WO2014185006A1

WO2014185006A1 - Lithium-ion-secondary-battery positive electrode and lithium-ion secondary battery

Info

Publication number: WO2014185006A1
Application number: PCT/JP2014/002096
Authority: WO
Inventors: 達哉江口; 仁愛清
Original assignee: 株式会社豊田自動織機
Priority date: 2013-05-13
Filing date: 2014-04-14
Publication date: 2014-11-20
Also published as: JP5610031B1; JP2014222572A

Abstract

Provided are a lithium-ion-secondary-battery positive electrode which exhibits excellent cycle properties and excellent discharge capacity when used at a high temperature and high voltage, and a lithium-ion secondary battery. A lithium-ion-secondary-battery positive electrode having a current collector, a positive-electrode-active-material layer containing a positive electrode active material and an adhesive and positioned on the surface of the current collector, and a coating layer positioned on at least part of the surface of the positive-electrode-active-material layer, the lithium-ion-secondary-battery positive electrode being characterized in that the coating layer has an insulating inorganic powder and a binder for the coating layer, and in that the coating layer is porous.

Description

Positive electrode for lithium ion secondary battery and lithium ion secondary battery

The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery.

2. Description of the Related Art In recent years, with the development of portable electronic devices such as mobile phones and laptop computers, and the commercialization of electric vehicles, a small, lightweight, high-capacity secondary battery is required. At present, lithium ion secondary batteries using lithium cobaltate (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material have been commercialized as high-capacity secondary batteries.

It is required that lithium ion secondary batteries can be used even in high temperature, high voltage use environments. In this specification, use at a voltage of 4.3 V or more is defined as high voltage use. However, when driven at high temperature and high voltage, the lithium ion secondary battery has a problem that the cycle characteristics are extremely deteriorated.

As the cause of this, when the lithium ion secondary battery is driven at high temperature and high voltage, the electrolytic decomposition occurs in the vicinity of the positive electrode during charging, and metal components such as the positive electrode active material are eluted by the acid generated by the oxidative decomposition. It is considered to be. When the metal component is eluted from the positive electrode active material, the capacity of the positive electrode decreases and the cycle characteristics deteriorate. In addition, decomposition products of the electrolyte solution or elution products of the metal components, which are generated by oxidative decomposition of the electrolyte solution or elution of the metal component, move from the positive electrode side to the negative electrode side and are reductively decomposed on the negative electrode surface. As a result, the decomposed matter is deposited on the surface of the negative electrode active material, and the deposit inhibits the insertion of lithium into the negative electrode active material, resulting in deterioration of cycle characteristics.

Various studies have been conducted to improve cycle characteristics under high temperature and high voltage use environments.

For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-53207) describes that on the positive electrode active material layer is formed a covering layer containing filler particles composed of inorganic particles containing magnesia and a binder. . In this technique, the covering layer provided on the positive electrode active material layer physically traps the decomposition product of the electrolytic solution and the elution product of the metal component. Patent Document 1 describes that the lithium ion secondary battery is excellent in cycle characteristics and storage characteristics under high temperature and high voltage use environment by having this covering layer.

However, Patent Document 1 does not describe how much the charge and discharge capacity is reduced by the formation of the covering layer. Moreover, it is described in patent document 1 that magnesia which comprises a coating layer reacts with water, and becomes alkaline, and it is estimated that magnesia which is a structural component of a coating layer is eluted. That is, it is assumed that the covering layer is gradually destroyed as the cycle is repeated. Therefore, it is required to protect the positive electrode active material layer with a stable material even in a high temperature, high voltage operating environment.

Further, in Patent Document 2 (Japanese Patent Application Laid-Open No. 6-168739), a positive electrode on the side facing the negative electrode is a film selected from an insulator, a semiconductor, and a composite of an insulator and a semiconductor capable of transmitting ions involved in the cell reaction. A surface-coated secondary battery is described. Patent Document 2 describes that by covering the positive electrode surface with the above-mentioned film, a short circuit inside the battery can be suppressed, and the cycle life of the battery can be extended. Patent Document 2 discloses that a nitride film is formed by sputtering on the surface of a positive electrode. In the technique described in Patent Document 2, since the protective film made of nitride is formed by sputtering, the protective film described in Patent Document 2 is a dense film without voids. Therefore, in the lithium battery having a protective film made of nitride described in Patent Document 2, it is presumed that the protective film becomes a resistance and the charge and discharge capacity is reduced as compared with a lithium battery having no protective film. Patent Document 2 does not describe how much the charge and discharge capacity is reduced by the formation of the protective film. Patent Document 2 does not describe use at high temperature and high voltage.

