WO2018096889A1 - Non-aqueous electrolyte solution and lithium ion secondary battery - Google Patents

Abstract

Provided is a non-aqueous electrolyte solution which can be used in a lithium ion secondary battery that uses a lithium-transition metal composite oxide, wherein the non-aqueous electrolyte solution contains a non-aqueous solvent and a pyridinium salt.

Description

Non-aqueous electrolyte and lithium ion secondary battery

The present invention relates to a non-aqueous electrolyte and a lithium ion secondary battery.

Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have the advantages of high energy density, low self-discharge, and long-term reliability. It has been put into practical use. In recent years, the application of lithium ion secondary batteries to electric vehicles, household storage batteries, and power storage has progressed.

In a general lithium ion secondary battery, a positive electrode and a negative electrode each having an active material layer including an active material formed on a current collector are stacked with a separator interposed therebetween, and a plurality of layers are stacked as necessary. It is set as a laminated body, and these are immersed and comprised by nonaqueous electrolyte solution.

As the positive electrode active material of the lithium ion secondary battery, for example, a lithium metal oxide such as lithium manganate (LiMn ₂ O ₄ , LiMnO ₂ ) is used. In addition, metal oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium iron phosphate (LiFePO ₄ ) are also used as the positive electrode active material of the lithium ion secondary battery.

Further, as the negative electrode active material of the lithium ion secondary battery, metal lithium (Li), silicon (Si) capable of occluding and releasing lithium ions, oxides such as silicon oxide, and carbon materials are used. In particular, lithium ion secondary batteries using carbon materials such as graphite (artificial graphite, natural graphite) and coke capable of occluding and releasing lithium ions have already been put into practical use.

And as a non-aqueous electrolyte of a lithium ion secondary battery, for example, a mixed solvent of a cyclic carbonate solvent and a chain carbonate solvent with a lithium salt added is used.

Examples of the cyclic carbonate solvent include ethylene carbonate and propylene carbonate. Examples of chain carbonate solvents include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Examples of the lithium salt include lithium hexafluorophosphate (LiPF ₆ ) and lithium borofluoride (LiBF ₄ ). In addition, LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) _{2, and the} like can be given as examples of lithium salts. Furthermore, Lithium bis (oxalate) borate (LiB (C ₂ O ₄ ) ₂ ) and the like can be cited as an example of a lithium salt.

In the secondary battery using the non-aqueous electrolyte as described above, the decomposition product of the oxidatively decomposed solvent is deposited on the surface of the positive electrode on the surface of the positive electrode to increase the resistance or is generated by the decomposition of the solvent. The battery may swell due to the gas. As a result, the storage characteristics of the battery and the cycle characteristics of the secondary battery are deteriorated, and the battery characteristics are deteriorated.

In order to prevent problems that occur in secondary batteries using a non-aqueous electrolyte, a compound having a protective film generating function in the non-aqueous electrolyte, such as vinylene carbonate, fluoroethylene carbonate, maleic anhydride, etc. Is added. Specifically, at the time of initial charging, the decomposition of the compound added to the electrolyte solution is intentionally promoted on the surface of the electrode active material, and the decomposition product forms a protective film having a protective function for preventing the decomposition of the solvent. It is known to do. A protective film having a protective function for preventing the decomposition of the solvent is called SEI (SEI: Solid Electrolyte Interface).

Non-Patent Document 1 has the effect of maintaining the battery characteristics of the secondary battery as a result of appropriately suppressing the chemical reaction and decomposition of the solvent on the electrode surface by forming a protective film on the negative electrode surface. It has been reported. However, these additives are generally considered to form SEI on the negative electrode surface, and are insufficient for suppressing gas generation due to oxidative decomposition of the solvent in the positive electrode.

Furthermore, recently, in order to realize a secondary battery having a high energy density, use of a positive electrode having a high potential has been studied.

Patent Document 1 _discloses Li _1.19 Mn _0.52 Fe _0.22 O _1.98 and Li _1.21 Mn _0.46 , which are lithium transition metal composite oxides that have lithium in excess of the stoichiometric composition. A lithium ion battery using a lithium metal composite oxide having a layered rock salt structure represented by Fe _0.15 Ni _0.15 O ₂ and Li _1.2 Mn _0.4 Ni _0.4 O ₂ as a positive electrode active material It is disclosed as a high potential battery.

Patent Document 2 discloses lithium ion using a lithium-excess metal composite oxide represented by 0.5Li ₂ MnO ₃ -0.5LiNi _0.37 Mn _0.37 Co _0.26 O ₂ as a positive electrode active material. The battery is disclosed as a high potential battery.

The high potential lithium ion secondary batteries as in Patent Documents 1 and 2 have a potential of 4.5 V or more, and the voltage (3.5-4. 2V), gas generation due to oxidative decomposition of the solvent is more likely to occur in the positive electrode. Therefore, there is a need for a technique for suppressing gas generated from the positive electrode in a high-potential battery.

Patent Document 3 discloses a method of suppressing gas generation from the positive electrode by forming a protective film on the positive electrode by coating the surface of the positive electrode active material with a silane coupling agent and an epoxy resin.

Patent Document 4 discloses a method for suppressing gas generation from the positive electrode by depositing a boric acid compound on the positive electrode active material.

Patent Document 5 describes using a lithium transition metal composite oxide having a layered structure, the surface of which is coated with fine particles of Y2O3, as a positive electrode active material for a non-aqueous electrolyte secondary battery. Thus, it is described that the high temperature durability is improved without reducing the battery capacity.

Patent Document 6 discloses that in a non-aqueous electrolyte secondary battery, the positive electrode active material includes a lithium-containing transition metal oxide having a rare earth compound (rare earth element is neodymium, samarium, or erbium) attached to the surface. And containing lithium boron tetrafluoride. This describes that the battery resistance can be reduced.

On the other hand, in Patent Document 7, in a non-aqueous electrolyte battery, the non-aqueous electrolyte is composed of a salt formed of a specific quaternary ammonium cation and an anion composed only of a non-metallic element, and only a lithium ion and a non-metallic element. It describes that it contains a lithium salt formed with an anion and a cyclic ester. And it is described that this quaternary ammonium cation includes a pyridinium cation represented by a specific chemical formula. Thus, it is described that high safety can be exhibited while maintaining excellent battery performance.

JP 2013-254605 A JP 2014-22335 A JP 2014-22276 A JP 2010-40382 A JP 2005-339886 A International Publication No. 2014/049977 JP 2002-367675 A

Journal. Power Sources, No. 162, Volume 2, p.1379-1394 (2006)

However, in a lithium ion secondary battery in which a lithium transition metal composite oxide having an excess of lithium having a high potential of 4.5 V or more is used as a positive electrode, There was a problem that gas generation could not be suppressed sufficiently.

An object of the present invention is to provide a lithium ion secondary battery that solves the above-described problems and can suppress gas generation during a charge / discharge cycle, and a non-aqueous electrolyte used in the lithium ion secondary battery.

According to one aspect of the invention,
There is provided a non-aqueous electrolyte used in a lithium ion secondary battery using a lithium transition metal composite oxide, which includes a non-aqueous solvent and a pyridinium salt.

According to another aspect of the invention,
A lithium ion secondary battery including a positive electrode including a positive electrode active material, a non-aqueous electrolyte, and a negative electrode including a negative electrode active material,
The non-aqueous electrolyte is the non-aqueous electrolyte described above,
Provided is a lithium ion secondary battery in which the positive electrode active material is a positive electrode active material in which at least a part of the surface of a lithium transition metal composite oxide containing excess lithium is coated with a metal oxide.

According to the embodiment of the present invention, it is possible to provide a lithium ion secondary battery capable of suppressing gas generation of the lithium ion secondary battery during a charge / discharge cycle, and a non-aqueous electrolyte used therefor.

It is sectional drawing which shows the structure of the lithium ion secondary battery by embodiment of this invention.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. However, the embodiments described below include technically preferable modes for carrying out the present invention, but do not limit the scope of the invention.

