JP2011060541A - Positive electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery using it - Google Patents

Abstract

Disclosed is a positive electrode for a non-aqueous secondary battery in which gas generation during high-temperature storage is suppressed.
A positive electrode for a non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode current collector and a positive electrode active material layer formed on a surface of the positive electrode current collector. The positive electrode active material layer includes a lithium transition metal oxide as a positive electrode active material. On the surface of the lithium transition metal oxide, at least one element A selected from the group consisting of alkali metals other than lithium and alkaline earth metals is supported, and at least a part of the element A forms a carbonate. Yes. The amount of element A is 8 mol to 34 μmol per 1 g of lithium transition metal oxide.
[Selection figure] None

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to improvement of the positive electrode.

2. Description of the Related Art In recent years, electronic devices have become increasingly portable and cordless, and as these driving power sources, secondary batteries that are small and light and have a high energy density are widely used. In addition to small secondary batteries used as power sources for portable devices, large-scale secondary batteries that require durability and safety for a long period of time have been actively developed. Large-sized secondary batteries are used for power storage and power sources for electric vehicles.
Among secondary batteries, non-aqueous electrolyte secondary batteries having high voltage and high energy density are expected.

The non-aqueous electrolyte secondary battery includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a separator disposed between the positive electrode and the negative electrode. As the separator, a microporous film made of polyolefin is mainly used. As the non-aqueous electrolyte, an aprotic organic solvent in which a lithium salt such as LiBF ₄ and LiPF ₆ is dissolved is used. As the positive electrode active material, lithium cobalt oxide (for example, LiCoO ₂ ) having a high potential with respect to lithium, excellent safety, and relatively easy synthesis is used. A carbon material such as graphite is used for the negative electrode active material.

A positive electrode active material for the purpose of increasing the capacity has been actively studied. Specifically, lithium nickel oxide having a high capacity (represented by LiNi _1-x M _x O ₂ , where M is, for example, Co, Mn, or Al) has been studied. However, when lithium nickel oxide is used, the high-temperature storage characteristics may deteriorate. When synthesizing a high-capacity lithium nickel oxide, it is necessary to add excess lithium and fire it. For this reason, surplus lithium carbonate is contained in the lithium nickel oxide after baking. This lithium carbonate easily reacts when the battery is stored at a high temperature, and a large amount of gas may be generated.

  Thus, for example, Patent Documents 1 to 3 propose a method of cleaning the lithium nickel oxide after firing and removing excess lithium carbonate.

JP 7-335215 A JP 2003-17054 A JP-A-6-342657

However, the methods described in Patent Documents 1 to 3 include a step of washing the positive electrode active material in the step of manufacturing the positive electrode active material. In the subsequent step of manufacturing the positive electrode, lithium carbonate may be formed on the active material surface. Specifically, the step of producing a positive electrode includes a step of preparing a slurry containing a positive electrode active material, a step of attaching the slurry to the surface of the positive electrode current collector and drying to form a coating film, Rolling the coating film to form a positive electrode active material layer. In the step of rolling the coating film, the positive electrode active material particles are broken to form an active surface. Lithium exposed on the active surface reacts with carbon dioxide in the atmosphere to form lithium carbonate. Therefore, in the methods described in Patent Documents 1 to 3, it is difficult to suppress gas generation when the battery is stored at high temperature.
Accordingly, an object of the present invention is to provide a positive electrode for a non-aqueous electrolyte secondary battery in which gas generation during high-temperature storage is suppressed.

One aspect of the present invention is a positive electrode for a nonaqueous electrolyte secondary battery comprising a positive electrode current collector and a positive electrode active material layer formed on a surface of the positive electrode current collector,
The positive electrode active material layer includes a lithium transition metal oxide as a positive electrode active material,
At least one element selected from the group consisting of alkali metals other than lithium and alkaline earth metals is supported on the surface of the lithium transition metal oxide,
At least a portion of the element forms a carbonate;
The amount of the element is 8 to 34 μmol per gram of the lithium transition metal oxide.

The amount of the positive electrode active material is preferably 3.2 to 3.6 g per 1 cm ^{3 of the} positive electrode active material layer.
The element is preferably at least one selected from the group consisting of Na, K, Rb, Mg, Ca, Sr, and Ba.
The transition metal contained in the lithium transition metal oxide preferably contains at least nickel.
The amount of nickel preferably accounts for 30 mol% or more of the transition metal.

The lithium transition metal oxide has the general formula (1):
Li _x Ni _w M _z Me _{1- (w + z)} O _{2 + d}
Represented by
M is at least one selected from Co and Mn,
Me is a metal different from M, or at least one selected from the group consisting of B, P, and S;
d represents an oxygen defect or oxygen excess;
x, w, and z are 0.98 ≦ x ≦ 1, 0.3 ≦ w ≦ 1.0, 0 ≦ z ≦ 0.7, and 0.9 ≦ (w + z) ≦ 1.0. preferable.
The metal different from M is preferably at least one selected from the group consisting of Mg, Sc, Y, Fe, Cu, Zn, Al, and Cr.

