  The reason is not clear, but even if moisture inevitably mixed in the battery reacts with the fluorine-containing electrolyte salt and hydrogen fluoride is generated, the oxide containing zirconium adsorbs hydrogen fluoride. It is thought that the elution of cobalt from the positive electrode active material can be prevented and the deterioration of the positive electrode can be prevented.

  Further, the element M in the general formula (2) contributes to the stability and the like of the zirconium-containing lithium cobalt composite oxide (A), so that the zirconium-containing lithium cobalt composite oxide (A) has a sufficient capacity. A range of s ≦ 0.03 is preferred.

  In particular, Al and Mg can stabilize the crystal structure of the lithium cobalt composite oxide.

  The average particle size of the zirconium-containing lithium cobalt composite oxide (A) is preferably 10 to 35 μm. Since stability is high if it is this range, even when it stores at high temperature, deterioration of a positive electrode active material can be prevented. Even if zirconium is present on the surface, the volume ratio of zirconium to the entire particle surface of the lithium cobalt composite oxide represented by the general formula (2) is small, so that hydrogen fluoride does not cause a decrease in capacity. Adsorption effect can be obtained.

The positive electrode active material may contain the lithium cobalt composite oxide (B) represented by the general formula (3).
Li _z Co _1-y A _y O ₂ (3)
(In general formula (3), A contains at least one element selected from Mg and Al. Z and y are in the range of 0.05 ≦ z ≦ 1.15 and 0 ≦ y ≦ 0.05, respectively. Within.)

The average particle size of the lithium cobalt composite oxide (B) is preferably 2 to 10 μm. If the particles are smaller than the zirconium-containing lithium cobalt composite oxide (A), the filling property of the positive electrode active material in the positive electrode mixture layer is increased, and the positive electrode mixture density is improved. By including Mg and / or Al that contributes to stabilization of the crystal structure, stability is high even when the size is small, and deterioration does not occur even during high-temperature storage.

  The positive electrode active material may contain the lithium-containing composite oxide (C) represented by the general formula (4).

Li _{1 + a} Ni _1-bcd Co _b Mn _c M ¹ _d O ₂ (4)

In the general formula (4), M ¹ is selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P, Ba, and Bi. At least one element, −0.15 ≦ a ≦ 0.15, 0.005 ≦ b ≦ 0.4, 0.005 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.03, and b + c + d ≦ 0.7.

  Since the lithium-containing composite oxide (C) has a larger capacity, when it is included in the positive electrode active material, the battery capacity can be increased.

  The average particle size of the lithium-containing composite oxide (C) can be 2 to 35 μm. From the viewpoint of increasing the filling property of the positive electrode active material in the positive electrode mixture layer, the average particle size of (A) <(C It is preferable to satisfy the relationship of the average particle size of () <average particle size of (B).

  The zirconium-containing lithium cobalt composite oxide (A) is preferably contained in an amount of 50% by mass or more based on all positive electrode active materials. This is because, within this range, even if hydrogen fluoride is generated, the adsorption can be performed sufficiently. Furthermore, when the lithium cobalt composite oxide (B) and the zirconium-containing lithium cobalt composite oxide (A) are used in combination, it is preferable that s <y. Since the lithium cobalt composite oxide (B) has a small particle diameter, the content of the element A is more stable, but the battery capacity may be reduced accordingly. However, since the zirconium-containing lithium cobalt composite oxide (A) has a relatively large particle size and high original stability, the amount of the element M may be small, and there is no concern that the battery capacity will be reduced accordingly. Therefore, the safety of the battery is improved by improving the stability of the lithium cobalt composite oxide (B), and the battery capacity can be increased by the zirconium-containing lithium cobalt composite oxide (A) occupying 50% by mass or more in the positive electrode active material. I can do it.

When the positive electrode active material is composed of a zirconium-containing lithium cobalt composite oxide (A) and a lithium cobalt composite oxide (B), a mixing ratio of 50:50 to 95: 5 is preferable. Moreover, when comprising a positive electrode active material with a zirconium containing lithium cobalt complex oxide (A) and a lithium containing complex oxide (C), the mixing ratio of 50: 50-95: 5 is preferable by mass ratio. Within these ranges, the positive electrode mixture density can be increased. In addition, within this range, even if hydrogen fluoride is generated, the adsorption can be sufficiently performed. Also, zirconium-containing lithium cobalt composite oxide (A), lithium cobalt composite oxide (B), and lithium-containing composite oxidation. When a positive electrode active material is comprised with a thing (C), it is preferable to mix by 50-90 mass%, 5-20 mass%, and 5-40 mass%, respectively.

  This is because the positive electrode mixture density can be increased within this range, and when it is within this range, sufficient adsorption can be performed even if hydrogen fluoride is generated.

The average particle diameter of the positive electrode active material referred to in this specification is a volume-based measurement measured by dispersing a measurement object in a medium that does not dissolve using a laser scattering particle size distribution meter (for example, “Microtrack HRA” manufactured by Nikkiso Co., Ltd.) It means D _50% , which is the value of the particle diameter at an integrated fraction of 50%.

  For example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like is preferably used for the binder related to the positive electrode mixture layer. In addition, as the conductive auxiliary agent related to the positive electrode mixture layer, for example, graphite (graphite carbon material) such as natural graphite (flaky graphite), artificial graphite; acetylene black, ketjen black, channel black, furnace black, Carbon materials such as carbon black such as lamp black and thermal black; carbon fiber;

  For the positive electrode, for example, a paste-like or slurry-like positive electrode mixture-containing composition in which a positive electrode active material, a binder, a conductive auxiliary agent, and the like are dispersed in a solvent such as N-methyl-2-pyrrolidone (NMP) is prepared ( However, the binder may be dissolved in a solvent), and this is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the positive electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other methods.

  Moreover, you may form the lead body for electrically connecting with the other member in a nonaqueous electrolyte secondary battery according to a conventional method as needed.

  The thickness of the positive electrode mixture layer is preferably, for example, 10 to 100 μm per one side of the current collector. Moreover, as a composition of a positive mix layer, it is preferable that the quantity of a positive electrode active material is 60-95 mass%, for example, it is preferable that the quantity of a binder is 1-15 mass%, and the quantity of a conductive support agent. Is preferably 3 to 20% by mass.

  As the current collector for the positive electrode, the same one as used for the positive electrode of a conventionally known non-aqueous electrolyte secondary battery can be used. For example, an aluminum foil having a thickness of 10 to 30 μm is preferable.

  The negative electrode according to the non-aqueous electrolyte secondary battery includes, for example, a negative electrode mixture layer made of a negative electrode active material and a binder, and a negative electrode mixture containing a conductive auxiliary agent as necessary, on one or both sides of the current collector. Can be used.

  Examples of the negative electrode active material include graphite, pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, activated carbon, and metals that can be alloyed with lithium (Si , Sn, etc.) or alloys thereof, oxides, etc., and one or more of these can be used.

For example, in order to increase the capacity of a battery, among the negative electrode active materials, a material containing Si and O as constituent elements (provided that the atomic ratio x of O to Si is 0.5 ≦ x ≦ 1.5) Hereinafter, the material can be referred to as “SiO _x ”.

The SiO _x may contain Si microcrystal or amorphous phase. In this case, the atomic ratio of Si and O is a ratio including Si microcrystal or amorphous phase Si. That is, SiO _x includes a structure in which Si (for example, microcrystalline Si) is dispersed in an amorphous SiO ₂ matrix, and this amorphous SiO ₂ is dispersed in the SiO ₂ matrix. It is sufficient that the atomic ratio x satisfies 0.5 ≦ x ≦ 1.5 in combination with Si. For example, in the case of a material in which Si is dispersed in an amorphous SiO ₂ matrix and the material has a molar ratio of SiO ₂ to Si of 1: 1, x = 1, so that the structural formula is represented by SiO. The In the case of a material having such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si Can be confirmed.