JP 2008-53207 A Japanese Patent Application Laid-Open No. 6-168739

The present inventors examined the material of the coating layer that is advantageous for obtaining a lithium ion secondary battery having excellent cycle characteristics and excellent charge and discharge capacity in a high temperature, high voltage use environment.

The present invention has been made in view of such circumstances, and a positive electrode for lithium ion secondary battery having excellent cycle characteristics and excellent charge and discharge capacity under high temperature and high voltage use environment, and lithium ion The purpose is to provide a secondary battery.

As a result of intensive studies by the present inventors, it is possible to provide a lithium ion secondary battery by arranging a coating layer having an insulating inorganic powder and a binder for a coating layer and having a void on the surface of a positive electrode active material layer. It has been found that it can exhibit excellent cycle characteristics and charge / discharge capacity even in a high temperature, high voltage use environment.

That is, the positive electrode for a lithium ion secondary battery of the present invention comprises a current collector, a positive electrode active material layer disposed on the surface of the current collector and containing a positive electrode active material and a binder, and a surface of the positive electrode active material layer. The coating layer is characterized in that the coating layer has an insulating inorganic powder and a binder for a coating layer, and the coating layer has a void.

In the image which observed the surface of the coating layer with the scanning electron microscope (Scanning Electron Microscope), it is preferable that the aperture ratio of the surface of a coating layer is 5% or more and 50% or less.

The thickness of the covering layer is preferably 1 μm or more and 20% or less of the thickness of the positive electrode active material layer.

It is preferable that the average particle size _{D 50} of the inorganic powder is 100nm or more 1μm or less.

The inorganic powder is preferably made of at least one selected from boron nitride, aluminum nitride and silicon nitride.

The lithium ion secondary battery of the present invention is characterized by including the above-described positive electrode for a lithium ion secondary battery, a negative electrode, and a non-aqueous electrolytic solution.

The positive electrode for a lithium ion secondary battery of the present invention has a covering layer disposed on at least a part of the surface of the positive electrode active material layer. This coating layer has an insulating inorganic powder and a coating layer binder. And a coating layer has a void.

Since the positive electrode having such a coating layer is used, the lithium ion secondary battery of the present invention exhibits excellent cycle characteristics and charge / discharge capacity even in a high temperature, high voltage use environment.

It is a schematic diagram explaining the positive electrode for lithium ion secondary batteries of this embodiment. It is a graph showing the cycle test result of the lamination type lithium ion secondary battery of Example 1, Example 2, Example 3 and Comparative Example 1.

<Positive electrode for lithium ion secondary battery>
The positive electrode for a lithium ion secondary battery of the present invention comprises a current collector, a positive electrode active material layer disposed on the surface of the current collector, and a covering layer disposed on at least a portion of the surface of the positive electrode active material layer; Have.

The current collector refers to a chemically inactive electron conductor for keeping current flowing to the electrode during discharge or charge of the lithium ion secondary battery. As a material of a collector, metal materials, such as stainless steel, titanium, nickel, aluminum, copper, or electroconductive resin can be mentioned, for example. Also, the current collector can take the form of a foil, a sheet, a film or the like. Therefore, as the current collector, for example, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be suitably used.

The thickness of the current collector is preferably 10 μm to 100 μm.

The positive electrode active material layer contains a positive electrode active material and a binder. The positive electrode active material layer may further contain a conductive aid, if necessary.

Examples of positive electrode active materials include lithium-containing compounds or other metal compounds. As the lithium-containing compound, for example, lithium cobalt composite oxide having a layered structure, lithium nickel composite oxide having a layered structure, lithium manganese composite oxide having a spinel structure, a general formula: Li _a Co _p Ni _q Mn _r D _s O _x (D is at least one selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe and Na, p + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 Lithium cobalt-containing composite metal oxide having a layered structure represented by ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) , An olivine-type lithium phosphate composite oxide represented by the general formula: LiMPO ₄ (M is at least one of Mn, Fe, Co, and Ni), represented by the general formula: Li ₂ MPO ₄ F Fluorinated olivine type lithium phosphate complex oxide (M is at least one of Mn, Fe, Co and Ni), silicate type lithium complex oxide represented by the general formula: Li ₂ MSiO ₄ (M is Mn And at least one of Fe, Co and Ni). Moreover, as other metal compounds, for example, oxides such as titanium oxide, vanadium oxide or manganese dioxide, or disulfides such as titanium sulfide or molybdenum sulfide can be mentioned.