  Another aspect of the present invention relates to a nonaqueous electrolyte secondary battery including the positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte.

ADVANTAGE OF THE INVENTION According to this invention, the gas generation from the positive electrode at the time of high temperature preservation | save of a battery is suppressed, and the nonaqueous electrolyte secondary battery excellent in the high temperature preservation | save characteristic can be provided.
In addition, gas generation from the positive electrode is also suppressed during the charge / discharge cycle of the battery. Therefore, the nonaqueous electrolyte secondary battery excellent in charge / discharge cycle characteristics can be provided.

It is a schematic longitudinal cross-sectional view of the nonaqueous electrolyte secondary battery which concerns on the Example of this invention.

  The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector. The positive electrode active material layer includes a lithium transition metal oxide as a positive electrode active material. At least one element selected from the group consisting of alkali metals other than lithium and alkaline earth metals (hereinafter referred to as element A) is supported on the surface of the lithium transition metal oxide. At least a part of the element A forms a carbonate. And the quantity of the element A is 8-34 micromol per gram weight of a lithium transition metal oxide.

When the above conditions are satisfied, gas generation during battery high-temperature storage is suppressed. The reason is considered as follows.
When lithium carbonate is present on the surface of the positive electrode active material, lithium carbonate reacts to generate gas when the battery is stored at a high temperature, which causes battery swelling.
On the other hand, when the element A is present on the surface of the positive electrode active material, the element A reacts with lithium carbonate adhering to the surface of the positive electrode active material, and changes to a carbonate of the element A that is chemically more stable than lithium carbonate. Therefore, adhesion of lithium carbonate on the surface of the positive electrode active material is suppressed. By attaching a carbonate that is chemically more stable than lithium carbonate to the positive electrode active material, the reaction of the carbonate during high-temperature storage of the battery can be suppressed, and gas generation can be reduced.

When the amount of the element A exceeds 34 μmol per gram of the lithium transition metal oxide, the amount of the element A increases, and the element A not only reacts with lithium carbonate but may react with the positive electrode active material. When the element A reacts with the positive electrode active material, lithium is desorbed from the positive electrode active material, and the capacity tends to decrease. When the amount of element A is less than 8 μmol per gram of lithium transition metal oxide, the effect of replacing lithium carbonate with carbonate of element A is reduced. Preferably, the amount of element A is 8 to 17 μmol per gram of lithium transition metal oxide.
Specifically, the element A is preferably at least one selected from the group consisting of Na, K, Rb, Mg, Ca, Sr, and Ba. These elements form carbonates that are chemically more stable than lithium carbonate. Among these, Na is particularly preferable from the viewpoint of the stability of the carbonate.

  The transition metal contained in the lithium transition metal oxide preferably contains at least nickel. In order to increase the capacity per active material weight, the proportion of nickel in the transition metal is preferably 30 mol% or more. More preferably, the proportion of nickel in the transition metal contained in the lithium transition metal oxide is 90 mol% or less.

The lithium transition metal oxide has the general formula (1):
Li _x Ni _w M _z Me _{1- (w + z)} O _{2 + d}
It is preferable to be represented by
M is preferably at least one selected from Co and Mn. Replacing a part of Ni with M improves the stability of the crystal structure during charge and discharge.

Me is preferably a metal different from M or at least one selected from the group consisting of B, P, and S. Replacing a part of Ni with Me improves the thermal stability during charging. The metal different from M is preferably at least one selected from the group consisting of Mg, Sc, Y, Fe, Cu, Zn, Al, and Cr.
Me may or may not contain at least one selected from the group consisting of alkali metals and alkaline earth metals. The element A is attached as a carbonate on the surface of the lithium transition metal oxide and is not an element constituting the lithium transition metal oxide. Therefore, the element A is distinguished from Me.
d represents an oxygen defect or oxygen excess, and is, for example, −0.20 to 0.20.

In the general formula (1), 0.98 ≦ x ≦ 1, 0.3 ≦ w ≦ 1.0, 0 ≦ z ≦ 0.7, and 0.9 ≦ (w + z) ≦ 1.0 are preferable.
The lithium amount (x) is a value immediately after the production of the positive electrode active material, and the x value changes due to charge / discharge.
In order to increase the capacity per active material weight, the Ni amount (w) is preferably 0.3 to 1.0. From the viewpoint of the capacity per active material weight, the M amount (z) is preferably 0.7 or less. From the viewpoint of the capacity per active material weight, the Me amount (1- (w + z)) is preferably 0.1 or less.