Then, SiO _x is preferably a complex complexed with carbon materials, for example, it is desirable that the surface of the SiO _x is coated with a carbon material. As described above, since SiO _x has poor conductivity, when using it as a negative electrode active material, from the viewpoint of securing good battery characteristics, a conductive material (conductive aid) is used, and SiO _x in the negative electrode is used. It is necessary to form an excellent conductive network by mixing and dispersing the material and the conductive material well. If complexes complexed with carbon material SiO _x, for example, simply than with a material obtained by mixing a conductive material such as SiO _x and the carbon material, good conductive network in the negative electrode Formed.

The complex of the SiO _x and the carbon material, as described above, other although the surface of the SiO _x coated with carbon material, such as granules of SiO _x and the carbon material can be cited.

In addition, since the composite in which the surface of SiO _x is coated with a carbon material is further combined with a conductive material (carbon material or the like), a better conductive network can be formed in the negative electrode. Thus, it is possible to realize a non-aqueous electrolyte secondary battery with higher capacity and better battery characteristics (for example, charge / discharge cycle characteristics). The complex of the SiO _x and the carbon material coated with a carbon material, for example, like granules the mixture was further granulated with SiO _x and the carbon material coated with a carbon material.

Further, as SiO _x whose surface is coated with a carbon material, the surface of a composite (for example, a granulated body) of SiO _x and a carbon material having a smaller specific resistance value is further coated with a carbon material. Those can also be preferably used. In a non-aqueous electrolyte secondary battery having a negative electrode containing SiO _x as a negative electrode active material, since a better conductive network can be formed when SiO _x and a carbon material are dispersed inside the granule, Battery characteristics such as heavy load discharge characteristics can be further improved.

Preferred examples of the carbon material that can be used to form a composite with SiO _x include carbon materials such as low crystalline carbon, carbon nanotubes, and vapor grown carbon fibers.

The details of the carbon material include at least one selected from the group consisting of fibrous or coiled carbon materials, carbon black (including acetylene black and ketjen black), artificial graphite, graphitizable carbon, and non-graphitizable carbon. A seed material is preferred. Fibrous or coil-like carbon materials are preferable in that they easily form a conductive network and have a large surface area. Carbon black (including acetylene black and ketjen black), graphitizable carbon, and non-graphitizable carbon have high electrical conductivity and high liquid retention, and even if SiO _x particles expand and contract. This is preferable in that it has a property of easily maintaining contact with the particles.

Further, it is preferable to use graphite as a negative electrode active material together with SiO _x , but it is also possible to use this graphite as a carbon material related to a composite of SiO _x and a carbon material. Graphite, like carbon black, has high electrical conductivity and high liquid retention, and also has the property of easily maintaining contact with the SiO _x particles even when they expand and contract. Therefore, it can be preferably used for forming a complex with SiO _x .

Among the carbon materials exemplified above, a fibrous carbon material is particularly preferable for use when the composite with SiO _x is a granulated body. Fibrous carbon material can follow the expansion and contraction of SiO _x with the charging and discharging of the battery due to the high shape is thin threadlike flexibility, also because bulk density is large, many and SiO _x particles It is because it can have a junction. Examples of the fibrous carbon include polyacrylonitrile (PAN) -based carbon fiber, pitch-based carbon fiber, vapor-grown carbon fiber, and carbon nanotube, and any of these may be used.

The fibrous carbon material can also be formed on the surface of the SiO _x particles by, for example, a vapor phase method.

The specific resistance value of SiO _x is usually 10 ³ to 10 ⁷ kΩcm, while the specific resistance value of the carbon material exemplified above is usually 10 ^{−5 to} 10 kΩcm.

The composite of SiO _x and the carbon material may further have a material layer (a material layer containing non-graphitizable carbon) that covers the carbon material coating layer on the particle surface.

When a composite of SiO _x and a carbon material is used for the negative electrode, the ratio of SiO _x and the carbon material is based on SiO _x : 100 parts by mass from the viewpoint of satisfactorily exerting the effect of the composite with the carbon material. The carbon material is preferably 5 parts by mass or more, and more preferably 10 parts by mass or more. Further, in the composite, if the ratio of the carbon material to be combined with SiO _x is too large, it may lead to a decrease in the amount of SiO _x in the negative electrode mixture layer, and the effect of increasing the capacity may be reduced. SiO _x: relative to 100 parts by weight, the carbon material, and more preferably preferably not more than 50 parts by weight, more than 40 parts by weight.

The composite of the SiO _x and the carbon material can be obtained, for example, by the following method.

First, a manufacturing method in the case of combining SiO _x will be described. A dispersion liquid in which SiO _x is dispersed in a dispersion medium is prepared, and sprayed and dried to produce composite particles including a plurality of particles. For example, ethanol or the like can be used as the dispersion medium. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. In addition to the above method, similar composite particles can be produced also by a granulation method by a mechanical method using a vibration type or planetary type ball mill or rod mill.

Incidentally, the SiO _x, in the case of manufacturing a granulated body with small carbon material resistivity value than SiO _x is adding the carbon material in the dispersion liquid of SiO _x are dispersed in a dispersion medium, the dispersion by using a liquid, by a similar method to the case of composite of SiO _x may be a composite particle (granule). Further, by granulation process according to the similar mechanical method, it is possible to produce a granular material of the SiO _x and the carbon material.

Next, when the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is coated with a carbon material to form a composite, for example, the SiO _x particles and the hydrocarbon-based material The gas is heated in the gas phase, and carbon generated by pyrolysis of the hydrocarbon-based gas is deposited on the surface of the particles. Thus, according to the vapor phase growth (CVD) method, the hydrocarbon-based gas spreads to every corner of the composite particle, and the surface of the particle and the pores on the surface are thin and contain a conductive carbon material. Since a uniform film (carbon material coating layer) can be formed, the SiO _x particles can be imparted with good conductivity with a small amount of carbon material.

In the production of SiO _x coated with a carbon material, the processing temperature (atmosphere temperature) of the vapor deposition (CVD) method varies depending on the type of hydrocarbon gas, but usually 600 to 1200 ° C. is appropriate. Among these, the temperature is preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the treatment temperature, the less the remaining impurities, and the formation of a coating layer containing carbon having high conductivity.

  As the liquid source of the hydrocarbon-based gas, toluene, benzene, xylene, mesitylene and the like can be used, but toluene that is easy to handle is particularly preferable. A hydrocarbon-based gas can be obtained by vaporizing them (for example, bubbling with nitrogen gas). Moreover, methane gas, acetylene gas, etc. can also be used.

In addition, after the surface of SiO _x particles (SiO _x composite particles or a granulated body of SiO _x and a carbon material) is covered with a carbon material by a vapor deposition (CVD) method, a petroleum-based pitch or a coal-based pitch is used. At least one organic compound selected from the group consisting of a thermosetting resin and a condensate of naphthalene sulfonate and aldehydes is attached to a coating layer containing a carbon material, and then the organic compound is attached. The obtained particles may be fired.

Specifically, a dispersion liquid in which a SiO _x particle (SiO _x composite particle or a granulated body of SiO _x and a carbon material) coated with a carbon material and the organic compound are dispersed in a dispersion medium is prepared, The dispersion is sprayed and dried to form particles coated with the organic compound, and the particles coated with the organic compound are fired.

  An isotropic pitch can be used as the pitch, and a phenol resin, a furan resin, a furfural resin, or the like can be used as the thermosetting resin. As the condensate of naphthalene sulfonate and aldehydes, naphthalene sulfonic acid formaldehyde condensate can be used.