The positive electrode active material is preferably made of a lithium-containing oxide represented by a chemical formula: LiMO ₂ (M is at least one selected from Ni, Co and Mn), and more preferably a general formula: Li _a Co _p Ni _q Mn _r D _s O _x (D is at least one selected from Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, and Na, p + q + r + s = 1, 0 <p <1, 0 ≦ Lithium having a layered structure represented by q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) It is preferable to consist of cobalt containing complex metal oxide.

As the lithium-containing oxide, for example, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi _0.5 Co _0.2 Mn _{0. _{_{_{3 O 2, LiCoO 2, LiNi}}}} 0.8 Co 0.2 O 2, LiCoMnO 2 and the like. Among them, LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiNi _0.5 Co _0.2 Mn _0.3 O ₂ are preferable as the lithium-containing oxide from the viewpoint of thermal stability.

The positive electrode active material is preferably in the form of powder having an average particle diameter D ₅₀ of 1 μm to 20 μm. When the average particle diameter D ₅₀ of the positive electrode active material is small, when the positive electrode active material of the same mass is used, the number of positive electrode active materials increases, and the surface area of the entire positive electrode active material becomes large. An increase in the surface area of the positive electrode active material results in an increase in the reaction area between the positive electrode active material and the electrolytic solution, which accelerates the decomposition of the electrolytic solution and may deteriorate the cycle characteristics of the lithium ion secondary battery. Thus the average particle size D ₅₀ of the positive electrode active material is not preferred is that too small an average particle diameter D ₅₀ of the positive electrode active material is preferably at least 1 [mu] m. The average particle diameter D ₅₀ of the positive electrode active material becomes large 20μm greater than the resistance of the lithium ion secondary battery, there is a concern that the output characteristics of the lithium ion secondary battery decreases. The average particle diameter D ₅₀ of the positive electrode active material can be measured by particle size distribution measurement method.

The binder plays a role of securing the positive electrode active material and the conductive auxiliary agent to the current collector. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene, polyethylene and polyvinyl acetate resins, imide resins such as polyimide and polyamideimide, and alkoxy Examples thereof include silyl group-containing resins and rubbers such as styrene butadiene rubber (SBR).

The conductive aid is added to the positive electrode active material layer as needed to enhance the conductivity of the electrode. As a conductive support agent, carbon black which is carbonaceous fine particles, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), and vapor grown carbon fiber (VGCF) can be mentioned. These conductive aids can be used alone or in combination of two or more. The amount of the conductive aid used is not particularly limited, but can be, for example, about 1 to 30 parts by mass with respect to 100 parts by mass of the active material contained in the positive electrode.

In order to dispose the positive electrode active material layer on the surface of the current collector, a composition for forming a positive electrode active material layer containing a positive electrode active material, a binder, and, if necessary, a conductive auxiliary, is prepared, and this composition A suitable solvent may be added to form a paste, which may be applied to the surface of the current collector and then dried. If necessary, the current collector provided with the positive electrode active material layer may be compressed to increase the electrode density.

As a method of applying the composition for forming a positive electrode active material layer, conventionally known methods such as roll coating method, dip coating method, doctor blade method, spray coating method and curtain coating method may be used.

As a solvent for viscosity adjustment, N-methyl-2-pyrrolidone (NMP), methanol, methyl isobutyl ketone (MIBK) and the like can be used.

The covering layer is disposed on at least a part of the surface of the positive electrode active material layer. Since at least a part of the surface of the positive electrode active material layer is covered by the covering layer, the positive electrode active material is not likely to be in direct contact with the electrolytic solution. Therefore, the decomposition reaction of the electrolytic solution by the positive electrode active material is suppressed. Therefore, deterioration of the cycle characteristics can be suppressed.

The covering layer has an air gap. The coating layer has an insulating inorganic powder and a coating layer binder. In the coating layer, the inorganic powder is disposed with gaps in some places, and the binder for the coating layer is disposed between the inorganic powders and between the inorganic powder and the positive electrode active material layer. The voids are formed between the inorganic powders, between the inorganic powder and the coating layer binder, or between the inorganic powder and the positive electrode active material layer. Because of the air gaps in the cover layer, lithium ions easily pass through the cover layer to reach the positive electrode active material layer. That is, the coating layer is unlikely to be resistant to lithium ion conduction, and even if the coating layer is disposed on the surface of the positive electrode active material layer, the charge / discharge capacity of the positive electrode is unlikely to be reduced.