The inventors of the present invention have made extensive studies in order to solve the above problems. As a result, a gas generated in a lithium ion secondary battery is obtained by using a positive electrode active material material in which a lithium transition metal composite oxide having an excess of lithium is coated with a metal oxide and containing a pyridinium salt in a non-aqueous electrolyte. And the present invention has been completed.

In the present embodiment, at least a part of the lithium transition metal composite oxide having an excess of lithium is coated with a metal oxide, and a non-aqueous electrolyte composed of a non-aqueous solvent and an electrolyte is made to contain a pyridinium salt.

In a lithium ion battery using a positive electrode active material in which a metal oxide is coated on at least a part of the surface of a lithium transition metal composite oxide having an excess of lithium, by containing a pyridinium salt in a non-aqueous electrolyte, Although details of the reason why the gas generation can be suppressed are unknown, for example, the decomposition of pyridinium ions on the surface of the coating oxide is promoted, and a film derived therefrom is formed on the entire surface of the positive electrode active material at the initial stage of charging. It is considered that the decomposition of is suppressed.

Hereinafter, the lithium ion secondary battery according to the present embodiment and the non-aqueous electrolyte used therein will be described in detail.

<Lithium ion secondary battery>
<Positive electrode>
As the positive electrode of the lithium ion secondary battery, one in which a positive electrode active material layer including a positive electrode active material and a binder is formed on a positive electrode current collector can be used.

<Positive electrode active material>
As the positive electrode active material, at least a part of the surface of a lithium transition metal composite oxide (hereinafter referred to as “lithium-excess transition metal composite oxide”) that has lithium in excess of the stoichiometric composition is a metal oxide ( Hereinafter, those appropriately coated with “coating oxide”) can be used. Here, the “stoichiometric composition” can be defined as a composition represented by LiMO ₂ (M represents a transition metal element or a metal element including at least a transition metal element). “Having an excess” means containing an excess of lithium with respect to the amount of lithium in this stoichiometric composition (LiMO ₂ ).

As such a lithium-excess transition metal composite oxide, one represented by any of the following composition formulas can be used.
Li _{1 + a} Ni _x Mn _y O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1) (composition formula A),
Li _{1 + a} Ni _x Mn _y M _z O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1, 0 <z <1, M is Co or Fe) (composition formula B).

It is preferable to have the following composition from the viewpoint of operating as a high-potential battery while sufficiently having the original performance of the lithium nickel manganese composite oxide.
In the composition formulas A and B, a is preferably 0.1 ≦ a ≦ 0.5, more preferably 0.2 ≦ a ≦ 0.5, and still more preferably 0.2 ≦ a ≦ 0.4.
In composition formula A, x and y are each independently preferably 0.05 or more, and more preferably 0.1 or more. Further, x + y ≧ 0.5 is preferable, and x + y ≧ 0.6 is more preferable.
In composition formula B, x, y and z are each independently preferably 0.01 or more, and more preferably 0.03 or more. Further, x + y ≧ 0.5 is preferable, and x + y ≧ 0.6 is more preferable.
In the composition formula A, 0.9 ≦ a + x + y ≦ 1.1 is preferable, 0.95 ≦ a + x + y ≦ 1.05 is more preferable, and a + x + y = 1 is still more preferable.
In the composition formula B, 0.9 ≦ a + x + y + z ≦ 1.1 is preferable, 0.95 ≦ a + x + y + z ≦ 1.05 is more preferable, and a + x + y + z = 1 is more preferable.

Furthermore, in order to improve cycle characteristics and safety, and to enable use at a high charging potential within a range that does not impair the desired effect, a part of the lithium transition metal composite oxide may be replaced with another element. Good. For example, at least one kind of metal selected from the group consisting of Sn, Mg, Ti, Al, Zr, Cr, V, Ga, Ge, Zn, Cu, Bi, Mo, and La is used as a part of nickel, manganese, cobalt, and iron Substitution with an element, or a part of oxygen can be substituted with S or F.

The lithium-excess transition metal composite oxide used in the embodiment of the present invention is preferably at least one selected from the above composition formulas A and B.

As a specific composition of the lithium-excess metal composite oxide in the present embodiment, for example, Li _1.2 Mn _0.4 Ni _0.4 O ₂ , Li _1.2 Mn _0.6 Ni _0.2 O ₂ , Li _1.21 Mn _0.46 Fe _0.15 Ni _0.15 O ₂ , Li _1.2 Mn _0.4 Fe _0.4 O ₂ , Li _1.26 Mn _0.37 Ni _0.22 Ti _0.15 O ₂ , Li _1.2 Mn _0.56 Ni _0.17 Co _0.07 O ₂ , Li _1.2 Mn _0.54 Ni _0.13 Co _0.13 O ₂ , Li _1.2 Mn _0.56 Ni _0.17 Co _0.07 O ₂ , Li _1.2 Mn _0.54 Ni _0.13 Co _0.13 O ₂ , Li _1.26 Fe _0.11 Ni _0.11 Mn _0.52 O ₂ , Li _{1 .29} Fe _0.07 Ni _0.07 Mn _{0. 7 O 2, Li 1.3 Fe 0.04} Ni 0.04 Mn 0.62 O 2, Li 1.2 Fe 0.20 Ni 0.20 Mn 0.40 O 2, Li 1.2 Ni 0.18 An example is Mn _0.54 Co _0.08 O ₂ .

The method for synthesizing the lithium-excess transition metal composite oxide represented by the above composition formula is not particularly limited, and a conventional method for synthesizing an oxide of related art can be applied.

The positive electrode active materials can be used singly or in combination of two or more.

The method of coating at least a part of the surface of the lithium-excess transition metal composite oxide with an oxide can be performed as follows.

Although the method for coating the oxide is not particularly limited, by mixing the lithium-excess transition metal composite oxide and a solution such as an aqueous solution of an inorganic source for coating, then removing the solvent, and firing the powder after drying, The oxide can be coated on at least part of the surface of the lithium-excess transition metal composite oxide.

As the inorganic source for coating, chloride, hydroxide, containing an element selected from the group consisting of La, Pr, Nd, Sm, Mg, Y, Ce, Eu, Ge, Mo, Zr, Al and V, Carbonates, nitrates, acetates, alcoholates and the like are preferred. These may use 1 type and may use 2 or more types together.

The concentration of each of these aqueous solutions or these alcohol solutions is not particularly limited, but is preferably 0.002 to 0.05% by mass. When this concentration is 0.002% by mass or more, the time required for evaporation of a solvent such as water or alcohol is shortened, so that the production efficiency is improved. Further, when the concentration is 0.05% by mass or less, the raw material is sufficiently dissolved, and a homogeneous mixed solution is obtained.

After evaporating a solvent such as water or alcohol, the dried product obtained by drying is fired.

This firing can be performed in a vacuum, an air atmosphere, an inert atmosphere, hydrogen or nitrogen, or a mixed atmosphere thereof, but it is preferably performed in an air atmosphere from the viewpoint of cost reduction. The firing temperature is preferably 350 to 800 ° C. When the firing temperature is 350 ° C. or higher, the reaction is completed and no reaction impurities or the like remain. Moreover, when the firing temperature is 800 ° C. or lower, the reaction with lithium in the lithium iron-manganese composite oxide can be suppressed, and mixing of lithium compounds as impurities can be prevented.

The drying method is not particularly limited, and examples thereof include a drying method using a rotary evaporator, a spray dryer and the like in addition to a normal drying method.

Further, in the oxide coating, the coating may be performed with two or more kinds of oxides. The method of coating two or more kinds of oxides is not particularly limited. For example, after coating one kind of oxide by the above method, the same method is used to coat two or more kinds of oxides again. can do.

The coating oxide is preferably an oxide of at least one metal selected from the group consisting of La, Pr, Nd, Sm, Mg, Y, Ce, Eu, Ge, Mo, Zr, Al, and V. Among these, Sm oxide and Al oxide are preferable from the viewpoint of easy handling and stability. In particular, Al oxide is more preferable from the viewpoint of suppressing gas generation by decomposition of pyridinium ions.