As a dispersion medium for dispersing the SiO _x particles coated with the carbon material and the organic compound, for example, water or alcohols (ethanol or the like) can be used. It is appropriate to spray the dispersion in an atmosphere of 50 to 300 ° C. The firing temperature is usually 600 to 1200 ° C., preferably 700 ° C. or higher, and more preferably 800 ° C. or higher. This is because the higher the processing temperature, the less the remaining impurities, and the formation of a coating layer containing a high-quality carbon material with high conductivity. However, the processing temperature needs to be lower than the melting point of SiO _x .

When using SiO _x as the negative electrode active material, it is preferable to use graphite as the negative electrode active material. By reducing the ratio of SiO _x in the negative electrode active material using graphite, the negative electrode (negative electrode mixture) associated with charging / discharging of the battery is suppressed as much as possible while suppressing the decrease in the capacity-enhancing effect due to the reduction of SiO _x. It is possible to suppress a change in volume of the layer) and to suppress a decrease in battery characteristics that may be caused by the volume change.

Examples of graphite used as the negative electrode active material together with SiO _x include natural graphite such as flaky graphite; graphitizable carbon such as pyrolytic carbons, mesophase carbon microbeads (MCMB), and carbon fibers at 2800 ° C. or more. Artificial graphite subjected to chemical treatment; and the like.

In the negative electrode, from the viewpoint of satisfactorily ensuring the effect of the high capacity by using a SiO _x, the content of SiO _x in the anode active material is preferably at least 0.01 wt%, It is more preferable that it is 3 mass% or more. Further, from the viewpoint of better avoiding the problem due to the volume change of the negative electrode due to charge / discharge, the content of SiO _x in the negative electrode active material is preferably 30% by mass or less, and 20% by mass or less. Is more preferable.

  Moreover, the same thing as what was illustrated previously as what can be used for a positive electrode can be used for the binder and conductive support agent of a negative electrode.

  For the negative electrode, for example, a negative electrode active material, a binder, and a conductive auxiliary agent used as necessary are prepared in a paste-like or slurry-like negative electrode mixture-containing composition in which a solvent such as NMP or water is dispersed. (However, the binder may be dissolved in a solvent), which is applied to one or both sides of the current collector, dried, and then subjected to a calendering process as necessary. However, the negative electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other methods.

  Moreover, you may form in the negative electrode the lead body for electrically connecting with the other member in a nonaqueous electrolyte secondary battery according to a conventional method as needed.

  The thickness of the negative electrode mixture layer is preferably 10 to 100 μm per one surface of the current collector, for example. Moreover, as a composition of a negative mix layer, it is preferable that a negative electrode active material shall be 80.0-99.8 mass% and a binder shall be 0.1-10 mass%, for example. Furthermore, when making a negative mix layer contain a conductive support agent, it is preferable that the quantity of the conductive support agent in a negative mix layer shall be 0.1-10 mass%.

  As the current collector for the negative electrode, a foil made of copper or nickel, a punching metal, a net, an expanded metal, or the like can be used, but a copper foil is usually used. In the negative electrode current collector, when the thickness of the entire negative electrode is reduced in order to obtain a battery having a high energy density, the upper limit of the thickness is preferably 30 μm, and the lower limit is 5 μm in order to ensure mechanical strength. Is desirable.

(Separator)
As the separator according to the non-aqueous electrolyte secondary battery of the present invention, a separator having sufficient strength and capable of holding a large amount of non-aqueous electrolyte is preferable, and has a thickness of 5 to 50 μm and an open area ratio of 30 to 70%. A microporous membrane made of polyolefin such as PE) or polypropylene (PP) can be used. The microporous membrane constituting the separator may be, for example, one using only PE or one using only PP, may contain an ethylene-propylene copolymer, and may be made of PE. A laminate of a membrane and a PP microporous membrane may be used.

  In addition, a separator for a non-aqueous electrolyte secondary battery includes a porous layer (a) mainly composed of a resin having a melting point of 140 ° C. or less, a resin having a melting point of 150 ° C. or more, or an inorganic filler having a heat resistant temperature of 150 ° C. or more. It is possible to use a laminated separator composed of a porous layer (b) containing as a main component. Here, “melting point” means a melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of JIS K 7121. “Heat resistant temperature is 150 ° C. or higher” means at least 150 ° C. This means that no deformation such as softening is observed.

  The porous layer (a) according to the multilayer separator is mainly for ensuring a shutdown function, and the non-aqueous electrolyte secondary battery is a resin that is a main component of the porous layer (a). When the temperature exceeds the melting point, the resin related to the porous layer (a) melts to close the pores of the separator, thereby causing a shutdown that suppresses the progress of the electrochemical reaction.

  Examples of the resin having a melting point of 140 ° C. or lower as the main component of the porous layer (a) include PE, and the form thereof is a microporous film used for a nonaqueous electrolyte secondary battery, a base such as a nonwoven fabric, and the like. The thing which apply | coated the particle | grains of PE to the material is mentioned. Here, in all the constituent components of the porous layer (a), the volume of the resin having a main melting point of 140 ° C. or less is 50% by volume or more, and more preferably 70% by volume or more. When the porous layer (a) is formed of the PE microporous film, the volume is 100% by volume.

  The porous layer (b) according to the multilayer separator has a function of preventing a short circuit due to direct contact between the positive electrode and the negative electrode even when the internal temperature of the nonaqueous electrolyte secondary battery is increased. The function is secured by a resin having a melting point of 150 ° C. or higher or an inorganic filler having a heat resistant temperature of 150 ° C. or higher. That is, when the battery becomes high temperature, even if the porous layer (a) shrinks, the porous layer (b) which does not easily shrink can directly generate the positive and negative electrodes which can be generated when the separator is thermally contracted. It is possible to prevent a short circuit due to the contact of. Moreover, since this heat-resistant porous layer (b) acts as a skeleton of the separator, the thermal contraction of the porous layer (a), that is, the thermal contraction of the entire separator itself can be suppressed.

  When the porous layer (b) is mainly formed of a resin having a melting point of 150 ° C. or higher, the form thereof is, for example, a microporous film formed of a resin having a melting point of 150 ° C. or higher (for example, the above-described battery made of PP The composition for forming a porous layer (b) containing fine particles of a resin having a melting point of 150 ° C. or higher is applied to the porous layer (a). And a laminated type form in which a porous layer (b) containing fine particles of a resin having a melting point of 150 ° C. or higher is laminated.

  As the resin constituting the fine particles of the resin having a melting point of 150 ° C. or more, crosslinked polymethyl methacrylate, crosslinked polystyrene, crosslinked polydivinylbenzene, crosslinked styrene-divinylbenzene copolymer, polyimide, melamine resin, phenol resin, benzoguanamine- And various cross-linked polymers such as formaldehyde condensates; heat-resistant polymers such as PP, polysulfone, polyether sulfone, polyphenylene sulfide, PTFE, polyacrylonitrile, aramid, and polyacetal.

The particle diameter of resin fine particles having a melting point of 150 ° C. or higher is an average particle diameter D of _50% measured by the same method as that for the positive electrode active material used in the nonaqueous electrolyte secondary battery of the present invention, for example, 0.01 μm or more. Preferably, it is 0.1 μm or more, more preferably 10 μm or less, and even more preferably 2 μm or less.

  Since the amount of fine particles of the resin having a melting point of 150 ° C. or higher is mainly contained in the porous layer (b), the total volume of the constituent components of the porous layer (b) (the total volume excluding the void portion) ), It is preferably 50% by volume or more, preferably 70% by volume or more, more preferably 80% by volume or more, and still more preferably 90% by volume or more.

  When the porous layer (b) is mainly formed of an inorganic filler having a heat resistant temperature of 150 ° C. or higher, a composition for forming the porous layer (b) containing the inorganic filler having a heat resistant temperature of 150 ° C. or higher (coating liquid) ) Is applied to the porous layer (a), and a porous layer (b) containing an inorganic filler having a heat resistant temperature of 150 ° C. or higher is laminated.