In addition, the coating layer has a complex surface formed of an inorganic powder and thus has a large surface area. Therefore, the coating layer is likely to physically trap the elution product of the metal component eluted from the positive electrode active material and the decomposition product of the electrolytic solution. Since the elution product of the metal component and the decomposition product of the electrolytic solution can be physically trapped on the surface of the positive electrode, deposition of the decomposition product and the like on the surface of the negative electrode active material can be suppressed. As a result, deterioration of the cycle characteristics of the lithium ion secondary battery can be suppressed.

Further, the binder for the coating layer binds the inorganic powders together and the inorganic powder and the positive electrode active material layer. Therefore, the coating layer is difficult to peel off from the positive electrode active material layer. Therefore, even if the cycle is repeated, the effect of the coating layer is likely to be sustained.

The binder for the coating layer may be contained in the coating layer in an amount of 3% by mass to 50% by mass with respect to the inorganic powder. When the binder for the coating layer is contained in the coating layer in an amount of more than 50% by mass with respect to the inorganic powder, the protective effect of the inorganic powder on the positive electrode active material layer is reduced, and voids are hardly formed. When the binder for the coating layer is contained in the coating layer in an amount of less than 3% by mass with respect to the inorganic powder, it is difficult to obtain the binding effect of the binder for the coating layer.

The voids may be anywhere in the coating layer. The voids may be located not only on the surface of the covering layer but also on the inside of the covering layer. One void may be in communication with another void.

When the surface of the coating layer is observed with an electron microscope, it can be seen that there is an opening in the coating layer. The opening points to a point where the positive electrode active material layer is observed from the surface through the voids included in the covering layer. Therefore, the aperture ratio can be determined by dividing the area of the opening by the area of the entire covering layer. At this time, the area of the entire covering layer includes the area of the opening.

In the image which observed the surface of the coating layer with the scanning electron microscope (Scanning Electron Microscope), it is preferable that the aperture ratio of the surface of a coating layer is 5% or more and 50% or less. When the aperture ratio is 5% or more, the lithium ion permeability of the coating layer is good. If the aperture ratio is 50% or less, direct contact of the electrolytic solution with the positive electrode active material can be reduced, so that decomposition of the electrolytic solution by the positive electrode active material can be favorably suppressed.

The thickness of the coating layer is substantially the average particle diameter D ₅₀ of the inorganic powder equal to or greater than. If a coating layer having a thickness at which the inorganic powder is at least one layer is disposed on the surface of the positive electrode active material layer, the positive electrode active material layer is well protected by the coating layer.

The thickness of the covering layer is preferably 1 μm or more and 20% or less of the thickness of the positive electrode active material layer. When the thickness of the covering layer is 1 μm or more, the cycle characteristics of the lithium ion secondary battery can be further improved even in a high temperature, high voltage use environment. If the thickness of the covering layer is 20% or less of the thickness of the positive electrode active material layer, the volume of the positive electrode active material layer, that is, the amount of the positive electrode active material is satisfactorily secured in the battery, and the charge and discharge capacity of the lithium ion secondary battery Can reduce the

The average particle diameter D ₅₀ of the inorganic powder is preferably smaller than the average particle diameter D ₅₀ of the positive electrode active material. When the average particle diameter D ₅₀ of the inorganic powder is smaller than the average particle diameter D ₅₀ of the positive electrode active material, the coating layer containing an inorganic powder is better to cover the positive electrode active material layer along the irregularities on the surface of the positive electrode active material layer be able to. In this case, the surface of the positive electrode active material present on the surface of the positive electrode active material layer is easily covered with the covering layer containing the inorganic powder. If at least a part of the surface of the positive electrode active material is covered with the covering layer containing the inorganic powder, direct contact of the surface of the positive electrode active material with the electrolytic solution can be suppressed, and the electrolytic solution is decomposed by the positive electrode active material. Can be suppressed. The inorganic powder preferably has an average particle diameter D ₅₀ of 100 nm or more and 1 μm or less. When the average particle diameter D ₅₀ of the inorganic powder is 100nm or 1μm or less, the coating layer containing an inorganic powder is more likely to cover the surface of the positive electrode active material layer.