  Lithium transition metal oxide reacts with carbon dioxide in the air, and lithium carbonate is easily generated. Therefore, in the case of a lithium transition metal oxide, in particular, in the case of the lithium transition metal oxide represented by the general formula (1), the effect of suppressing the residual lithium carbonate in the positive electrode appears significantly.

  Presence of carbonate radicals (carbonate of element A) on the surface of the positive electrode can be confirmed by, for example, X-ray photoelectron spectroscopy. This confirms that at least a part of the element A forms a carbonate.

The amount of the element A adhering to the surface of the lithium transition metal oxide is, for example, removing the element A adhering to the surface of the lithium transition metal oxide by washing with ion-exchanged water, and the amount of the element A removed It is obtained by seeking. More specifically, a lithium transition metal oxide is put into ion-exchanged water, and the ion-exchanged water is sufficiently stirred. Thereafter, the ion-exchanged water is filtered to obtain a filtrate containing the element A. Using this filtrate, the amount of element A is determined by ion chromatography.
Since the lithium transition metal oxide itself does not dissolve in the ion-exchanged water, even when the lithium transition metal oxide itself contains the element A, the amount of the element A attached can be easily determined by the above method.

Hereinafter, although the example of the manufacturing method of the positive electrode of this invention is shown, a manufacturing method is not limited to this.
The method for producing the positive electrode of the present invention described above is, for example,
(1) preparing a slurry containing a positive electrode active material containing a lithium transition metal oxide;
(2) The slurry is attached to the surface of the positive electrode current collector and dried to form a coating film,
Rolling the coating film to form a positive electrode active material layer;
(3) After the step (2), the step of washing the positive electrode active material layer with an alkaline washing liquid;
(4) After the step (3), a step of heating the positive electrode active material layer is included.

In the step (3), it is possible to remove lithium carbonate (Li ₂ CO ₃ ) without causing side reactions such as elution of Li of the positive electrode active material by appropriately selecting the cleaning conditions. Lithium carbonate attached to the surface of the positive electrode active material can be replaced with alkali metal and alkaline earth metal carbonate, which is more stable than lithium carbonate, so that lithium carbonate can be easily removed.
Since the positive electrode is washed after the rolling process, the active material particles are cracked in the rolling process to form an active surface, and even if lithium carbonate adheres to the surface, lithium carbonate can be reliably removed.
In particular, when a large force is required during rolling and particle breakage of the active material is likely to occur, for example, a high-density positive electrode having a positive electrode active material amount of 3.2 to 3.6 g per 1 cm ^{3 of} the positive electrode active material layer is obtained. Even in this case, adhesion of lithium carbonate can be suppressed.

The cleaning liquid preferably contains at least one element selected from the group consisting of alkali metals other than lithium and alkaline earth metals.
The temperature and pH of the cleaning liquid and the cleaning time may be appropriately set according to the degree of cleaning of the positive electrode. From the viewpoint of the stability of the positive electrode current collector and the positive electrode active material and the cleaning effect, the pH of the cleaning liquid is 9 -12, the cleaning time is preferably 5 seconds or more and 30 seconds or less, and the temperature of the cleaning liquid is preferably 20 to 60 ° C.

By the step (4), moisture contained in the positive electrode can be removed and the positive electrode active material can be coated with a binder contained in the positive electrode. Thereby, it is possible to further suppress carbon dioxide from adhering to the positive electrode active material.
The positive electrode active material layer is preferably heated at a temperature lower than the decomposition temperature of the binder. When the heating temperature exceeds the decomposition temperature of the binder, the binding force due to the binder may be significantly reduced, and the positive electrode active material layer may be easily peeled off from the positive electrode.
In order to obtain a good coating state of the binder on the positive electrode active material, the heating temperature of the positive electrode active material layer is preferably 160 ° C. or higher and 300 ° C. or lower, and the heating time is preferably 5 seconds or longer and 60 seconds or shorter.

It is preferable that the above manufacturing method further includes a step of washing the lithium transition metal oxide with water before the step (1). It is preferable to use a lithium transition metal oxide that is an active material from which lithium carbonate (Li ₂ Co ₃ ) has been removed in advance by washing so that gas generation can be further reduced.

  The slurry in step (1) is obtained, for example, by adding a liquid component such as N-methyl-2-pyrrolidone to a positive electrode mixture composed of a mixture of a positive electrode active material, a binder, and a conductive agent. The positive electrode active material layer in the step (2) includes, for example, a positive electrode active material, a binder, and a conductive agent.

Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyethylene, polypropylene, aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, polyacrylic acid methyl ester, and polyethyl acrylate. Ester, Polyacrylic acid hexyl ester, Polymethacrylic acid, Polymethacrylic acid methyl ester, Polymethacrylic acid ethyl ester, Polymethacrylic acid hexyl ester, Polyvinyl acetate, Polyvinylpyrrolidone, Polyether, Polyethersulfone, Hexafluoropolypropylene, Styrene Butadiene rubber and carboxymethyl cellulose are used.
The binder includes, for example, a group consisting of tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethyl vinyl ether, acrylic acid, and hexadiene. You may use the copolymer which consists of at least 2 sorts selected more. A binder may be used individually by 1 type and may be used in combination of 2 or more type.