  The inorganic filler according to the porous layer (b) has a heat-resistant temperature of 150 ° C. or higher, is stable with respect to the non-aqueous electrolyte of the non-aqueous electrolyte secondary battery, and further in the operating voltage range of the non-aqueous electrolyte secondary battery. Any electrochemically stable material that is difficult to be oxidized or reduced may be used, but fine particles are preferable from the viewpoint of dispersion and the like, and alumina, silica, and boehmite are preferable. Alumina, silica and boehmite have high oxidation resistance, and the particle size and shape can be adjusted to desired values, etc., so it is easy to accurately control the porosity of the porous layer (b). It becomes. In addition, as for the inorganic filler whose heat-resistant temperature is 150 degreeC or more, the thing of the said illustration may be used individually by 1 type and may use 2 or more types together, for example. Also, an inorganic filler having a heat resistant temperature of 150 ° C. or higher and a resin fine particle having a melting point of 150 ° C. or higher may be used in combination.

  There is no restriction | limiting in particular about the shape of the inorganic filler whose heat-resistant temperature which concerns on a porous layer (b) is 150 degreeC or more, A substantially spherical shape (a true spherical shape is included), a substantially ellipsoid shape (an ellipsoid shape is included), plate shape, etc. Various shapes can be used.

Moreover, if the average particle diameter of the inorganic filler having a heat resistant temperature of 150 ° C. or higher (the average particle diameter of the plate-like filler and the other-shaped filler. The same applies hereinafter) related to the porous layer (b) is too small, the ion permeability is high. Since it falls, it is preferable that it is 0.3 micrometer or more, and it is more preferable that it is 0.5 micrometer or more. In addition, if the inorganic filler having a heat resistant temperature of 150 ° C. or higher is too large, the electrical characteristics are likely to be deteriorated. Therefore, the average particle diameter is preferably 5 μm or less, and more preferably 2 μm or less. The average particle diameter of the inorganic filler having a heat resistant temperature of 150 ° C. or higher as used herein is an average particle diameter (D _50% ) determined by the same method as the average particle diameter of the negative electrode active material.

  Since the inorganic filler having a heat resistant temperature of 150 ° C. or higher in the porous layer (b) is mainly contained in the porous layer (b), the amount in the porous layer (b) is the amount of the porous layer (b). Is 50% by volume or more, preferably 70% by volume or more, more preferably 80% by volume or more, and 90% by volume or more. More preferably it is. By making the content of the inorganic filler in the porous layer (b) high as described above, it is possible to satisfactorily suppress the thermal contraction of the entire separator even when the nonaqueous electrolyte secondary battery becomes high temperature. And the occurrence of a short circuit due to direct contact between the positive electrode and the negative electrode can be more effectively suppressed.

  In the case where the inorganic filler having a heat resistant temperature of 150 ° C. or higher and the resin fine particles having a melting point of 150 ° C. or higher are used in combination, it suffices that these two together form the main body of the porous layer (b), Specifically, the total amount of these components may be 50% by volume or more in the total volume of the constituent components of the porous layer (b) (total volume excluding the pores), and 70% by volume or more. It is preferable to make it 80% by volume or more, more preferably 90% by volume or more. Thereby, the effect similar to the case where the inorganic filler in a porous layer (b) is made into high content as mentioned above is securable.

  In the porous layer (b), resin fine particles having a melting point of 150 ° C. or higher or inorganic fillers having a heat resistant temperature of 150 ° C. or higher are bound, or the porous layer (b) and the porous layer (a) For example, an organic binder is preferably contained in order to integrate them. Examples of organic binders include ethylene-vinyl acetate copolymers (EVA, those having a structural unit derived from vinyl acetate of 20 to 35 mol%), ethylene-acrylic acid copolymers such as ethylene-ethyl acrylate copolymers, and fluorine-based binders. Examples include rubber, SBR, CMC, hydroxyethyl cellulose (HEC), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), polyvinyl pyrrolidone (PVP), cross-linked acrylic resin, polyurethane, and epoxy resin. A heat-resistant binder having a heat-resistant temperature is preferably used. As the organic binder, those exemplified above may be used singly or in combination of two or more.

  Among the organic binders exemplified above, highly flexible binders such as EVA, ethylene-acrylic acid copolymer, fluorine-based rubber, and SBR are preferable. Specific examples of such highly flexible organic binders include Mitsui DuPont Polychemical's “Evaflex Series (EVA)”, Nihon Unicar's EVA, Mitsui DuPont Polychemical's “Evaflex-EAA Series (Ethylene). -Acrylic acid copolymer) ", Nippon Unicar EEA, Daikin Industries" DAI-EL Latex Series (Fluororubber) ", JSR" TRD-2001 (SBR) ", Nippon Zeon" BM-400B " (SBR) ".

  When the organic binder is used in the porous layer (b), it may be used in the form of an emulsion dissolved or dispersed in a solvent of a composition for forming a porous layer (b) described later.

  The coated laminate type separator is, for example, a porous layer (b) forming composition (liquid composition such as slurry) containing fine particles of a resin having a melting point of 150 ° C. or higher or an inorganic filler having a heat resistant temperature of 150 ° C. or higher. Etc.) may be applied to the surface of the microporous membrane for constituting the porous layer (a) and dried at a predetermined temperature to form the porous layer (b).

  The composition for forming the porous layer (b) contains fine particles of a resin having a melting point of 150 ° C. or higher or an inorganic filler having a heat resistant temperature of 150 ° C. or higher, and an organic binder as necessary. Including the medium. The same shall apply hereinafter.) The organic binder can be dissolved in a solvent. The solvent used in the composition for forming the porous layer (b) is not particularly limited as long as it can uniformly disperse the inorganic filler and can uniformly dissolve or disperse the organic binder. Common organic solvents such as hydrocarbons, furans such as tetrahydrofuran, and ketones such as methyl ethyl ketone and methyl isobutyl ketone are preferably used. In addition, for the purpose of controlling the interfacial tension, alcohols (ethylene glycol, propylene glycol, etc.) or various propylene oxide glycol ethers such as monomethyl acetate may be appropriately added to these solvents. In addition, when the organic binder is water-soluble or used as an emulsion, water may be used as a solvent. In this case, alcohols (methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, etc.) are appropriately added. It is also possible to control the interfacial tension.

  The composition for forming the porous layer (b) has a solid content containing, for example, 10 to 80% by mass of resin fine particles having a melting point of 150 ° C. or higher, an inorganic filler having a heat resistant temperature of 150 ° C. or higher, and an organic binder. It is preferable to do.

  In the laminated separator, the porous layer (a) and the porous layer (b) do not have to be one each, and a plurality of layers may be present in the separator. For example, it is good also as a structure which has arrange | positioned the porous layer (a) on both surfaces of the porous layer (b), or has arrange | positioned the porous layer (b) on both surfaces of the porous layer (a). However, increasing the number of layers may increase the thickness of the separator and increase the internal resistance of the battery or decrease the energy density. Therefore, it is not preferable to increase the number of layers. The total number of layers of the porous layer (a) and the porous layer (b) is preferably 5 or less.

  The thickness of the separator (a separator made of a microporous membrane made of polyolefin or the laminated separator) according to the nonaqueous electrolyte secondary battery is more preferably 10 to 30 μm.

  In the laminated separator, the thickness of the porous layer (b) [when the separator includes a plurality of porous layers (b), the total thickness] From the viewpoint of exhibiting more effectively, it is preferably 3 μm or more. However, if the porous layer (b) is too thick, the energy density of the battery may be lowered. Therefore, the thickness of the porous layer (b) is preferably 8 μm or less.