Since the inorganic powder is insulating, when the covering layer containing the inorganic powder is disposed on the surface of the positive electrode active material layer, short circuit between the positive electrode and the negative electrode can be suppressed.

For example, nitrides, carbides, metal oxides can be used as the insulating inorganic powder. Examples of the nitride include boron nitride, aluminum nitride, silicon nitride and carbon nitride. Examples of carbides include silicon carbide and boron carbide. Examples of the metal oxide include silica, alumina, magnesium oxide, zirconium oxide and hafnium oxide.

Particularly preferred inorganic powders are those which are thermally stable. For that reason, the inorganic powder is preferably a nitride. In general, nitrides are thermally stable up to about 1500.degree. Since 1500 ° C. is a temperature sufficiently higher than the thermal runaway range of the lithium ion secondary battery, the nitride can suppress the thermal runaway of the lithium ion secondary battery.

In general, nitrides have high chemical stability and can be arranged without causing an extra side reaction accompanying the cell reaction. The inorganic powder is preferably made of at least one selected from boron nitride, aluminum nitride and silicon nitride because of high chemical stability of the substance.

The covering layer is disposed on the surface of the positive electrode active material layer. It is desirable that the coating layer be disposed after the positive electrode active material layer is formed so that the coating layer and the positive electrode active material layer are not mixed.

The method of arranging the coating layer on the positive electrode active material layer is not particularly limited. For example, the coating layer can be disposed on the positive electrode active material layer by the following method. The material of the coating layer is dissolved in an organic solvent or water to form a solution, and the solution is sprayed onto the coated surface of the positive electrode active material layer using a sprayer to volatilize and remove the organic solvent or water. Can be placed. As the organic solvent in this case, ethanol, N-methyl-2-pyrrolidone (NMP), methanol, butanol, methyl isobutyl ketone (MIBK) and the like can be used. Water is preferably one from which impurities such as distilled water or ion exchange water have been removed.

In addition, the material of the coating layer is dissolved in an organic solvent or water for viscosity adjustment to form a paste-like mixture, the paste-like mixture is applied on the positive electrode active material layer, and dried after application. A covering layer can be arranged on the layer. As a coating method, conventionally known methods such as roll coating method, dip coating method, doctor blade method, spray coating method and curtain coating method may be used. As an organic solvent for viscosity adjustment, ethanol, NMP, methanol, butanol, MIBK and the like can be used. Water is preferably one from which impurities such as distilled water or ion exchange water have been removed.

In the above-described method of arranging the covering layer, the adjustment of the opening ratio in the covering layer can be performed by adjusting the amount of the covering layer applied. The larger the coating amount of the coating layer, the smaller the aperture ratio.

As the binder for the covering layer, an inorganic binder containing an element such as silicon element, aluminum element or zirconium element, phenol resin, polyimide resin, epoxy resin or melamine resin can be used.

In order to simplify the arrangement of the coating layer, it is preferable to use, as a binder for the coating layer, a binder that can be dissolved in an organic solvent and that solidifies at room temperature if the organic solvent volatilizes. When such a binder is used, it is not necessary to heat the electrode to form the coating layer. It is preferable to use the inorganic binder which can be solidified, for example at normal temperature as a binder for coating layers. Since the inorganic binder is chemically stable, the decomposition reaction involved in the battery reaction can be suppressed. As the inorganic binder, an inorganic binder containing zirconium element, particularly zirconium oxide can be preferably used.

The schematic diagram explaining the positive electrode for lithium ion secondary batteries of this embodiment in FIG. 1 is shown. In FIG. 1, a positive electrode active material 3 is bound by a binder 2 on a current collector 1. The positive electrode active material layer 4 is composed of the positive electrode active material 3 and the binder 2. The covering layer 5 is disposed on the positive electrode active material layer 4.

In the covering layer 5 of FIG. 1, the inorganic powder 51 is disposed with a gap in some places, and the coating layer binder 52 is disposed between the inorganic powders 51 and between the inorganic powder 51 and the positive electrode active material layer 4 There is. The inorganic powder 51 and the inorganic powder 51 and the positive electrode active material layer 4 are bound to each other by the coating layer binder 52. As shown in FIG. 1, the voids 6 are formed between the inorganic powders 51, between the inorganic powders 51 and the binder 52 for a covering layer, or between the inorganic powders 51 and the positive electrode active material layer 4. In addition, the inorganic powder 51 is disposed along the irregularities of the surfaces of the positive electrode active material 3 and the binder 2.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention is characterized by having the above-described positive electrode for a lithium ion secondary battery. The lithium ion secondary battery which has the said positive electrode for lithium ion secondary batteries has the outstanding cycle performance also in the use environment of high temperature and a high voltage.