Moreover, the effect of suppressing gas generation can be enhanced by using two or more kinds of coating oxides in combination rather than one kind. Among these, a composite oxide of Sm oxide and Al oxide is preferable from the viewpoint of suppressing gas generation by decomposition of pyridinium ions. In particular, a double layer composite oxide in which the Sm oxide is in the inner shell and the Al oxide is in the outer shell is preferable. The ratio (mass ratio) between Sm oxide and Al oxide is preferably 1: 0.5 to 1: 2.0, more preferably 1: 0.8 to 1: 1.2, and 1: 0.9. ˜1: 1.1 is more preferred, and 1: 1 is particularly preferred.

The content of the coating oxide (hereinafter also referred to as “coating amount”) with respect to the lithium-excess transition metal composite oxide is preferably in the range of 0.1% by mass to 15% by mass. From the viewpoint of sufficiently suppressing the amount of gas generated, the content (coating amount) of the coating oxide is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and further preferably 1% by mass or more. From the standpoint of obtaining a sufficient initial discharge capacity by suppressing a decrease in the initial discharge capacity, it is preferably 15% by mass or less, more preferably 5% by mass or less, and further preferably 3% by mass or less.

When coated with two or more kinds of metal oxides (preferably Sm oxide and Al oxide), the content (coating amount) of the coating oxide is independently in the range of 0.1 to 3% by mass. The range of 0.5 to 3% by mass is more preferable, and the range of 0.5 to 1.5% by mass is particularly preferable.

Whether or not at least a part of the surface of the lithium-excess transition metal composite oxide is covered with the specific oxide or composite oxide is determined by scanning electron microscope observation (energy dispersive X-ray analysis), transmission type The determination can be made by electron microscope observation, X-ray photoelectron spectroscopic analysis, Auger electron spectroscopic analysis, and electron energy loss spectroscopic analysis.

In the positive electrode active material layer containing the positive electrode active material, a conductive additive may be added for the purpose of reducing impedance. Examples of the conductive auxiliary agent include graphites such as natural graphite and artificial graphite, and carbon blacks such as acetylene black, ketjen black, furnace black, channel black, and thermal black. A plurality of types of conductive assistants may be appropriately mixed and used. The amount of the conductive auxiliary agent is preferably 1 to 10% by mass with respect to the positive electrode active material.

The binder for the positive electrode is not particularly limited, and examples thereof include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and the like. Further, styrene-butadiene copolymer rubber, polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide, or the like may be used as the positive electrode binder. In particular, from the viewpoint of versatility and low cost, it is preferable to use polyvinylidene fluoride as the binder for the positive electrode. The amount of the positive electrode binder used is preferably 2 to 10 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. .

As the positive electrode current collector, a general one can be arbitrarily used. For example, an aluminum foil or a stainless lath plate can be used.

The positive electrode is, for example, a mixture obtained by mixing a positive electrode active material, a conductive additive and a binder with a solvent such as N-methylpyrrolidone added and kneaded to the current collector by the doctor blade method or the die coater method. And can be produced by drying.

<Negative electrode>
As the negative electrode of the lithium ion secondary battery, for example, a negative electrode active material layer including a negative electrode active material and a binder formed on a negative electrode current collector can be used.

Examples of the negative electrode active material include lithium metal, a metal or alloy that can be alloyed with lithium, an oxide that can occlude and release lithium, a carbon material, and a negative electrode active material containing silicon.

Examples of the metal or alloy that can be alloyed with lithium include lithium-silicon and lithium-tin. Examples of oxides capable of inserting and extracting lithium include niobium pentoxide (Nb ₂ O ₅ ), lithium titanium composite oxide (Li _4/3 Ti _5/3 O ₄ ), and titanium dioxide (TiO ₂ ). ) And the like. Examples of the carbon material capable of inserting and extracting lithium include carbonaceous materials such as graphite material, carbon black, coke, mesocarbon microbeads, hard carbon, and graphite. Examples of the graphite material include artificial graphite and natural graphite. Examples of carbon black include acetylene black and furnace black.

Among the above-described negative electrode active materials, a carbonaceous material is preferable from the viewpoints of excellent cycle characteristics and safety, and excellent continuous charge characteristics. In addition, the negative electrode active material may be used alone or in combination of two or more in any combination and ratio.

In this embodiment, a negative electrode active material containing silicon may be used. For example, examples of the negative electrode active material containing silicon include silicon and silicon compounds. Examples of silicon include simple silicon. Examples of the silicon compound include a compound of a transition metal such as silicon oxide, silicate, nickel silicide, and cobalt silicide and silicon.

A silicon compound is preferably used from the viewpoint of charge / discharge cycle characteristics because the silicon compound relaxes expansion and contraction due to repeated charge / discharge of the negative electrode active material itself. Further, depending on the type of silicon compound, there is a function of ensuring conduction between silicon. From such a viewpoint, silicon oxide is preferably used as the silicon compound of the negative electrode active material.

The silicon oxide is not particularly limited, but is represented by, for example, SiO _x (0 <x ≦ 2). The silicon oxide may include lithium. The silicon oxide containing lithium is represented by, for example, SiLi _y O _z (y> 0, 2>z> 0).

Further, the silicon oxide may contain a trace amount of a metal element or a non-metal element. The silicon oxide can contain, for example, 0.1 to 5% by mass of one or more elements selected from nitrogen, boron and sulfur. By containing a small amount of metal element or non-metal element in silicon oxide, the electrical conductivity of silicon oxide can be improved. Further, the silicon oxide may be crystalline or amorphous.

In addition to the silicon or silicon oxide, the negative electrode active material preferably contains a carbon material capable of inserting and extracting lithium ions. The carbon material can also be contained in a composite state with silicon or silicon oxide. Similar to silicon oxide, the carbon material has a function of relaxing expansion and contraction due to repeated charging and discharging of the negative electrode active material itself and ensuring conduction between silicon as the negative electrode active material. By the coexistence of silicon and / or silicon oxide and a carbon material, better cycle characteristics can be obtained.

As the carbon material, graphite, amorphous carbon, diamond-like carbon, carbon nanotube, or the like can be used alone or in combination. Graphite with high crystallinity has high electrical conductivity, and is excellent in adhesion to a positive electrode current collector made of a metal such as copper and voltage flatness. On the other hand, since amorphous carbon having low crystallinity has a relatively small volume expansion, it has a high effect of relaxing the volume expansion of the entire negative electrode, and deterioration due to non-uniformity such as crystal grain boundaries and defects hardly occurs. The content of the carbon material in the negative electrode active material is preferably 2% by mass or more and 50% by mass or less, and more preferably 2% by mass or more and 30% by mass or less.

When using silicon oxide as the silicon compound, the negative electrode active material containing silicon and a silicon compound is prepared, for example, by mixing simple silicon and silicon oxide and sintering under high temperature and reduced pressure. be able to. When a transition metal and silicon compound is used as the silicon compound, for example, by mixing and melting simple silicon and the transition metal, or by coating the transition metal on the surface of the simple silicon by vapor deposition or the like. Can be made.

In addition to the above-described method for producing a negative electrode active material, a composite with carbon can be combined. For example, in a high-temperature non-oxygen atmosphere, by introducing a mixed sintered product of elemental silicon and silicon compound into a gas atmosphere of an organic compound, a coating layer containing carbon is formed around the core of elemental silicon and silicon oxide. can do. Also, for example, by mixing a mixed sintered product of single silicon and silicon oxide and a carbon precursor resin in a high temperature non-oxygen atmosphere, a coating layer made of carbon around the core of the single silicon and silicon oxide Can be formed. Thus, by forming the coating layer containing carbon around the cores of the simple silicon and the silicon oxide, it is possible to obtain a further improvement effect of suppression of volume expansion with respect to charge / discharge and cycle characteristics.

When silicon is used as the negative electrode active material, a composite containing silicon, silicon oxide and a carbon material (hereinafter also referred to as “Si / SiO / C composite”) is preferable.