  Further, in the laminated separator, the thickness of the porous layer (a) [when the separator has a plurality of porous layers (I), the total thickness thereof. same as below. ] Is preferably 6 μm or more, more preferably 10 μm or more, from the viewpoint of more effectively exerting the above-described action (particularly the shutdown action) due to the use of the porous layer (a). However, if the porous layer (a) is too thick, there is a possibility that the energy density of the battery may be lowered. In addition, the force that the porous layer (a) tends to shrink is increased, and the heat of the entire separator is increased. There is a possibility that the action of suppressing the shrinkage becomes small. Therefore, the thickness of the porous layer (a) is preferably 25 μm or less, more preferably 20 μm or less, and still more preferably 14 μm or less.

  The porosity of the separator as a whole is preferably 30% or more in a dried state in order to secure the amount of electrolyte solution retained and to improve ion permeability. On the other hand, from the viewpoint of securing separator strength and preventing internal short circuit, the separator porosity is preferably 70% or less in a dry state. The porosity of the separator: P (%) can be calculated by obtaining the sum for each component i from the thickness of the separator, the mass per area, and the density of the constituent components using the following equation (5).

P = {1- (m / t) / (Σa _i · ρ _i )} × 100 (5)
Here, in the above formula, a _i : ratio of component i when the total mass is 1, ρ _i : density of component i (g / cm ³ ), m: mass per unit area of the separator (g / cm ² ), t: thickness of separator (cm).

In the case of the multilayer separator, in the formula (5), m is the mass per unit area (g / cm ² ) of the porous layer (a), and t is the thickness of the porous layer (a) ( (cm), the porosity: P (%) of the porous layer (a) can also be obtained using the formula (5). It is preferable that the porosity of the porous layer (a) calculated | required by this method is 30 to 70%.

Further, in the case of the laminated separator, in the formula (5), m is the mass per unit area (g / cm ² ) of the porous layer (b), and t is the thickness of the porous layer (b) ( cm), the porosity: P (%) of the porous layer (b) can also be obtained using the above equation (5). It is preferable that the porosity of the porous layer (b) calculated | required by this method is 20 to 60%.

  The separator preferably has high mechanical strength. For example, the puncture strength is preferably 3N or more. As described above, the negative electrode active material has a large volume expansion / contraction during charging / discharging, and mechanical damage is also applied to the facing separator due to expansion / contraction of the entire negative electrode due to repeated charging / discharging cycles. If the piercing strength of the separator is 3N or more, good mechanical strength is ensured, and mechanical damage to the separator can be reduced.

  Examples of the separator having a puncture strength of 3N or more include the above-described laminated separator, and in particular, a porous layer (a) mainly composed of a resin having a melting point of 140 ° C. or lower and an inorganic filler having a heat resistant temperature of 150 ° C. or higher. A separator in which a porous layer (b) containing as a main component is laminated is preferable. This is probably because the mechanical strength of the inorganic filler is high, so that the mechanical strength of the entire separator can be increased by supplementing the mechanical strength of the porous layer (a).

The puncture strength can be measured by the following method. A separator is fixed on a plate having a hole with a diameter of 2 inches so as not to be wrinkled or bent, and a semispherical metal pin having a tip diameter of 1.0 mm is lowered onto a measurement sample at a speed of 120 mm / min. Measure the force when making a hole in the separator 5 times. And an average value is calculated | required about the measurement of 3 times except the maximum value and the minimum value among the said measurement values of 5 times, and this is made into the piercing strength of a separator.
[Non-aqueous electrolyte]
The nonaqueous electrolyte secondary battery of the present invention uses a fluorine-containing electrolyte salt and a phosphonoacetate compound represented by the general formula (1).

In the general formula (1), R ¹ , R ² and R ³ each independently represents an alkyl group having 1 to 12 carbon atoms, an alkenyl group or an alkynyl group which may be substituted with a halogen atom, and n Represents an integer of 0-6.

  Examples of the phosphonoacetate compound include the following compounds.

<Compound with n = 0 in the general formula (1)>
Trimethyl phosphonoformate, methyl diethylphosphonoformate, methyl dipropylphosphonoformate, methyl dibutylphosphonoformate, triethyl phosphonoformate, ethyl dimethylphosphonoformate, ethyl dipropylphosphonoformate, ethyl Dibutyl phosphonoformate, tripropyl phosphonoformate, propyl dimethylphosphonoformate, propyl diethylphosphonoformate, propyl dibutylphosphonoformate, tributyl phosphonoformate, butyl dimethylphosphonoformate, butyl diethylphospho Noformate, butyl dipropylphosphonoformate, methyl bis (2,2,2-trifluoroethyl) phosphonoformate, ethyl bis (2 2,2-trifluoroethyl) phosphonoacetate formate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate, butyl bis (2,2,2-trifluoroethyl) phosphonoacetate formate and the like.

<Compound with n = 1 in the general formula (1)>
Trimethyl phosphonoacetate, methyl diethyl phosphonoacetate, methyl dipropyl phosphonoacetate, methyl dibutyl phosphonoacetate, triethyl phosphonoacetate, ethyl dimethylphosphonoacetate, ethyl dipropylphosphonoacetate, ethyl dibutylphosphonoacetate, tripropyl Phosphonoacetate, propyl dimethylphosphonoacetate, propyl diethylphosphonoacetate, propyl dibutylphosphonoacetate, tributyl phosphonoacetate, butyldimethylphosphonoacetate, butyldiethylphosphonoacetate, butyldipropylphosphonoacetate, methylbis (2 , 2,2-trifluoroethyl) phosphonoacetate, ethyl bis (2,2,2-trifluoroethyl) phos No acetate, propyl bis (2,2,2-trifluoroethyl) phosphonoacetate, butyl bis (2,2,2-trifluoroethyl) phosphonoacetate, allyl dimethylphosphonoacetate, allyl diethylphosphonoacetate, 2 -Propynyl dimethylphosphonoacetate, 2-propynyl diethylphosphonoacetate, etc.

<Compound with n = 2 in the general formula (1)>
Trimethyl 3-phosphonopropionate, methyl 3- (diethylphosphono) propionate, methyl 3- (dipropylphosphono) propionate, methyl 3- (dibutylphosphono) propionate, triethyl 3-phosphonopropionate, ethyl 3- (dimethylphosphono) propionate, ethyl 3- (dipropylphosphono) propionate, ethyl 3- (dibutylphosphono) propionate, tripropyl 3-phosphonopropionate, propyl 3- (dimethylphosphono) propionate, Propyl 3- (diethylphosphono) propionate, propyl 3- (dibutylphosphono) propionate, tributyl 3-phosphonopropionate, butyl 3- (dimethylphosphono) propionate, butyl 3- (diethylphosphono) prop Pionate, butyl 3- (dipropylphosphono) propionate, methyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, ethyl 3- (bis (2,2,2-trifluoroethyl) phosphono ) Propionate, propyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate, butyl 3- (bis (2,2,2-trifluoroethyl) phosphono) propionate and the like.

<Compound with n = 3 in the general formula (1)>
Trimethyl 4-phosphonobutyrate, methyl 4- (diethylphosphono) butyrate, methyl 4- (dipropylphosphono) butyrate, methyl 4- (dibutylphosphono) butyrate, triethyl 4-phosphonobutyrate, ethyl 4- (Dimethylphosphono) butyrate, ethyl 4- (dipropylphosphono) butyrate, ethyl 4- (dibutylphosphono) butyrate, tripropyl 4-phosphonobutyrate, propyl 4- (dimethylphosphono) butyrate, propyl 4- (Diethylphosphono) butyrate, propyl dibutylphosphono) butyrate, tributyl 4-phosphonobutyrate, butyl 4- (dimethylphosphono) butyrate, butyl 4- (diethylphosphono) butyrate, butyl 4- (dipropylphosphono) ) Butyrate etc.

  Among the exemplified phosphonoacetate compounds, ethyldiethylphosphonoacetate (EDPA) and 2-propynyl (diethylphosphono) acetate (PDEA) are particularly preferable.