The lithium ion secondary battery of the present invention has, as a battery component, a negative electrode, a separator and an electrolytic solution in addition to the above-described positive electrode for lithium ion secondary battery.

The negative electrode includes a current collector and a negative electrode active material layer bonded to the surface of the current collector. The negative electrode active material layer contains a negative electrode active material and a binder, and optionally contains a conductive auxiliary. The current collector, the binder, and the conductive additive are the same as those described for the positive electrode.

Examples of the negative electrode active material include carbon-based materials capable of occluding and releasing lithium, compounds having an element capable of being alloyed with lithium, and compounds having an element capable of being alloyed with lithium, and polymer materials.

Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, cokes, graphites, glassy carbons, an organic polymer compound fired body, carbon fiber, activated carbon or carbon blacks. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenol or furan at an appropriate temperature.

As elements that can be alloyed with lithium, for example, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge , Sn, Pb, Sb and Bi. Among them, silicon (Si) or tin (Sn) is preferable as an element capable of being alloyed with lithium.

As a compound having an element capable of alloying with lithium, for example, ZnLiAl, AlSb, SiB ₄ , SiB ₆ , Mg ₂ Si, Mg ₂ Sn, Ni ₂ Si, TiSi ₂ , MoSi ₂ , CoSi ₂ , NiSi ₂ , CaSi _{_{_{_{2, CrSi 2, Cu 5 Si}}}} , FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2, SiC, Si 3 N 4, Si 2 N 2 O, SiO v (0 <v ≦ 2) SnO _w (0 <w ≦ 2), SnSiO ₃ , LiSiO and LiSnO. The compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. As a silicon compound, SiO _x (0.5 ≦ x ≦ 1.5) is preferable. Examples of tin compounds include tin alloys (Cu--Sn alloy, Co--Sn alloy, etc.).

Examples of the polymer material include polyacetylene and polypyrrole.

The separator separates the positive electrode and the negative electrode, and allows lithium ions to pass while preventing the short circuit of the current due to the contact of the both electrodes. As the separator, for example, a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene or polyethylene, or a porous film made of ceramic can be used.

As the electrolyte, an electrolyte that can be used for a lithium ion secondary battery can be used. The electrolytic solution contains a solvent and an electrolyte dissolved in the solvent.

As the solvent, for example, cyclic esters, linear esters, ethers can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma-butyrolactone, vinylene carbonate, 2-methyl-gamma-butyrolactone, acetyl-gamma-butyrolactone and gamma-valerolactone. Examples of chain esters include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, methyl ethyl carbonate, propionic acid alkyl ester, malonic acid dialkyl ester and acetic acid alkyl ester. As the ethers, for example, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane can be mentioned.

Examples of the electrolyte to be dissolved in the above electrolyte include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , and LiN (CF ₃ SO ₂ ) ₂ .

As an electrolytic solution, for example, 0.5 mol / l to 1.7 mol / l of lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ and the like in a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, dimethyl carbonate There may be mentioned solutions dissolved at a certain concentration.

The lithium ion secondary battery can be mounted on a vehicle. Since the lithium ion secondary battery has excellent cycle performance, a vehicle equipped with the lithium ion secondary battery has high performance in terms of life and output.

Any vehicle may be used as long as it uses electric energy from batteries for all or part of the power source. For example, electric vehicles, hybrid vehicles, plug-in hybrid vehicles, hybrid railway vehicles, electric forklifts, electric wheelchairs, electric assists There are bicycles and electric motorcycles.

As mentioned above, although embodiment of the positive electrode for lithium ion secondary batteries of this invention and lithium ion secondary battery were described, this invention is not limited to the said embodiment. In the range which does not deviate from the summary of the present invention, it can carry out with various forms which gave change, improvement, etc. which a person skilled in the art can make.

Hereinafter, the present invention will be described in more detail by way of examples.