Furthermore, it is preferable that all or part of the silicon oxide has an amorphous structure. The silicon oxide having an amorphous structure can suppress the volume expansion of a carbon material or silicon which is another negative electrode active material. Although this mechanism is not clear, it is presumed that the formation of a film on the interface between the carbon material and the electrolytic solution has some influence due to the amorphous structure of silicon oxide. The amorphous structure is considered to have relatively few elements due to non-uniformity such as crystal grain boundaries and defects.

Note that it can be confirmed by X-ray diffraction measurement that all or part of silicon oxide has an amorphous structure. When silicon oxide does not have an amorphous structure, a peak intrinsic to silicon oxide is strongly observed in X-ray diffraction measurement. On the other hand, when all or part of the silicon oxide has an amorphous structure, a peak unique to the silicon oxide becomes broad in the X-ray diffraction measurement.

In the Si / SiO / C composite, it is preferable that all or part of silicon is dispersed in silicon oxide. By dispersing at least a part of silicon in silicon oxide, volume expansion as a whole of the negative electrode can be further suppressed, and decomposition of the electrolytic solution can also be suppressed. In addition, it can confirm that all or one part of silicon | silicone is disperse | distributing in silicon oxide by using a transmission electron microscope observation and energy dispersive X-ray spectroscopy measurement together. Specifically, the cross section of the sample is observed with a transmission electron microscope, and the oxygen concentration of the silicon portion dispersed in the silicon oxide is measured by energy dispersive X-ray spectroscopy measurement. As a result, it can be confirmed that silicon dispersed in silicon oxide is not an oxide.

In the Si / SiO / C composite, for example, all or part of silicon oxide has an amorphous structure, and all or part of silicon is dispersed in silicon oxide. Such a Si / SiO / C composite can be produced, for example, by a method disclosed in Japanese Patent Application Laid-Open No. 2004-47404. That is, the Si / SiO / C composite can be obtained, for example, by performing a CVD process on silicon oxide in an atmosphere containing an organic gas such as methane gas. The Si / SiO / C composite obtained by such a method has a form in which the surface of particles made of silicon oxide containing silicon is coated with carbon. Silicon is nanoclustered in silicon oxide.

In the Si / SiO / C composite, the ratio of silicon, silicon oxide and carbon material is not particularly limited, but the following composition is preferable. Silicon is preferably 5% by mass or more and 90% by mass or less, and more preferably 20% by mass or more and 50% by mass or less with respect to the Si / SiO / C composite. The silicon oxide is preferably 5% by mass or more and 90% by mass or less, and more preferably 40% by mass or more and 70% by mass or less with respect to the Si / SiO / C composite. The carbon material is preferably 2% by mass to 50% by mass and more preferably 2% by mass to 30% by mass with respect to the Si / SiO / C composite.

Further, the Si / SiO / C composite may be a mixture of simple silicon, silicon oxide and carbon material, or may be prepared by mixing simple silicon, silicon oxide and carbon material by mechanical milling. it can. For example, the Si / SiO / C composite can be obtained by mixing particulate silicon, silicon oxide and carbon materials.

For example, the average particle size of simple silicon can be made smaller than the average particle size of the carbon material and silicon oxide. In this way, single silicon having a large volume change during charge and discharge has a relatively small particle size, and carbon materials and silicon oxides having a small volume change have a relatively large particle size. Is more effectively suppressed. In addition, in the charge / discharge process, lithium is occluded and released in the order of large particle size, small particle size, and large particle size. From this point, residual stress and residual strain are generated. It is suppressed.

The average particle size of the single silicon is preferably 20 μm or less, and more preferably 15 μm or less. The average particle diameter of silicon oxide is preferably 1/2 or less of the average particle diameter of the carbon material, and the average particle diameter of simple silicon is 1/2 or less of the average particle diameter of silicon oxide. preferable. Furthermore, the average particle diameter of the silicon oxide is 1/2 or less of the average particle diameter of the carbon material, and the average particle diameter of the single silicon is more than 1/2 of the average particle diameter of the silicon oxide. preferable. By controlling the average particle diameter within the above range, the effect of reducing the volume expansion can be obtained more effectively, so that a secondary battery excellent in the balance of energy density, cycle life and efficiency can be obtained. More specifically, graphite is used as the carbon material, the average particle diameter of silicon oxide is set to 1/2 or less of the average particle diameter of graphite, and the average particle diameter of elemental silicon is set to 1 / (average particle diameter of silicon oxide). It is preferable to set it to 2 or less. Note that the average particle diameter of simple silicon or silicon oxide is measured by a measuring method such as a laser diffraction scattering method or a dynamic light scattering method.

Further, as the negative electrode active material, a material obtained by treating the surface of the above-mentioned Si / SiO / C composite with a silane coupling agent or the like may be used.

The active material layer of the negative electrode preferably contains the negative electrode active material capable of occluding and releasing lithium ions as a main component. Specifically, the content of the negative electrode active material is preferably 55% or more of the total weight of the active material layer including the negative electrode active material, the negative electrode binder, and various auxiliary agents as necessary. 65% or more is more preferable.

The binder for the negative electrode is not particularly limited, and examples thereof include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, and styrene-butadiene copolymer. Rubber (SBR), polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamideimide and the like can be used. Moreover, polyacrylic acid or carboxymethylcellulose containing lithium salt, sodium salt, or potassium salt neutralized with an alkali can be used as the binder for the negative electrode. Among these, polyimide, polyamideimide, SBR, and polyacrylic acid or carboxymethylcellulose containing lithium salt, sodium salt, potassium salt neutralized with alkali are preferable because of their strong binding properties. The amount of the binder for the negative electrode to be used is preferably 5 to 25 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoints of “sufficient binding force” and “high energy” which are in a trade-off relationship. .

As the material for the current collector of the negative electrode, a general material can be arbitrarily used. For example, metal materials such as copper, nickel, and stainless steel are used. Among these, copper is particularly preferable from the viewpoint of ease of processing and cost. The surface of the current collector of the negative electrode is preferably subjected to a roughening treatment in advance. Furthermore, the shape of the current collector is arbitrary, and examples thereof include a foil shape, a flat plate shape, and a mesh shape. Also, a perforated current collector such as expanded metal or punching metal can be used.

For example, as in the case of the positive electrode active material layer, the negative electrode is made into a slurry by adding a solvent to a mixture of the above-described negative electrode active material, a binder, and various auxiliary agents as necessary. It can be manufactured by applying a coating solution to a current collector and drying it.

<Non-aqueous electrolyte>
The electrolyte used in the lithium ion secondary battery according to the embodiment of the present invention is mainly composed of an electrolyte salt containing a non-aqueous organic solvent (also referred to as “non-aqueous solvent”) and a pyridinium salt.

As this non-aqueous solvent, a general one can be appropriately selected. For example, examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, chain esters, lactones, ethers, sulfones, nitriles, and phosphate esters. Among these, cyclic carbonates and chain carbonates are preferable.

Specific examples of cyclic carbonates include propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and the like.

Specific examples of the chain carbonates include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, and methyl butyl carbonate.

Specific examples of chain esters include methyl formate, methyl acetate, methyl propionate, ethyl propionate, methyl pivalate, ethyl pivalate and the like.

Specific examples of lactones include γ-butyrolactone, δ-valerolactone, α-methyl-γ-butyrolactone, and the like.

Specific examples of ethers include tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1, Examples include 2-dibutoxyethane.

Specific examples of sulfones include sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, and the like.

Specific examples of nitriles include acetonitrile, propionitrile, succinonitrile, glutaronitrile, adiponitrile and the like.

Specific examples of the phosphate esters include trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, and the like.

In addition, the said non-aqueous solvent can be used individually by 1 type or in combination of 2 or more types. Examples of combinations of a plurality of types of nonaqueous solvents include, for example, a combination of cyclic carbonates and chain carbonates, cyclic carbonates and chain carbonates, a third solvent as a chain ester, lactones, and ethers. , Nitriles, sulfones, and combinations in which phosphate esters are added. Especially, in order to implement | achieve the outstanding battery characteristic, the combination containing at least cyclic carbonate and chain carbonate is more preferable.