  The phosphonoacetate compound represented by the general formula (1) forms a film on the surface of the negative electrode to prevent the negative electrode and the electrolyte solvent from reacting during high temperature storage of the battery, thereby preventing blistering due to gas generation and deterioration of the negative electrode active material. Thus, the cycle characteristics can be improved by reducing the change in battery thickness.

  From the above viewpoint, the content of the phosphonoacetate compound represented by the general formula (1) in the nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery is 0.1% by mass or more and 5% by mass or less. preferable. More preferably, it is 0.5 mass% or more and 2.5 mass% or less.

  Further, the zirconium-containing lithium cobalt composite oxide (A) described above contains 50% by mass or more with respect to all the positive electrode active materials, or 0.0001 or more in terms of a molar ratio (Zr / Co) to cobalt (Co). When the non-aqueous electrolyte that the phosphonoacetate compound contains in this range is used within the following range, deterioration in the positive electrode and the negative electrode can be prevented in a more balanced manner. This is preferable because the phenomenon in which the rate increases can be delayed more slowly.

  In addition, the nonaqueous electrolyte may also contain a halogen-substituted cyclic carbonate. The halogen-substituted cyclic carbonate also acts on the negative electrode and has an action of suppressing reaction with the electrolyte solvent. Among the cyclic carbonates substituted with a halogen element, 4-fluoro-1,3-dioxolan-2-one (FEC) is particularly preferable.

  The content of the halogen-substituted cyclic carbonate in the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery is preferably 0.1% by mass or more from the viewpoint of better ensuring the effect of its use. More preferably, it is 5% by mass or more. However, if the content of the halogen-substituted cyclic carbonate in the non-aqueous electrolyte is too large, the effect of improving storage characteristics may be reduced. Therefore, the content of the halogen-substituted cyclic carbonate in the non-aqueous electrolyte used for the non-aqueous electrolyte secondary battery is preferably 10% by mass or less, and more preferably 5% by mass or less.

  Further, it is preferable to use a non-aqueous electrolyte that also contains vinylene carbonate (VC). VC acts on a negative electrode (particularly a negative electrode using a carbon material as a negative electrode active material) and has a function of suppressing a reaction between the negative electrode and a nonaqueous electrolyte component. Therefore, a nonaqueous electrolyte secondary battery with better charge / discharge cycle characteristics can be obtained by using a nonaqueous electrolyte that also contains VC.

  The content of VC in the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery is preferably 0.1% by mass or more, more preferably 1.0% by mass or more, from the viewpoint of better ensuring the effect of the use. It is more preferable that However, when there is too much content of VC in a nonaqueous electrolyte, there exists a possibility that the improvement effect of a storage characteristic may become small. Therefore, the content of VC in the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery is preferably 10% by mass or less, and more preferably 4.0% by mass or less.

The lithium salt used for the non-aqueous electrolyte is a fluorine-containing electrolyte salt. For example, inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , Examples thereof include organic lithium salts such as LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2), LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group]. These fluorine-containing electrolyte salts react with moisture inevitably mixed in the battery to generate hydrogen fluoride, causing the above-described problems.

  The concentration of the lithium salt in the nonaqueous electrolyte is preferably 0.5 to 1.5 mol / l, and more preferably 0.9 to 1.25 mol / l.

  The organic solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the lithium salt and does not cause a side reaction such as decomposition in a voltage range used as a battery. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; chain esters such as methyl propionate; cyclic esters such as γ-butyrolactone; dimethoxyethane, Chain ethers such as diethyl ether, 1,3-dioxolane, diglyme, triglyme and tetraglyme; cyclic ethers such as 1,4-dioxane, tetrahydrofuran and 2-methyltetrahydrofuran; acetonitrile, propionitrile, methoxypropionitrile and the like Nitriles; sulfites such as ethylene glycol sulfite; and the like. These may be used in combination of two or more. In order to obtain a battery with better characteristics, it is desirable to use a combination that can obtain high conductivity, such as a mixed solvent of ethylene carbonate and chain carbonate.

  In addition, non-aqueous electrolytes used in non-aqueous electrolyte secondary batteries include acid anhydrides and sulfonate esters for the purpose of further improving charge / discharge cycle characteristics and improving safety such as high-temperature storage and prevention of overcharge. , Additives such as dinitriles such as succinonitrile and adiponitrile, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, t-butylbenzene, 1,3-dioxane (including derivatives thereof) Can also be added as appropriate.

  Further, as the nonaqueous electrolyte of the nonaqueous electrolyte secondary battery, a gel (gel electrolyte) obtained by adding a known gelling agent such as a polymer to the nonaqueous electrolyte can be used.

Example 1
<Creation of positive electrode>
Formula _Li t Co _{1-S M} S in _{O 2 t = 1.03, s =} 0.01 (Mg) , and the content of zirconium (Zr), the molar ratio of cobalt (Co) (Zr / Co) Then, commercially available lithium carbonate, cobalt oxide, zirconium oxide, and magnesium carbonate were mixed so as to be 0.0005, and these were fired in the air to prepare a powdery zirconium-containing lithium cobalt composite oxide. In this example, the zirconium-containing lithium cobalt composite oxide (A) has a lithium cobalt composite oxide represented by Li _1.03 Co _0.99 Mg _0.01 O ₂ , and zirconium oxide is present on the crystal surface. The average particle size was 20 μm.

Then, lithium-cobalt composite oxide and a _{Li 1.04 Co 0.98 Al 0.01 Mg 0.01} O 2 ( average particle size 5 [mu] m) is (B), at a mass ratio (A) :( B) = A mixture of positive electrode active materials mixed at a blending ratio of 85:15 was prepared.

  100 parts by mass of a mixture of positive electrode active materials in the upper stage, 20 parts by mass of an NMP solution containing PVDF as a binder at a concentration of 10% by mass, 1 part by mass of artificial graphite and 1 part by mass of Ketjen Black as a conductive additive Was kneaded using a biaxial kneader, and NMP was further added to adjust the viscosity to prepare a positive electrode mixture-containing paste.

After coating the positive electrode mixture-containing paste on both surfaces of an aluminum foil (positive electrode current collector) having a thickness of 15 μm, vacuum drying is performed at 120 ° C. for 12 hours to form a positive electrode mixture layer on both surfaces of the aluminum foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the positive electrode mixture layer, and a nickel lead body was welded to the exposed portion of the aluminum foil to produce a strip-like positive electrode having a length of 375 mm and a width of 43 mm. . The positive electrode mixture layer in the obtained positive electrode had a thickness of 55 μm per one side.
<Production of negative electrode>
Graphite: 97.5 parts by mass, SBR as a binder: 1.5 parts by mass, and CMC as a thickener: 1 part by mass, water was added and mixed to prepare a negative electrode mixture-containing paste.

After applying the negative electrode mixture-containing paste to both sides of a copper foil (negative electrode current collector) having a thickness of 8 μm, vacuum drying is performed at 120 ° C. for 12 hours to form a negative electrode mixture layer on both sides of the copper foil. did. Thereafter, press treatment was performed to adjust the thickness and density of the negative electrode mixture layer, and a nickel lead body was welded to the exposed portion of the copper foil to produce a strip-shaped negative electrode having a length of 380 mm and a width of 44 mm. . The negative electrode mixture layer in the obtained negative electrode had a thickness of 65 μm per one surface.
<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1, and further 2-propynyl diethyl A non-aqueous electrolyte was prepared by adding phosphonoacetate, 1,3-dioxane, VC and FEC in amounts of 1% by mass.
<Preparation of separator>
5 kg of ion-exchanged water and 0.5 kg of a dispersant (aqueous polycarboxylic acid ammonium salt, solid content concentration 40% by mass) are added to 5 kg of boehmite secondary aggregates having an average particle size of 3 μm. A dispersion was prepared by crushing for 10 hours with a ball mill at 40 times / min. When a part of the treated dispersion was vacuum dried at 120 ° C. and observed with a scanning electron microscope (SEM), the shape of boehmite was almost plate-like. The average particle size of the boehmite after the treatment was 1 μm.