  Examples of the conductive agent include graphites such as natural graphite and artificial graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon fiber and metal fiber Conductive fibers such as carbon fluoride, metal powders such as aluminum powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, phenylene derivatives, etc. An organic conductive material is used.

  The content of the positive electrode active material in the positive electrode active material layer is preferably 80 to 87% by weight. The content of the conductive agent in the positive electrode active material layer is preferably 1 to 20% by weight. The content of the binder in the positive electrode active material layer is preferably 1 to 10% by weight.

For the positive electrode current collector in the step (2), for example, a long conductive substrate made of stainless steel, aluminum, or titanium and having porosity or non-porosity is used.
Although the thickness of a positive electrode electrical power collector is not specifically limited, Preferably it is 1-500 micrometers, More preferably, it is 5-20 micrometers. By setting the thickness of the positive electrode current collector in the above range, it is possible to reduce the weight while maintaining the strength of the positive electrode.

  The present invention relates to a nonaqueous electrolyte secondary battery comprising the positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte. By using the positive electrode, gas generation from the positive electrode is suppressed during high-temperature storage characteristics and charge / discharge cycles, and the high-temperature storage characteristics and charge / discharge cycle characteristics of the nonaqueous electrolyte secondary battery are improved.

The negative electrode has a negative electrode current collector and a negative electrode active material layer formed on the surface thereof. The negative electrode active material layer includes, for example, a negative electrode active material and a binder. This negative electrode is obtained, for example, by the following method. A slurry is prepared by adding a liquid component such as N-methyl-2-pyrrolidone to a negative electrode mixture composed of optional components such as a negative electrode active material and a binder. This slurry is applied to the negative electrode current collector and dried to form a coating film. Thereafter, the coating film is rolled to form a negative electrode active material layer.
Further, the negative electrode active material layer may be composed of only the negative electrode active material. This negative electrode is obtained, for example, by forming a film-like negative electrode active material on the negative electrode current collector by a vapor phase method.

  As the negative electrode active material, for example, a carbon material, an alloy-based material, an oxide, a nitride, or the like can be used. As the carbon material, for example, various natural graphite, coke, graphitized carbon, various artificial graphite, and amorphous carbon are used, and the shape thereof is, for example, particulate or fibrous. As the alloy material, for example, a material containing an element that can be alloyed with Li, such as Si and Sn, is used. Since the capacity density is high, the alloy-based material is preferably silicon (Si), tin (Sn), an alloy, compound, or solid solution containing Si, or an alloy, compound, or solid solution containing Sn.

The compound containing Si is preferably SiO _x (0.05 <x <1.95).
In addition to Si, compounds, alloys, and solid solutions containing Si include B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, and N. , And at least one selected from the group consisting of Sn may be included.
In addition to Sn, compounds, alloys, and solid solutions containing Sn include B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N , And at least one selected from the group consisting of Si may be included.
Examples of the tin-containing compound include Ni ₂ Sn ₄ , Mg ₂ Sn, SnO _x (0 <x <2), SnO ₂ , and SnSiO ₃ .
A negative electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type.

  The content of the negative electrode active material in the negative electrode is preferably 90 to 99% by weight. The content of the binder in the negative electrode is preferably 1 to 10% by weight. The materials mentioned for the positive electrode binder are used for the negative electrode binder.

For the negative electrode current collector, for example, a long conductive substrate made of stainless steel, nickel, or copper and having porosity or non-porosity is used.
Although the thickness of a negative electrode collector is not specifically limited, Preferably it is 1-500 micrometers, More preferably, it is 5-20 micrometers. By setting the thickness of the negative electrode current collector in the above range, it is possible to reduce the weight while maintaining the strength of the negative electrode.

For the separator, for example, a microporous thin film, a woven fabric, or a non-woven fabric having a predetermined mechanical strength and insulating property excellent in ion permeability is used. As the material of the separator, polyolefin resins such as polypropylene and polyethylene are preferable. A separator made of a polyolefin resin is excellent in durability and has a shutdown function, so that the safety of the nonaqueous electrolyte secondary battery is improved.
The thickness of a separator is 10-300 micrometers, for example, Preferably it is 40 micrometers or less, More preferably, it is 15-30 micrometers, More preferably, it is 10-25 micrometers.
Examples of the microporous thin film include a single layer film made of one kind of material, a composite film containing two or more kinds of materials, or a multilayer film in which two or more single layer films or composite films are laminated.

As the non-aqueous electrolyte, for example, a liquid non-aqueous electrolyte, a gel-like non-aqueous electrolyte, or a solid (polymer electrolyte) non-aqueous electrolyte is used.
A nonaqueous solvent in which a supporting salt (for example, a lithium salt) is dissolved is used for the liquid nonaqueous electrolyte.