Further, a fluorinated ether solvent, a fluorinated carbonate solvent, a fluorinated phosphate ester or the like may be added as a third solvent to the combination of the cyclic carbonate and the chain carbonate.

Examples of the fluorinated ether solvent include fluorinated ethers represented by the following general formula (2).

(In the formula, R ¹ and R ² are each independently an alkyl group or a fluorine-containing alkyl group, and at least one of R ¹ and R ² is a fluorine-containing alkyl group.)

Specific examples of the fluorinated ether solvent include CF ₃ OCH ₃ , CF ₃ OC ₂ H ₅ , F (CF ₂ ) ₂ OCH ₃ , F (CF ₂ ) ₂ OC ₂ H ₅ , and F (CF ₂ ) _3. OCH ₃ , F (CF ₂ ) ₃ OC ₂ H ₅ , F (CF ₂ ) ₄ OCH ₃ , F (CF ₂ ) ₄ OC ₂ H ₅ , F (CF ₂ ) ₅ OCH ₃ , F (CF ₂ ) ₅ OC ₂ H ₅ , F (CF ₂ ) ₈ OCH ₃ , F (CF ₂ ) ₈ OC ₂ H ₅ , F (CF ₂ ) ₉ OCH ₃ , CF ₃ CH ₂ OCH ₃ , CF ₃ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ OCH ₃ , CF ₃ CF ₂ CH ₂ OCHF ₂ , CF ₃ CF ₂ CH ₂ O (CF ₂ ) ₂ H, CF ₃ CF ₂ CH ₂ O (CF ₂ ) ₂ F, HCF ₂ CH ₂ OCH ₃ , H _{(CF 2) 2} OC _{2 CH 3, H (CF 2} ) 2 OCH 2 CF 3, H (CF 2) 2 CH 2 OCHF 2, H (CF 2) 2 CH 2 O (CF 2) 2 H, H (CF 2) 2 CH 2 O (CF ₂ ) ₃ H, H (CF ₂ ) ₃ CH ₂ O (CF ₂ ) ₂ H, H (CF ₂ ) ₄ CH ₂ O (CF ₂ ) ₂ H, (CF ₃ ) ₂ CHOCH ₃ , (CF ₃ ) ₂ CHCF ₂ OCH ₃ , CF ₃ CHFCF ₂ OCH ₃ , CF ₃ CHFCF ₂ OCH ₂ CH ₃ , CF ₃ CHFCF ₂ CH ₂ OCHF ₂ , CF ₃ CHFCF ₂ CH ₂ OCH ₂ CF ₂ CF ₃ , H (CF _{2 ) 2 CH 2 OCF 2 CHFCF 3} , CHF 2 CH 2 OCF 2 CFHCF 3, F (CF 2) 2 CH 2 OCF 2 CFHCF 3, CF 3 (CF 2) 3 OCH Fluorinated ethers such as _2.

Examples of the fluorinated carbonate solvent include fluoroethylene carbonate, fluoromethyl methyl carbonate, 2-fluoroethyl methyl carbonate, ethyl- (2-fluoroethyl) carbonate, (2,2-difluoroethyl) ethyl carbonate, bis (2 Fluorinated carbonates such as -fluoroethyl) carbonate and ethyl- (2,2,2-trifluoroethyl) carbonate.

Fluorinated phosphate ester solvents include tris phosphate (2,2,2-trifluoroethyl), tris phosphate (trifluoromethyl), tris phosphate (2,2,3,3-tetrafluoropropyl) And fluorinated phosphoric acid esters.

Examples of pyridinium salts contained in the electrolytic solution according to the embodiment of the present invention include those represented by the following general formula (1).

R ¹ of the pyridinium ion represented by the formula (1) is a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and R ² to R ⁶ are each independently a hydrogen atom or 1 to 4 carbon atoms. It is an alkyl group.

Examples of the alkyl group having 1 to 8 carbon atoms include methyl group, ethyl group, n-propyl group, iso-propyl group, n-butyl group, t-butyl group, n-pentyl group, n-hexyl group, n -Heptyl group, n-octyl group and the like can be mentioned. Examples of the alkyl group having 1 to 4 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an iso-propyl group, and an n-butyl group.

Examples of X ⁻ in the formula (1) include PF ₆ ⁻ and BF ₄ ^— .

From the standpoint that pyridinium ions efficiently form a film on the surface of the positive electrode and sufficiently suppress the decomposition of the solvent, R ¹ = C ₄ H ₉ , R ² to R ⁵ = H, R ⁶ = CH ₃ Or a pyridinium salt represented by the formula (1) in which R ¹ = C ₄ H ₉ , R ² to R ⁴ = H, R ⁵ = CH ₃ , R ⁶ = H is preferable. Further, X ⁻ in the formula (1) is preferably PF ₆ ^— .

The pyridinium salt contained in the nonaqueous electrolytic solution according to the present embodiment is preferably contained in the nonaqueous electrolytic solution in the range of 0.005% by mass to 10% by mass, and in the range of 0.01% by mass to 5% by mass. It is more preferable that it is contained. A film formed by decomposition of pyridinium ions can be confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) or the like. Further, the content of the pyridinium salt in the non-aqueous electrolyte in the battery immediately after production (before activation treatment) is preferably in the range of 0.5 to 10% by mass, and in the range of 1 to 6% by mass. More preferably, it is more preferably in the range of 1 to 5% by mass. In addition, content (mass%) of the pyridinium salt contained in a non-aqueous electrolyte is the mass percentage of the pyridinium salt with respect to a non-aqueous electrolyte.

In addition, specific examples of the electrolyte salt used other than the pyridinium salt are not particularly limited, but LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , _{LiN (SO 2 C 2 F 5} ) 2, CF 3 SO 3 Li, C 4 F 9 SO 3 Li, and _{LiAsF 6, LiAlCl 4, LiSbF 6} , LiPF 4 (CF 3) 2, LiPF 3 (C 2 F 5 ) ₃ , LiPF ₃ (CF ₃ ) ₃ , (CF ₂ ) ₂ (SO ₂ ) ₂ NLi, (CF ₂ ) ₃ (SO ₂ ) ₂ Li, lithium bisoxalatoborate (Lithium bis (oxalate) borate), lithium Oxalato difluoroborate (Lithium difluoro (oxalato) borate) It is. Particularly preferred among the above electrolytes are LiPF ₆ , LiBF ₄ , LiN (SO ₂ F) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ . The electrolyte salts described above can be used alone or in combination of two or more.

The concentration of the above-described electrolyte salt dissolved in the non-aqueous solvent in the non-aqueous electrolyte is preferably 0.1 to 3M (mol / L) with respect to the non-aqueous solvent, and 0.5 to 2M. More preferably.

In addition, the non-aqueous electrolyte can optionally contain general components as other components. For example, vinylene carbonate, maleic anhydride, ethylene sulfite, boronic acid ester, 1,3-propane sultone, 1,5,2,4-dioxadithian-2,2,4,4-tetraoxide and other components As mentioned.

<Separator>
Although it does not restrict | limit especially as a separator, Polyolefin, such as a polypropylene and polyethylene, Single layer or laminated porous films, such as an aramid, a polyimide, and a nonwoven fabric can be used. Moreover, the separator using glass fiber is mentioned with an inorganic material. Further, a coating of different materials on the polyolefin or a laminated film can also be used. As an example of the coating of the different material on the polyolefin, a polyolefin base material coated with a fluorine compound or inorganic fine particles may be mentioned. Examples of the laminated film include those obtained by laminating a polyethylene substrate and a polypropylene layer, and those obtained by laminating a polyolefin substrate and an aramid layer.

The thickness of the separator is preferably 5 to 50 μm, more preferably 10 to 40 μm from the viewpoint of the energy density of the battery and the mechanical strength of the separator.

<Structure of lithium ion secondary battery>
The structure of the lithium ion secondary battery is not particularly limited, and the above-described configuration can be used for a coin battery, a cylindrical battery, a laminated battery, or the like having a single-layer or multilayer separator.