  To 500 g of the above dispersion, 0.5 g of xanthan gum as a thickener and 17 g of a resin binder dispersion (modified polybutyl acrylate, solid content 45% by mass) as a binder are added and stirred with a three-one motor for 3 hours to form a uniform slurry. [Porous layer (b) forming slurry, solid content ratio 50 mass%] was prepared.

PE microporous separator for lithium ion secondary battery [Porous layer (a): thickness 12 μm, porosity 40%, average pore diameter 0.08 μm, PE melting point 135 ° C.] 40 W · min / m ² ), and a slurry for forming a porous layer (b) is applied to the treated surface with a micro gravure coater and dried to form a porous layer (b) having a thickness of 4 μm, Got. The mass per unit area of the porous layer (b) in this separator was 5.5 g / m ² , the boehmite volume content was 95% by volume, and the porosity was 45%.
<Battery assembly>
The belt-like positive electrode is overlapped with the belt-like negative electrode via the separator, wound in a spiral shape, and then pressed so as to be flattened to form an electrode winding body having a flat winding structure. The wound body was fixed with an insulating tape made of polypropylene. Next, the electrode winding body is inserted into a rectangular battery case made of aluminum alloy having an outer dimension of 4.0 mm in thickness, 34 mm in width, and 50 mm in height to weld the lead body, and a lid made of aluminum alloy The plate was welded to the open end of the battery case. Thereafter, the nonaqueous electrolyte is injected from the inlet provided on the cover plate, and left for 1 hour, and then the inlet is sealed. The nonaqueous electrolyte secondary having the structure shown in FIG. A battery was obtained.

  Here, the battery shown in FIGS. 1 and 2 will be described. FIG. 1A is a plan view, and FIG. 1B is a partial cross-sectional view thereof. As shown in FIG. 2 is spirally wound through a separator 3 and then pressed so as to be flattened and accommodated in a rectangular (square tube) battery case 4 together with a nonaqueous electrolyte as a flat wound electrode body 6. Has been. However, in FIG. 1, in order to avoid complication, a metal foil, a non-aqueous electrolyte, and the like as a current collector used for manufacturing the positive electrode 1 and the negative electrode 2 are not illustrated.

  The battery case 4 is made of an aluminum alloy and constitutes an outer package of the battery. The battery case 4 also serves as a positive electrode terminal. And the insulator 5 which consists of PE sheets is arrange | positioned at the bottom part of the battery case 4, and it connects to each one end of the positive electrode 1 and the negative electrode 2 from the flat wound electrode body 6 which consists of the positive electrode 1, the negative electrode 2, and the separator 3. The positive electrode lead body 7 and the negative electrode lead body 8 thus drawn are drawn out. A stainless steel terminal 11 is attached to a sealing lid plate 9 made of aluminum alloy for sealing the opening of the battery case 4 via a polypropylene insulating packing 10, and an insulator 12 is attached to the terminal 11. A stainless steel lead plate 13 is attached.

  And this cover plate 9 is inserted in the opening part of the battery case 4, and the opening part of the battery case 4 is sealed by welding the joint part of both, and the inside of the battery is sealed. Further, in the battery of FIG. 1, a non-aqueous electrolyte inlet 14 is provided in the cover plate 9, and a sealing member is inserted into the non-aqueous electrolyte inlet 14, for example, laser welding or the like. (See FIG. 1 and FIG. 2, in practice, the non-aqueous electrolyte inlet 14 is actually sealed with the non-aqueous electrolyte inlet.) Although it is a member, for ease of explanation, it is shown as a non-aqueous electrolyte inlet 14). Further, the lid plate 9 is provided with a cleavage vent 15 as a mechanism for discharging the internal gas to the outside when the temperature of the battery rises.

  In the battery of this Example 1, the outer can 5 and the cover plate 9 function as a positive electrode terminal by directly welding the positive electrode lead body 7 to the lid plate 9, and the negative electrode lead body 8 is welded to the lead plate 13, The terminal 11 functions as a negative electrode terminal by conducting the negative electrode lead body 8 and the terminal 11 through the lead plate 13, but depending on the material of the battery case 4, the sign may be reversed. There is also.

FIG. 2 is a perspective view schematically showing the external appearance of the battery shown in FIG. 1. FIG. 2 is shown for the purpose of showing that the battery is a square battery. FIG. 1 schematically shows a battery, and only specific members of the battery are shown. Also in FIG. 1, the inner peripheral portion of the electrode body is not cross-sectional.
(Example 2)
<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1, and further 2-propynyl diethyl A non-aqueous electrolyte was prepared by adding 0.1% by mass of phosphonoacetate, 1,3-dioxane, VC and FEC in amounts of 1% by mass, respectively.

A non-time electrolyte secondary battery was produced in the same manner as in Example 1 except that the non-aqueous electrolyte used for assembling the battery was changed as described above.
(Example 3)
<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1, and further 2-propynyl diethyl A non-aqueous electrolyte was prepared by adding 5% by mass of phosphonoacetate, 1,3-dioxane, VC and FEC in amounts of 1% by mass.

A non-time electrolyte secondary battery was produced in the same manner as in Example 1 except that the non-aqueous electrolyte used for assembling the battery was changed as described above.
Example 4
<Preparation of non-aqueous electrolyte>
LiPF ₆ was dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1, and further 2-propynyl diethyl A non-aqueous electrolyte was prepared by adding 3% by mass of phosphonoacetate, 1,3-dioxane, VC and FEC in amounts of 1% by mass, respectively.

A non-time electrolyte secondary battery was produced in the same manner as in Example 1 except that the non-aqueous electrolyte used for assembling the battery was changed as described above.
(Example 5)
Formula _Li t Co _{1-S M} S in _{O 2 t = 1.03, s =} 0.01 (Mg) , and the content of zirconium (Zr), the molar ratio of cobalt (Co) (Zr / Co) Commercially available lithium carbonate, cobalt oxide, zirconium oxide and magnesium carbonate were mixed so as to be 0.0001, and these were fired in the atmosphere to prepare a powdery zirconium-containing lithium cobalt composite oxide. In this example, the zirconium-containing lithium cobalt composite oxide (A) has a lithium cobalt composite oxide represented by Li _1.03 Co _0.99 Mg _0.01 O ₂ , and zirconium oxide is present on the crystal surface. The average particle size was 20 μm.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the zirconium-containing lithium cobalt composite oxide (A) was produced as described above.

(Example 6)
Formula _Li t Co _{1-S M} S in _{O 2 t = 1.03, s =} 0.01 (Mg) , and the content of zirconium (Zr), the molar ratio of cobalt (Co) (Zr / Co) Commercially available lithium carbonate, cobalt oxide, zirconium oxide and magnesium carbonate were mixed so as to be 0.001, and these were fired in the atmosphere to prepare a powdery zirconium-containing lithium cobalt composite oxide. In this example, the zirconium-containing lithium cobalt composite oxide (A) has a lithium cobalt composite oxide represented by Li _1.03 Co _0.99 Mg _0.01 O ₂ , and zirconium oxide is present on the crystal surface. The average particle size was 20 μm.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the zirconium-containing lithium cobalt composite oxide (A) was produced as described above.
(Example 7)
Nickel sulfate, cobalt sulfate, manganese sulfate and magnesium sulfate, ^{respectively, 3.78mol / dm 3, 0.25mol /} dm 3, 0.08mol / dm 3, a mixed aqueous solution containing a concentration of 0.08 mol / dm ³ Prepared. Next, ammonia water whose pH was adjusted to about 12 by addition of sodium hydroxide was put into a reaction vessel, and while stirring it vigorously, the mixed aqueous solution and 25% by mass concentration of ammonia water were respectively added thereto. , 23 cm ³ / min, and 6.6 cm ³ / min at a rate of dropwise addition using a metering pump to synthesize a coprecipitation compound of Ni, Co, Mn, and Mg (spherical coprecipitation compound). At this time, the temperature of the reaction solution is maintained at 50 ° C., and a 3 mol / dm ³ concentration sodium hydroxide aqueous solution is simultaneously dropped so that the pH of the reaction solution is maintained at around 12, and further nitrogen is added. Gas was bubbled at a flow rate of 1 dm ³ / min.