A known nonaqueous solvent may be used as the nonaqueous solvent. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, or a cyclic carboxylic acid ester is used.
For the cyclic carbonate, for example, propylene carbonate (PC) or ethylene carbonate (EC) is used. For example, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or dimethyl carbonate (DMC) is used as the chain carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). A non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

Examples of the supporting salt include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiCl , LiBr, LiI, chloroborane lithium, borates and imide salts are used.
Examples of borates include bis (1,2-benzenediolate (2-)-O, O ′) lithium borate, bis (2,3-naphthalenedioleate (2-)-O, O ′). Lithium borate, bis (2,2′-biphenyldiolate (2-)-O, O ′) lithium borate, bis (5-fluoro-2-olate-1-benzenesulfonic acid-O, O ′) boron A lithium acid is mentioned.
Examples of the imide salts include lithium bistrifluoromethanesulfonate imide ((CF ₃ SO ₂ ) ₂ NLi), lithium trifluoromethanesulfonate nonafluorobutanesulfonate (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO) ₂ )), lithium bispentafluoroethanesulfonate imide ((C ₂ F ₅ SO ₂ ) ₂ NLi).
A support salt may be used individually by 1 type, and may be used in combination of 2 or more type.
The concentration of the supporting salt in the liquid non-aqueous electrolyte is preferably 0.5 to 2 mol / L.

In addition to the liquid non-aqueous electrolyte, a material that can be decomposed on the surface of the negative electrode to form a film having high lithium ion conductivity and can increase charge / discharge efficiency may be added as an additive. Examples of the additive include vinylene carbonate (VC), 4-methyl vinylene carbonate, 4,5-dimethyl vinylene carbonate, 4-ethyl vinylene carbonate, 4,5-diethyl vinylene carbonate, 4-propyl vinylene carbonate, 4 , 5-dipropyl vinylene carbonate, 4-phenyl vinylene carbonate, 4,5-diphenyl vinylene carbonate, vinyl ethylene carbonate (VEC), and divinyl ethylene carbonate. These may be used alone or in combination of two or more.
Among the above additives, at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate is preferable.
In the above additive, a hydrogen atom may be substituted with a fluorine atom in the molecule.

A known non-aqueous electrolyte may be added to a known non-aqueous electrolyte that decomposes during overcharge to form a film on the electrode and inactivate the battery. As this benzene derivative, those having a phenyl group and a cyclic compound group adjacent to the phenyl group are preferred.
The cyclic compound group is preferably a phenyl group, a cyclic ether group, a cyclic ester group, a cycloalkyl group, or a phenoxy group.
Examples of the benzene derivative include cyclohexylbenzene, biphenyl, and diphenyl ether. These may be used alone or in combination of two or more.
The proportion of the benzene derivative in the non-aqueous solvent is preferably 10% by volume or less.

The shape of the nonaqueous electrolyte secondary battery of the present invention is not particularly limited. Various shapes such as a coin type, a cylindrical type, a square type, a sheet type, a button type, a flat type, and a laminated type may be appropriately selected according to the use of the nonaqueous electrolyte secondary battery.
Further, the present invention is not limited to a small non-aqueous electrolyte secondary battery used for a power source of a small device. For example, it is also suitably used for a large-sized and large-capacity secondary battery used for power supply and power storage of an electric vehicle.

Examples of the present invention will be described in detail below, but the present invention is not limited to these examples.
Example 1
(1) Preparation of positive electrode active material Nickel composite oxide represented by Ni _0.80 Co _0.15 Al _0.05 O and lithium hydroxide monohydrate were mixed at a molar ratio of 1: 1.03, and this mixture was mixed. Firing was performed at 750 ° C. for 5 hours to obtain LiNi _0.80 Co _0.15 Al _0.05 O ₂ as a positive electrode active material. After adding 1 kg of this positive electrode active material to 1 L of water, this was stirred for 10 minutes, and the lithium carbonate contained excessively in the positive electrode active material was removed.

(2) Production of positive electrode 1 kg of the positive electrode active material obtained in (1) above and 0.5 kg of an NMP solution of polyvinylidene fluoride (PVDF) (manufactured by Kureha Chemical Co., Ltd., # 7208, solid content concentration 8% by weight) And 30 g of acetylene black together with an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to a double-arm kneader, and then stirred at 30 ° C. for 30 minutes to prepare a positive electrode slurry.
This positive electrode slurry was applied to both surfaces of an aluminum foil (thickness 20 μm) as a positive electrode current collector, and then dried at 120 ° C. for 15 minutes to form a coating film. Then, the coating film was compressed with a roll press machine to form a positive electrode active material layer to obtain a positive electrode. At this time, the total thickness of the positive electrode current collector and the positive electrode active material layers formed on both surfaces thereof was 120 μm.
Sodium hydroxide was dissolved in water to obtain a pH 10 alkaline cleaning solution (cleaning solution A1). The positive electrode was immersed in a cleaning solution at 20 ° C. for 10 seconds to wash the positive electrode. Thereafter, the positive electrode was heat-treated at 190 ° C. for 10 seconds in a constant temperature bath.
The positive electrode after the heat treatment was cut into a size (longitudinal dimension 500 mm, width dimension 43 mm) suitable for accommodation in a rectangular battery case (height 50 mm, width 34 mm, thickness 5 mm). The amount of the positive electrode active material was 3.4 g per 1 cm ^{3 of} the positive electrode active material layer.