For example, in the case of a laminated laminate type lithium ion battery, positive electrodes, separators, and negative electrodes are alternately laminated, and each electrode is connected to a metal terminal called a tab, and placed in a container composed of a laminate film as an outer package. The electrolyte solution can be injected and sealed.

FIG. 1 shows an example of the structure of a lithium ion secondary battery according to an embodiment of the present invention.

The positive electrode is formed by forming the positive electrode active material layer 1 containing the positive electrode active material on the positive electrode current collector 1A. As such a positive electrode, there are a single-sided electrode in which the positive electrode active material layer 1 is formed on one side of the positive electrode current collector 1A, and a double-sided electrode in which the positive electrode current collector 1A is formed on both sides of the positive electrode current collector 1A. It is used.

The negative electrode is formed by forming the negative electrode active material layer 2 containing the negative electrode active material on the negative electrode current collector 2A. As such a negative electrode, a single-sided electrode in which the negative electrode active material layer 2 is formed on one side of the negative electrode current collector 2A, and a double-sided electrode in which the negative electrode active material layer 2 is formed on both sides of the negative electrode current collector 2A are provided. It is used.

These positive electrode and negative electrode are arranged opposite to each other with a separator 3 interposed therebetween as shown in FIG. The two positive electrode current collectors 1A are connected to each other on one end side, and the positive electrode tab 1B is connected to this connection part. The two negative electrode current collectors 2A are connected to each other on the other end side, and the negative electrode tab 2B is connected to this connection part. The laminated body (power generation element) including the positive electrode and the negative electrode is accommodated in the exterior body 4 and is in a state impregnated with the electrolytic solution. The positive electrode tab 1 </ b> B and the negative electrode tab 2 </ b> B are exposed to the outside of the exterior body 4. The exterior body 4 is formed by using two rectangular laminate sheets, overlapping so as to wrap the power generating element, and fusing and sealing the four sides.

The laminate film can be appropriately selected as long as it is stable to the electrolyte and has a sufficient water vapor barrier property. For example, polypropylene, polyethylene or the like coated with aluminum, silica, or alumina can be used as the laminate film. In particular, from the viewpoint of suppressing volume expansion, it is preferably used as a laminate film coated with aluminum.

As a typical layer structure of a laminate film, a structure in which a metal thin film layer and a heat-fusible resin layer are laminated can be mentioned. In addition, as a typical layer structure of the laminate film, a protective layer made of a film of polyester or polyamide such as polyethylene terephthalate is further provided on the surface of the metal thin film layer opposite to the heat-sealing resin layer. A laminated layer structure can be mentioned. When the battery element is sealed, the battery element is surrounded by facing the heat-fusible resin layer. As the metal thin film layer, for example, a foil of Al, Ti, Ti alloy, Fe, stainless steel, Mg alloy or the like having a thickness of 10 to 100 μm is used.

The resin used for the heat-fusible resin layer of the laminate film is not particularly limited as long as it can be heat-sealed. For example, polypropylene, polyethylene, an acid-modified product of polypropylene or polyethylene, polyphenylene sulfide, polyester such as polyethylene terephthalate, polyamide, ethylene-vinyl acetate copolymer, and the like are used as the heat-fusible resin layer. Further, an ionomer resin obtained by intermolecularly bonding an ethylene-methacrylic acid copolymer or an ethylene-acrylic acid copolymer with metal ions is also used as the heat-fusible resin layer. The thickness of the heat-fusible resin layer is preferably 10 to 200 μm, and more preferably 30 to 100 μm.

Hereinafter, the non-aqueous electrolyte and the lithium ion secondary battery according to the present embodiment will be described more specifically with reference to examples.

Example 1
The lithium ion secondary battery of Example 1 and the non-aqueous electrolyte used for this were produced as follows.

<Positive electrode>
0.2 g of samarium nitrate was dissolved in 10 ml of water. Lithium iron manganese composite oxide Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ positive electrode material (positive electrode base material) having a low specific surface area (BET value 4.4 m ² / g) for the obtained solution 20 g of A) was mixed and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder was heat treated in an air atmosphere at 400 ° C. for 3 hours to obtain a positive electrode active material in which a part of the surface of the lithium iron manganese composite oxide was coated with Sm oxide.

A mixture containing 92% by mass of this positive electrode active material, 4% by mass of ketjen black, 0.5% by mass of carbon nanofibers, and 3.5% by mass of polyvinylidene fluoride was prepared, and a solvent was added thereto to add slurry. Was prepared. And this slurry was apply | coated on the positive electrode electrical power collector which consists of aluminum foil (thickness 20 micrometers), and then it dried, and produced the positive electrode of thickness 175 micrometers (including the thickness of an electrical power collector). Moreover, the double-sided electrode which apply | coated the slurry on both surfaces of the positive electrode electrical power collector, and was dried was also produced in the same procedure.

<Negative electrode>
A mixture containing 75% by mass of SiO having an average particle size of 15 μm, 4% by mass of graphite, 6% by mass of carbon black, and 15% by mass of polyamic acid was prepared, and a solvent was added to prepare a slurry. And it apply | coated on the negative electrode electrical power collector which consists of copper foil (thickness 10 micrometers), and then dried, and produced the negative electrode of thickness 46 micrometers (including the thickness of an electrical power collector). The produced negative electrode was annealed at 350 ° C. for 3 hours in a nitrogen atmosphere to cure the binder. Moreover, the double-sided electrode which apply | coated the slurry and dried it on both surfaces of the negative electrode collector was produced in the same procedure.

<Non-aqueous electrolyte>
A solvent in which ethylene carbonate (EC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) were mixed at a volume ratio of 28.5: 66.5: 5 was prepared. Then, obtained by dissolving LiPF ₆ as a 1.0M to prepare the solvent. Further, 1-butyl-2-methylpyridinium hexafluorophosphate (hereinafter referred to as “B2MPY”) was dissolved to 2% by mass to prepare a nonaqueous electrolytic solution.

<Lithium ion battery>
After molding the positive electrode and the negative electrode produced by the above method, a battery element as shown in FIG. 1 was produced. A porous film separator 3 was sandwiched between the positive electrode active material layer 1 of the positive electrode and the negative electrode active material layer 2 of the negative electrode and laminated. A positive electrode tab 1B and a negative electrode tab 2B were welded to the positive electrode current collector 1A and the negative electrode current collector 2B, respectively.

The produced battery element was wrapped with an aluminum laminate film outer package 4 and sealed on three sides of the outer package 4 by heat fusion, and then impregnated with the above electrolyte at an appropriate degree of vacuum.

Then, under reduced pressure, one side of the outer package 4 that had not been heat-sealed was heat-sealed and sealed to produce a lithium ion battery before activation treatment.

<Activation process>
About the produced lithium ion battery before the activation process, it charged to 4.5V with the electric current of 20 mA (20 mA / g) per 1 g of positive electrode active materials. Thereafter, the battery was discharged to 1.5 V at a current of 20 mA (20 mA / g) per 1 g of the positive electrode active material.

After discharging to 1.5 V, similarly, after charging to 4.5 V at 20 mA / g, it was discharged to 1.5 V. That is, the activation process which repeats a charging / discharging cycle twice was performed.

Then, the lithium ion secondary battery of this example was fabricated by breaking the sealing portion and depressurizing it to vent the gas inside the battery and resealing the broken portion.

(Example 2)
A positive electrode active material in which a part of the surface of the lithium iron manganese composite oxide was coated with Al oxide was obtained in the same manner as in Example 1 except that samarium nitrate was replaced with aluminum nitrate. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode active material was used.

(Example 3)
0.2 g of samarium nitrate was dissolved in 10 ml of water. Lithium iron manganese composite oxide Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ positive electrode material (positive electrode base material) having a low specific surface area (BET value 4.4 m ² / g) for the obtained solution 20 g of A) was mixed and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder was heat-treated at 400 ° C. for 3 hours in an air atmosphere to obtain a coated composite oxide in which a part of the surface of the lithium iron manganese composite oxide was coated with Sm oxide.