The coprecipitated compound was washed with water, filtered and dried to obtain a hydroxide. This hydroxide, LiOH.H ₂ O, BaSO ₄ and Al (OH) ₃ are mixed in ethanol so that the molar ratio is 1: 1: 0.01: 0.01. After being dispersed to form a slurry, the mixture was mixed for 40 minutes with a planetary ball mill and dried at room temperature to obtain a mixture. Next, the mixture is put in an alumina crucible, heated to 600 ° C. in a dry air flow of 2 dm ³ / min, kept at that temperature for 2 hours for preheating, further heated to 900 ° C. and heated to 12 ° C. Lithium-containing composite oxide (C) was synthesized by firing for a period of time.

  The obtained lithium-containing composite oxide (C) was washed with water and then heat-treated at 700 ° C. for 12 hours in the air (oxygen concentration of about 20 vol%), and then pulverized in a mortar to obtain a powder. The lithium-containing composite oxide after pulverization was stored in a desiccator.

About the said lithium containing complex oxide (C), the composition analysis was performed as follows using ICP (Inductively Coupled Plasma) method. First, 0.2 g of the lithium-containing composite oxide was collected and placed in a 100 mL container. Thereafter, 5 mL of pure water, 2 mL of aqua regia, and 10 mL of pure water were added in order and dissolved by heating. After cooling, the mixture was further diluted 25 times and analyzed by ICP (“ICP-757” manufactured by JARRELASH) (calibration). Line method). From the obtained results, the composition of the lithium-containing composite oxide was derived. As a result, Li _1.0 Ni _0.89 Co _0.05 Mn _0.02 Mg _0.02 Ba _0.01 Al _0.01 O ₂ It was found to be the represented composition.

  Zirconium-containing lithium-cobalt composite oxide (A), lithium-cobalt composite oxide (B) and lithium-containing composite oxide (C) prepared in Example 1 were used as the positive electrode active material in a mass ratio (A). : (B) :( C) = 65: 15: A mixture of positive electrode active materials mixed at a blending ratio of 15:20 was prepared.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the mixture of the positive electrode active materials described above was used.

(Comparative Example 1)
<Preparation of non-aqueous electrolyte>
LiPF ₆ is dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1. Dioxane, VC and FEC were added in amounts of 1% by mass, respectively, to prepare a non-aqueous electrolyte.

A non-time electrolyte secondary battery was produced in the same manner as in Example 1 except that the non-aqueous electrolyte used for assembling the battery was changed as described above.
(Comparative Example 2)
A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the zirconium-containing lithium cobalt composite oxide (A) was changed to Li _1.03 Co _0.99 Mg _0.01 O ₂ .
(Comparative Example 3)
<Preparation of non-aqueous electrolyte>
LiPF ₆ is dissolved at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC) in a volume ratio of 2: 3: 1. Dioxane, VC and FEC were added in amounts of 1% by mass, respectively, to prepare a non-aqueous electrolyte.

A non-time electrolyte secondary battery was prepared in the same manner as in Comparative Example 2 except that the non-aqueous electrolyte used for assembling the battery was changed as described above.
<Initial capacity measurement>
About each nonaqueous electrolyte secondary battery of an Example and a comparative example, constant current charge was performed to 4.35V with the electric current value of 1.0C, and then constant voltage charge was performed with the voltage of 4.35V. The total charging time for constant current charging and constant voltage charging was 2.5 hours. Thereafter, the battery was discharged to 3.0 V at a current value of 0.2 C, and the initial capacity was measured.
<Charge / discharge cycle characteristics evaluation>
For each of the nonaqueous electrolyte secondary batteries of the examples and comparative examples (batteries different from those evaluated above), constant current charging and constant voltage charging were performed under the same conditions as in the initial capacity measurement, A series of operations for discharging to 3.0 V at a current value of 1.0 C was set as one cycle, and the discharge capacity was repeated until the discharge capacity reached 50% with respect to the initial capacity, thereby obtaining the capacity 50% cycle number.

  Then, the value obtained by dividing the discharge capacity of each battery at the elapse of 200 cycles by the initial capacity was expressed as a percentage, and the 200 cycle capacity maintenance rate was obtained.

The charge / discharge cycle characteristics were evaluated in an environment at room temperature (25 ° C.) and an environment at 45 ° C.
<High temperature storage characteristics evaluation>
For each non-aqueous electrolyte secondary battery of Example and Comparative Example (battery different from the ones that were evaluated above), after performing constant current charging and constant voltage charging under the same conditions as the initial capacity measurement, The battery was stored for 4 hours in an environment of 85 ° C., and the change in thickness (swelling amount) of the battery before and after storage was determined.

Although all the negative electrodes having the same configuration were verified this time, for example, a composite in which a known SiO _x is combined with a carbon material as a negative electrode active material and graphite can be used in combination. This is preferable because the battery capacity is improved. Even when a composite in which SiO _x is combined with a carbon material as a negative electrode active material and graphite are used in combination, it has been confirmed that the phosphonoacetate compound of the present invention forms a film, and the same effects as in the examples Can be expected.

Claims (5)

A positive electrode, a negative electrode, a non-aqueous electrolyte and a separator;
The non-aqueous electrolyte uses a fluorine-containing electrolyte salt and a phosphonoacetate compound represented by the general formula (1),
The positive electrode has a positive electrode active material,
The positive electrode active material is
The lithium cobalt composite oxide represented by the general formula (2) includes a zirconium-containing lithium cobalt composite oxide (A) in which an oxide containing zirconium is present on the crystal surface,
The zirconium (Zr) content of the zirconium-containing lithium cobalt composite oxide (A) is 0. 0 by molar ratio (Zr / Co) to cobalt (Co) of the lithium cobalt composite oxide represented by the general formula (2). A nonaqueous electrolyte secondary battery characterized by being in the range of 0001 to 0.01.

In the general formula (1), R ¹ , R ² and R ³ each independently represent an alkyl group having 1 to 12 carbon atoms, an alkenyl group or an alkynyl group which may be substituted with a halogen atom, and n is An integer of 0 to 6 is shown.
Li _t Co _1-s M _s O ₂ (2)
(In the general formula (2), M is at least one element selected from Fe, V, Cr, Ti, Mg, Al, B, Ca, Ba. S and t are 0 ≦ s ≦ 0. 03, 0.05 ≦ t ≦ 1.15.)
  2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the zirconium-containing lithium cobalt composite oxide (A) is contained in an amount of 50 mass% or more with respect to all the positive electrode active materials. The nonaqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material further contains a lithium cobalt composite oxide (B) represented by the general formula (3).
Li _z Co _1-y A _y O 2 (3)
(In general formula (3), A contains at least one element selected from Mg and Al. Z and y are in the range of 0.05 ≦ z ≦ 1.15 and 0 ≦ y ≦ 0.03, respectively. Within.)   2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the zirconium-containing lithium cobalt composite oxide (A) has an average particle size of 10 to 35 μm. The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material further contains a lithium-containing composite oxide (C) represented by the general formula (4).
Li _{1 + a} Ni _1-bcd Co _b Mn _c M ¹ _d O ₂ (4)
(In the general formula (4), M ¹ is selected from the group consisting of Mg, Al, Ti, Fe, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Sn, W, B, P, Ba and Bi. At least one element, −0.15 ≦ a ≦ 0.15, 0.005 ≦ b ≦ 0.4, 0.005 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.03, and b + c + d ≦ 0.7.)

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