(3) Production of negative electrode 3 kg of artificial graphite, 200 g of a modified styrene-butadiene rubber dispersion (manufactured by Nippon Zeon Co., Ltd., BM-400B, solid content 40% by weight) and 50 g of carboxymethyl cellulose together with an appropriate amount of water The mixture was put into a double arm kneader and stirred to prepare a negative electrode slurry. This negative electrode slurry was applied to both surfaces of a copper foil (thickness 12 μm) as a negative electrode current collector, and then dried at 120 ° C. for 15 minutes to form a coating film. Then, the coating film was rolled with a roll press to form a negative electrode active material layer, thereby obtaining a negative electrode. At this time, the total thickness of the negative electrode current collector and the negative electrode active material layers formed on both surfaces of the negative electrode current collector was 145 μm. The negative electrode was cut into a size (dimension 425 mm in the longitudinal direction and 45 mm in the width direction) suitable for housing in the battery case.

(4) Preparation of nonaqueous electrolyte Ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 3 to obtain a nonaqueous solvent. In this non-aqueous solvent, lithium hexafluorophosphate (LiPF ₆ ) was dissolved to obtain a non-aqueous electrolyte. At this time, the concentration of LiPF ₆ in the nonaqueous electrolyte was 1.4 mol / L.
In order to increase the charge / discharge efficiency of the battery, vinylene carbonate was added to the nonaqueous electrolyte. The content ratio of vinylene carbonate in the non-aqueous solvent was 5 parts by weight per 100 parts by weight of the non-aqueous solvent.

(5) Assembly of non-aqueous electrolyte secondary battery The positive electrode obtained in (2) above, the negative electrode obtained in (3) above, the non-aqueous electrolyte obtained in (4) above, polyethylene and polypropylene A square nonaqueous electrolyte secondary battery shown in FIG. 1 was assembled by the following procedure using a separator made of a composite film (manufactured by Celgard, product number “2300”, thickness 25 μm). FIG. 1 is a schematic longitudinal sectional view of a rectangular nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

  One end of the positive electrode lead 22 was electrically connected to the positive electrode, and one end of the negative electrode lead 23 was electrically connected to the negative electrode. A negative electrode, a positive electrode, and a separator (A089 (trade name) manufactured by Celgard Co., Ltd.) made of a polyethylene microporous film with a thickness of 20 μm interposed therebetween are wound, and an electrode having a substantially elliptical cross section Group 21 was configured. A bottomed rectangular tube-shaped aluminum battery can 20 having a bottom portion 20a and side portions 20b was prepared. The electrode group 21 was accommodated in the battery can 20. Thereafter, an insulator 24 for preventing a short circuit between the battery can 20 and the positive electrode lead 22 or the negative electrode lead 23 was disposed on the electrode group 21. Next, a sealing plate 25 having a negative electrode terminal 27 surrounded by an insulating gasket 26 at the center was disposed in the opening of the battery can 20. The other end of the negative electrode lead 23 was electrically connected to the negative electrode terminal 27. The other end of the positive electrode lead 22 was electrically connected to the lower surface of the sealing plate 25. The end of the opening of the battery can 20 and the sealing plate 25 were welded by laser to seal the opening of the battery can 20. Thereafter, a non-aqueous electrolyte was injected into the battery can 20 from the injection hole of the sealing plate 25. Finally, the liquid injection hole was closed by welding with a plug 29 to complete a square lithium ion secondary battery A (height 50 mm, width 34 mm, thickness 5 mm) having a design capacity of 900 mAh.

<< Comparative Example 1 >>
A battery B1 was produced in the same manner as in Example 1 except that the positive electrode was not washed.

<< Comparative Example 2 >>
Lithium hydroxide was dissolved in water to obtain an alkaline cleaning liquid (cleaning liquid B2) having a pH of 10. A battery B2 was produced in the same manner as in Example 1 except that this cleaning liquid B2 was used instead of the cleaning liquid A1.

[Evaluation]
(6) Qualitative analysis of carbonate of element A adhering to the surface of the positive electrode active material The surface of the positive electrode was analyzed by X-ray photoelectron spectroscopy. In the X-ray photoelectron spectrum obtained by the analysis, the binding spectrum of element A and carbon was confirmed, and it was confirmed that the carbonate of element A was formed on the surface of the positive electrode.