Thereafter, this coated composite oxide and 0.2 g of aluminum nitrate were added to 10 ml of water (the aluminum nitrate was dissolved) and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder is heat-treated at 400 ° C. for 3 hours in an air atmosphere, so that a part of the surface of the lithium iron manganese complex oxide is coated with Sm oxide and the outside is coated with Al oxide. An oxide (positive electrode active material) was obtained. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode active material was used.

Example 4
0.2 g of aluminum nitrate was dissolved in 10 ml of water. Lithium iron manganese composite oxide Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ positive electrode material (positive electrode base material) having a low specific surface area (BET value 4.4 m ² / g) for the obtained solution 20 g of A) was mixed and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder was heat-treated at 400 ° C. for 3 hours in an air atmosphere to obtain a coated composite oxide in which a part of the surface of the lithium iron manganese composite oxide was coated with Al oxide.

Thereafter, this coated composite oxide and 0.2 g of samarium nitrate were added to 10 ml of water (samarium nitrate was dissolved) and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder is heat-treated at 400 ° C. for 3 hours in an air atmosphere, so that a part of the surface of the lithium iron manganese complex oxide is coated with Al oxide and the outside is coated with Sm oxide. An oxide (positive electrode active material) was obtained. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 1 except that this positive electrode active material was used.

(Example 5)
The samarium nitrate is replaced with aluminum nitrate, and the positive electrode base material A is a lithium iron manganese-based composite oxide Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 with a high specific surface area (BET value 5.6 m ² / g). A positive electrode active material in which a part of the surface of the lithium iron manganese composite oxide was coated with an Al oxide was used in the same manner as in Example 1 except that the O ₂ positive electrode material (positive electrode base material B) was used. Obtained. Thereafter, this positive electrode active material is used, and in the preparation of the electrolytic solution, 1-butyl-3-methylpyridinium hexafluorophosphate (hereinafter referred to as “B3MPY”) is used instead of B2MPY as the pyridinium salt, A lithium ion secondary battery was produced in the same manner as in Example 1 except that the content was changed from 2% by mass to 5% by mass.

(Example 6)
A positive electrode active material in which a part of the surface of the lithium iron manganese composite oxide was coated with an Al oxide in the same manner as in Example 5 except that the amount of aluminum nitrate was changed from 0.2 g to 0.4 g. Obtained material. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 5 except that this positive electrode active material was used.

(Example 7)
A positive electrode active material in which a part of the surface of the lithium iron manganese composite oxide was coated with Al oxide in the same manner as in Example 5 except that the amount of aluminum nitrate was changed from 0.2 g to 0.6 g. Obtained material. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 5 except that this positive electrode active material was used.

(Example 8)
0.2 g of samarium nitrate was dissolved in 10 ml of water. Lithium iron manganese composite oxide Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ positive electrode material (positive electrode base material) having a high specific surface area (BET value 5.6 m ² / g) for the obtained solution 20 g of B) was mixed and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder was heat-treated at 400 ° C. for 3 hours in an air atmosphere to obtain a coated composite oxide in which a part of the surface of the lithium iron manganese composite oxide was coated with Sm oxide.

Thereafter, this coated composite oxide and 0.2 g of aluminum nitrate were added to 10 ml of water (the aluminum nitrate was dissolved) and stirred for 30 minutes. The obtained slurry-like mixture was dried overnight in a 120 ° C. air constant temperature bath. The powder is heat-treated at 400 ° C. for 3 hours in an air atmosphere, so that a part of the surface of the lithium iron manganese complex oxide is coated with Sm oxide and the outside is coated with Al oxide. An oxide (positive electrode active material) was obtained. Thereafter, a lithium ion secondary battery was produced in the same manner as in Example 5 except that this positive electrode active material was used.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the positive electrode base material A (uncoated product) was used as the positive electrode active material.

(Comparative Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that an electrolytic solution not containing B2MPY was used as the nonaqueous electrolytic solution.

(Comparative Example 3)
A lithium ion secondary battery was prepared in the same manner as in Example 1 except that the positive electrode base material A (uncoated product) was used as the positive electrode active material and the electrolyte solution containing no B2MPY was used as the nonaqueous electrolyte solution. Produced.

(Comparative Example 4)
A lithium ion battery was produced in the same manner as in Example 5 except that the positive electrode base material B (uncoated product) was used as the positive electrode active material.

(Comparative Example 5)
A lithium ion battery was produced in the same manner as in Example 5 except that an electrolyte containing no B3MPY was used as the nonaqueous electrolyte.

(Comparative Example 6)
A lithium ion secondary battery was produced in the same manner as in Example 5 except that the positive electrode base material B was used as the positive electrode active material and the electrolyte solution containing no B3MPY was used as the nonaqueous electrolyte solution.

<Evaluation method of lithium ion battery>
About the lithium ion secondary battery produced by the above method, it is charged at a constant current of 40 mA / g to 4.5 V in a constant temperature bath at 45 ° C., and further charged at a constant voltage of 4.5 V until a current of 5 mA / g is reached. Continued. Then, it discharged to 1.5V with the electric current of 5 mA / g, and conditioned.

After that, the lithium ion secondary battery after conditioning was charged to 4.5 V at a constant current of 40 mA / g in a constant temperature bath at 45 ° C., and further charged at a constant voltage of 4.5 V until a current of 5 mA / g was reached. Thereafter, the battery was discharged to 1.5 V at a current of 40 mA / g. The charge / discharge cycle of the lithium ion battery was repeated 10 times in total.

Then, the capacity retention rate after 10 cycles was determined from the ratio of the initial discharge capacity obtained in the first cycle and the discharge capacity obtained in the 10th cycle. Moreover, the gas generation amount after 10 cycles was calculated | required regarding each Example and the comparative example. The amount of gas generated was measured by the Archimedes method.

<Evaluation results of lithium ion secondary battery>
In each example and comparative example, the type of positive electrode base material, the coating and its coating amount, the type and addition amount of pyridinium salt, and the initial capacity after activation of the obtained battery, and the amount of gas generated after 10 cycles. The results are summarized in Table 1.

“Positive electrode base” in the table means the following.
Positive electrode base material A: Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ (BET value 4.4 m ² / g),
Positive electrode base material B: Li _1.26 Ni _0.11 Mn _0.52 Fe _0.11 O ₂ (BET value 5.6 m ² / g)

From the comparison between Examples 1 and 2 using the pyridinium salt B2MPY and Comparative Example 1, it can be seen that the amount of gas generated is reduced by the coating of the oxide. From the comparison between Example 1 and Comparative Example 2, It can be seen that the amount of gas generation is reduced by adding the pyridinium salt B2MPY.

Furthermore, Examples 1 to 4 show that the Sm and Al double oxide coating generates less gas than the single coating of Al or Sm oxide. Furthermore, the initial capacity of Example 3 of Sm oxide (inner shell) / Al oxide (outer shell) is higher than Example 4 of Al oxide (inner shell) / Sm oxide (outer shell). I understand.

In addition, it can be seen from the comparison between Example 5 using the pyridinium salt B3MPY and Comparative Example 4 that the amount of gas generated is reduced by the oxide coating. From the comparison between Example 5 and Comparative Example 5, the use of the pyridinium salt is confirmed. It can be seen that the amount of gas generated decreases.

Also, from Examples 5 to 7, it can be seen that the amount of gas generation decreases as the Al oxide coating amount increases. Further, from the comparison between Example 8 and Examples 5 to 7, the gas generation amount of Example 8 of Sm oxide (inner shell) / Al oxide (outer shell) is larger than that of the example in which only Al oxide is coated. I understand that it is small.

As described above, according to the embodiment of the present invention, lithium using a positive electrode active material in which at least a part of a lithium transition metal composite oxide having lithium in excess of the stoichiometric composition is coated with a metal oxide. In an ion secondary battery, it turns out that gas generation at the time of a cycle can be controlled by making a non-aqueous electrolyte contain a pyridinium salt. Furthermore, it was found that the amount of gas generated was remarkably reduced by forming a double coating of Sm oxide (inner shell) / Al oxide (outer shell) and containing a pyridinium salt in the non-aqueous electrolyte. This is because a uniform protective film is formed on the entire positive electrode active material because the decomposition of pyridinium ions is very efficiently decomposed on the surface of the Sm oxide (inner shell) / Al oxide (outer shell). Seem.