(7) Quantification of element A adhering to the surface of the positive electrode active material The element A adhering to the surface of the positive electrode active material was quantified by the following method.
The central part of the positive electrode was cut into a size of 2.0 cm in length and 2.0 cm in width to obtain a sample piece. After putting a sample piece into a 50 mL sample bottle containing 25 mL of ion-exchanged water, the ion-exchanged water was stirred for 30 minutes in this state. Within 10 minutes after stirring, the ion-exchanged water in the sample bottle was filtered with a filter having a pore diameter of 0.45 μm. The filtrate was used as a measurement sample. The content of element A in the measurement sample was quantified by ion chromatography.

(8) Evaluation of Battery (i) Evaluation of Charging / Discharging Cycle Characteristics Under a 45 ° C. environment, charging / discharging was performed under the following conditions to determine the discharge capacity at the first cycle.
<Charging / discharging conditions>
(A) Charging Constant current charging: Charging current value 900 mA / end-of-charge voltage 4.2 V
Constant voltage charging: Charging voltage value 4.2V / end-of-charge current 100mA
(B) Discharge Constant current discharge: discharge current value 900 mA / discharge end voltage 2.5 V

Thereafter, charging and discharging were repeated 500 cycles under the above conditions in a 45 ° C. environment, and the discharge capacity at the 500th cycle was determined.
The capacity retention rate A was determined by the following formula.
Capacity maintenance rate A (%)
= Discharge capacity at 500th cycle / Discharge capacity at the first cycle × 100

Moreover, the thickness of the battery after 1 cycle and the thickness of the battery after 500 cycles were measured.
The battery thickness increase rate (%) was determined by the following formula.
Increase rate of battery thickness (%)
= Battery thickness after 500 cycles / Battery thickness after 1 cycle x 100

(Ii) Evaluation of high-temperature storage characteristics Two batteries were prepared.
About one battery, it charged / discharged on the said conditions (a) and (b) in 25 degreeC environment, and calculated | required the discharge capacity. This was the discharge capacity before storage.
The other battery was charged under the above condition (a) in a 25 ° C. environment and then stored in an 85 ° C. environment for 2 days.
Then, it discharged on the said conditions (b) in 25 degreeC environment, and calculated | required the discharge capacity. This was the discharge capacity after storage.
The capacity maintenance rate B was determined by the following formula.
Capacity retention ratio B (%) = discharge capacity after storage / discharge capacity before storage × 100

In addition, the battery thickness before and after storage was measured. The increase rate (%) of the battery thickness after storage with respect to the battery thickness before storage was determined by the following formula.
Increase rate of battery thickness (%) = battery thickness after storage / battery thickness before storage × 100
The evaluation results are shown in Table 1.

In the battery A of Example 1, compared with the batteries B1 and B2 of Comparative Examples 1 and 2, the battery thickness increase rate is greatly reduced, battery swelling is suppressed, and excellent charge / discharge cycle characteristics and high temperature storage characteristics are obtained. It was.
This is presumably because the lithium carbonate was removed by washing the positive electrode with an aqueous sodium hydroxide solution, and sodium carbonate more stable than lithium carbonate was produced on the active material surface.

Example 2
In the positive electrode cleaning method, a positive electrode was produced in the same manner as in Example 1 except that the amount of sodium hydroxide dissolved in water was changed and the pH of the cleaning solution was changed to the values shown in Table 2. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 2.

In any case, the adhesion amount of the element A on the positive electrode was in the range of 8 to 34 μmol / g, and all the batteries exhibited good charge / discharge cycle characteristics and high temperature storage characteristics.
In particular, in batteries D and E, excellent charge / discharge cycle characteristics and high-temperature storage characteristics were obtained, and it was found that the pH of the alkaline liquid in positive electrode cleaning is preferably 9-12.

Example 3
A positive electrode was produced in the same manner as in Example 1 except that the positive electrode immersion time was changed to the values shown in Table 3 in the positive electrode cleaning method. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 3.

  The battery G in which the positive electrode immersion time was 30 seconds exhibited good charge / discharge cycle characteristics and high-temperature storage characteristics. In the battery H in which the positive electrode immersion time was longer than 30 seconds, the reaction between the active material and the alkaline liquid was promoted, the amount of the element A deposited on the positive electrode was increased, and the charge / discharge cycle characteristics and the high-temperature storage characteristics were deteriorated.

Example 4
In the positive electrode cleaning method, a positive electrode was produced in the same manner as in Example 1 except that the temperature of the cleaning solution was changed to the values shown in Table 4. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 4.

  Batteries I and J having a cleaning liquid temperature of 60 ° C. or less exhibited good charge / discharge cycle characteristics and high-temperature storage characteristics. In the battery K in which the temperature of the cleaning liquid is higher than 60 ° C., the side reaction between the active material and the cleaning liquid has progressed, and thus the positive electrode cleaning effect is considered to be reduced.

Example 5
In the positive electrode cleaning method, a positive electrode was produced in the same manner as in Example 1 except that the metal species of the cleaning liquid (pH 10) was changed to the metal species shown in Table 5. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 5.

  All the positive electrodes had an adhesion amount of element A in the range of 8 to 17 μmol / g, and all the batteries exhibited excellent charge / discharge cycle characteristics and high-temperature storage characteristics. Since carbonates more stable than lithium carbonate were formed, the amount of gas generated during charge / discharge cycles and high-temperature storage decreased.

Example 6
In the heat treatment method after washing the positive electrode, a positive electrode was produced in the same manner as in Example 1 except that the heat treatment temperature and time were changed to the values shown in Table 6. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 6.

  In the batteries R to T, V, and W, the adhesion amount of the element A on the positive electrode was 8 to 34 μmol / g, and good charge / discharge cycles and high temperature storage characteristics were obtained. In particular, in the batteries S, T, and W, the adhesion amount of the element A on the positive electrode was 8 to 17 μmol / g, and excellent charge / discharge cycle characteristics and high-temperature storage characteristics were obtained. When heat treatment was carried out under these conditions, the active material was sufficiently covered with the binder, and the amount of carbonate adhered decreased, so the amount of gas generated during charge / discharge cycles and high temperature storage decreased.

Example 7
A positive electrode was produced in the same manner as in Example 1 except that the active material density of the positive electrode (active material weight per 1 cm ^{3 of the} positive electrode active material layer) was changed to the values shown in Table 7. The active material density was adjusted by changing the linear pressure when the coating film was compressed with a roll press. Using this positive electrode, a battery was produced in the same manner as in Example 1. The positive electrode and the battery were evaluated by the same method as described above. The evaluation results are shown in Table 7 together with the results of the battery A of Example 1.

In the batteries X, A, and Y, the adhesion amount of the positive electrode element A was 8 to 20 μmol / g, and good charge / discharge cycles and high-temperature storage characteristics were obtained. When the active material density of the positive electrode was in the range of 3.2 to 3.6 g / cm ³ , it was possible to suppress the adhesion of lithium carbonate, and the amount of gas generated during charge / discharge cycles and high-temperature storage decreased.
In the above embodiment, a square battery is used, but the same effect can be obtained by using a battery of another shape such as a cylindrical battery.

  Since the non-aqueous electrolyte secondary battery of the present invention is excellent in charge / discharge cycle characteristics and high-temperature storage characteristics, it is required to have a power source for small electronic devices such as notebook computers, mobile phones, digital still cameras, and high output. It is suitably used for power storage and power sources for electric vehicles.

20 Battery Can 21 Electrode Group 22 Positive Electrode Lead 23 Negative Electrode Lead 24 Insulator 25 Sealing Plate 26 Insulating Gasket 27 Negative Electrode Terminal 29 Sealing

Claims (8)

A positive electrode current collector, and a positive electrode active material layer formed on the surface of the positive electrode current collector,
The positive electrode active material layer includes a lithium transition metal oxide as a positive electrode active material,
At least one element selected from the group consisting of alkali metals other than lithium and alkaline earth metals is supported on the surface of the lithium transition metal oxide,
At least a portion of the element forms a carbonate;
The positive electrode for a non-aqueous electrolyte secondary battery, wherein the amount of the element is 8 to 34 μmol per 1 g of the weight of the lithium transition metal oxide. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the amount of the positive electrode active material is 3.2 to 3.6 g per 1 cm ^{3 of the} positive electrode active material layer.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the element is at least one selected from the group consisting of Na, K, Rb, Mg, Ca, Sr, and Ba.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the transition metal contained in the lithium transition metal oxide contains at least nickel.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 4, wherein the amount of nickel occupies 30 mol% or more of the transition metal. The lithium transition metal oxide has the general formula (1):
Li _x Ni _w M _z Me _{1- (w + z)} O _{2 + d}
Represented by
M is at least one selected from Co and Mn,
Me is a metal different from M, or at least one selected from the group consisting of B, P and S;
d represents an oxygen defect or oxygen excess;
0.98 ≦ x ≦ 1, 0.3 ≦ w ≦ 1.0, 0 ≦ z ≦ 0.7, and 0.9 ≦ (w + z) ≦ 1.0. The positive electrode for nonaqueous electrolyte secondary batteries as described.   The positive electrode for a nonaqueous electrolyte secondary battery according to claim 6, wherein the metal different from M is at least one selected from the group consisting of Mg, Sc, Y, Fe, Cu, Zn, Al, and Cr. .   A nonaqueous electrolyte secondary battery comprising: the positive electrode according to claim 1; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and a nonaqueous electrolyte.

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