Although the present invention has been described above with reference to the embodiments and examples, the present invention is not limited to the above embodiments and examples. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

Some or all of the above embodiments can be described as in the following supplementary notes, but are not limited thereto.

(Appendix 1)
A non-aqueous electrolyte used for a lithium ion secondary battery using a lithium transition metal composite oxide, comprising a non-aqueous solvent and a pyridinium salt.

(Appendix 2)
The nonaqueous electrolytic solution according to appendix 1, wherein the pyridinium salt is represented by the formula (1).

(In Formula 1, R ¹ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, R ² to R ⁶ each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, wherein X ^- PF ₆ ^- represents a) ^- or BF ₄

(Appendix 3)
The nonaqueous electrolytic solution according to appendix 1 or 2, wherein the content of the pyridinium salt in the nonaqueous electrolytic solution is in the range of 0.005 mass% to 5 mass%.

(Appendix 4)
The nonaqueous electrolytic solution according to any one of appendices 1 to 3, wherein the nonaqueous electrolytic solution contains at least one of a chain carbonate solvent and a cyclic carbonate solvent as the nonaqueous solvent.

(Appendix 5)
The nonaqueous electrolytic solution according to any one of appendices 1 to 4, wherein the nonaqueous electrolytic solution further includes at least one selected from a fluorinated ether solvent, a fluorinated carbonate solvent, and a fluorinated phosphate ester solvent.

(Appendix 6)
A lithium ion secondary battery including a positive electrode including a positive electrode active material, a non-aqueous electrolyte, and a negative electrode including a negative electrode active material,
The nonaqueous electrolytic solution is the nonaqueous electrolytic solution according to any one of appendices 1 to 5,
The lithium ion secondary battery, wherein the positive electrode active material is a positive electrode active material in which at least a part of the surface of the lithium transition metal composite oxide having an excess of lithium is coated with a metal oxide.

(Appendix 7)
The oxide to be coated is an oxide of at least one metal selected from the group consisting of La, Pr, Nd, Sm, Mg, Y, Ce, Eu, Ge, Mo, Zr, Al, and V. The lithium ion secondary battery according to appendix 6.

(Appendix 8)
The lithium ion secondary battery according to appendix 6 or 7, wherein a content of the oxide to be coated with respect to the lithium transition metal composite oxide is in a range of 0.1% by mass to 15% by mass.

(Appendix 9)
The lithium ion secondary battery according to any one of appendices 6 to 8, wherein the oxide to be coated is at least one of Sm oxide and Al oxide.

(Appendix 10)
The lithium ion secondary battery according to any one of appendices 6 to 8, wherein the oxide to be coated is a composite oxide film of Sm oxide and Al oxide.

(Appendix 11)
The lithium ion secondary battery according to any one of appendices 6 to 8, wherein the oxide to be coated contains Sm oxide in the inner shell and Al oxide in the outer shell.

(Appendix 12)
The content of the oxide of Sm and the oxide of Al with respect to the lithium transition metal composite oxide is independently in the range of 0.1% by mass to 3% by mass, according to Supplementary Note 10 or 11. Lithium ion secondary battery.

(Appendix 13)
The lithium transition metal composite oxide is
Lithium transition metal composite oxide represented by any of the following composition formulas:
Li _{1 + a} Ni _x Mn _y O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1),
Li _{1 + a} Ni _x Mn _y M _z O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1, 0 <z <1, M is Co or Fe), and the lithium transition metal Part of the transition metal of the composite oxide is replaced with at least one metal element selected from the group consisting of Sn, Mg, Ti, Al, Zr, Cr, V, Ga, Ge, Zn, Cu, Bi, Mo, and La Food,
The lithium ion secondary battery according to any one of supplementary notes 6 to 12, including at least one selected from the above.

(Appendix 14)
The lithium ion secondary battery according to any one of appendices 6 to 13, wherein the negative electrode contains a silicon oxide and a carbon material as the negative electrode active material.

As mentioned above, although this invention was demonstrated with reference to embodiment and an Example, this invention is not limited to the said embodiment and Example. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

The non-aqueous electrolyte and the lithium ion secondary battery using the non-aqueous electrolyte according to the embodiment of the present invention are used in, for example, all industrial fields that require a power source and industrial fields related to transportation, storage, and supply of electrical energy. be able to. Specifically, it can be used as a power source for mobile devices such as mobile phones, notebook computers, tablet terminals, and portable game machines. Further, it can be used as a power source for moving / transporting media such as electric vehicles, hybrid cars, electric motorcycles, and electric assist bicycles. Furthermore, it can be used for household power storage systems, backup power sources such as UPS, and power storage facilities for storing power generated by solar power generation or wind power generation.

This application claims priority based on Japanese Patent Application No. 2016-227676 filed on November 24, 2016, the entire disclosure of which is incorporated herein.

1: Positive electrode active material layer 1A: Positive electrode current collector 1B: Positive electrode tab 2: Negative electrode active material layer 2A: Negative electrode current collector 2B: Negative electrode tab 3: Separator 4: Exterior body

Claims (10)

A non-aqueous electrolyte used for a lithium ion secondary battery using a lithium transition metal composite oxide, which contains a non-aqueous solvent and a pyridinium salt. The nonaqueous electrolytic solution according to claim 1, wherein the pyridinium salt is represented by the formula (1).

(In Formula 1, R ¹ represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, R ² to R ⁶ each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms, wherein X ^- PF ₆ ^- represents a) ^- or BF ₄ The nonaqueous electrolytic solution according to claim 1 or 2, wherein the content of the pyridinium salt in the nonaqueous electrolytic solution is in the range of 0.005 mass% to 5 mass%. The nonaqueous electrolytic solution according to any one of claims 1 to 3, wherein the nonaqueous electrolytic solution contains at least one of a chain carbonate solvent and a cyclic carbonate solvent as the nonaqueous solvent. The non-aqueous electrolyte according to any one of claims 1 to 4, wherein the non-aqueous electrolyte further contains at least one selected from a fluorinated ether solvent, a fluorinated carbonate solvent, and a fluorinated phosphate ester solvent. Electrolytic solution. A lithium ion secondary battery including a positive electrode including a positive electrode active material, a non-aqueous electrolyte, and a negative electrode including a negative electrode active material,
The nonaqueous electrolytic solution is the nonaqueous electrolytic solution according to any one of claims 1 to 5,
The lithium ion secondary battery, wherein the positive electrode active material is a positive electrode active material in which at least a part of the surface of the lithium transition metal composite oxide having an excess of lithium is coated with a metal oxide. The oxide to be coated is an oxide of at least one metal selected from the group consisting of La, Pr, Nd, Sm, Mg, Y, Ce, Eu, Ge, Mo, Zr, Al and V;
The lithium ion secondary battery according to claim 6, wherein a content of the oxide to be coated with respect to the lithium transition metal composite oxide is in a range of 0.1 mass% to 15 mass%. The lithium ion secondary battery according to 6 or 7, wherein the oxide to be coated is at least one of Sm oxide and Al oxide. The lithium transition metal composite oxide is
Lithium transition metal composite oxide represented by any of the following composition formulas:
Li _{1 + a} Ni _x Mn _y O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1),
Li _{1 + a} Ni _x Mn _y M _z O ₂ (0 <a ≦ 0.5, 0 <x <1, 0 <y <1, 0 <z <1, M is Co or Fe), and the lithium transition metal Part of the transition metal of the composite oxide is replaced with at least one metal element selected from the group consisting of Sn, Mg, Ti, Al, Zr, Cr, V, Ga, Ge, Zn, Cu, Bi, Mo, and La Food,
The lithium ion secondary battery as described in any one of Claims 6-8 containing at least 1 sort (s) chosen from. The lithium ion secondary battery according to any one of claims 6 to 9, wherein the negative electrode contains a silicon oxide and a carbon material as the negative electrode active material.

Images