<Preparation of positive electrode for lithium ion secondary battery>
(Positive electrode A)
First, as a positive electrode active material, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ having an average particle diameter D ₅₀ of 5 μm, acetylene black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder, respectively 94 parts by mass, 3 parts by mass, and 3 parts by mass were mixed to obtain a mixture. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to make a slurry.
An aluminum foil having a thickness of 20 μm was prepared as a current collector. The slurry was placed on the current collector, and was applied to the current collector using a doctor blade so that the slurry became a film. The resulting sheet was dried at 80 ° C. for 20 minutes to volatilize off the NMP. After that, the current collector and the coating on the current collector were firmly and closely bonded by a roll press machine. At this time, the electrode density was adjusted to 3.2 g / cm ² . The assembly was heated in a vacuum oven at 120 ° C. for 6 hours. The joined product after heating was cut into a predetermined shape (25 mm × 30 mm rectangular shape) to obtain a positive electrode A. The thickness of the positive electrode A was about 50 μm.

(Positive electrode B)
As insulating inorganic powder, the average particle diameter D ₅₀ was prepared powder of boron nitride 500 nm. Tetrabutoxy zirconium was prepared as a coating layer binder. The boron nitride powder and tetrabutoxyzirconium were weighed and mixed so as to have a mass ratio of 3: 2. The mixture was dispersed in butanol. The mixture dispersed in butanol described above was sprayed onto the surface of the positive electrode A using the sprayer as the number of times of spraying was once. The spraying time for one spraying was 5 seconds per unit area in the spraying range. By evaporating and removing butanol, a coating layer composed of a 1 μm thick boron nitride powder and a binder for coating layer was formed on the surface of the positive electrode A. Let this be the positive electrode B. When the surface of the positive electrode B was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed to follow the surface of the positive electrode active material layer.

(Positive electrode C)
A coating layer consisting of a 2 μm thick boron nitride powder and a coating layer binder was formed on the surface of the positive electrode A in the same manner as the positive electrode B except that the number of times of spraying was two. This is called positive electrode C. Specifically, the number of times of spraying is set to 2 times, after spraying once over the entire spraying range and then again performing the second spraying on the first spraying location as in the first spraying. The When the surface of the positive electrode C was observed with a scanning electron microscope (SEM), it could be confirmed that the coating layer was disposed to follow the surface of the positive electrode active material layer.

(Positive electrode D)
A coating layer of 5 μm thick boron nitride powder and a binder for a coating layer was formed on the surface of the positive electrode A in the same manner as the positive electrode B except that the number of times of spraying was 5 times. This is referred to as a positive electrode D. When the surface of the positive electrode D was observed with a scanning electron microscope (SEM), it could be confirmed that the coating layer was disposed to follow the surface of the positive electrode active material layer.

<Image observation of coating layer by scanning electron microscope (SEM)>
Each coating layer of the said positive electrode B, the positive electrode C, and the positive electrode D was observed with the scanning electron microscope, and the aperture ratio in the surface of a coating layer was measured. The focal length of the SEM was set such that a surface depth of 1 μm to 5 μm could be observed according to the thickness of the coating layer. The opening indicates a point where the positive electrode active material layer is observed from the surface through the voids included in the covering layer. The open area ratio (%) = area of opening / area of entire covering layer × 100. The opening ratio of the covering layer of the positive electrode B was 50%, the opening ratio of the covering layer of the positive electrode C was 25%, and the opening ratio of the covering layer of the positive electrode D was 5%.

<Production of laminate type lithium ion secondary battery>
Example 1
A laminate type lithium ion secondary battery of Example 1 using the positive electrode B as a positive electrode was produced as follows.

The negative electrode was produced as follows. 97 parts by mass of graphite powder, 1 part by mass of acetylene black as a conductive additive, 1 part by mass of styrene-butadiene rubber (SBR) as a binder, and 1 part by mass of carboxymethylcellulose (CMC) were mixed to obtain a mixture . The mixture was dispersed in an appropriate amount of ion exchange water to prepare a slurry. This slurry was applied to a copper foil having a thickness of 20 μm, which is a current collector for a negative electrode, using a doctor blade so as to form a film. The slurry-coated current collector was dried and pressed, and the assembly was heated at 120 ° C. for 6 hours in a vacuum dryer. The bonded article after heating was cut into a predetermined shape (25 mm × 30 mm rectangular shape) to obtain a negative electrode. The thickness of the negative electrode was about 45 μm.

A laminated lithium ion secondary battery was produced using the positive electrode B and the negative electrode. Specifically, a rectangular sheet (27 × 32 mm, 25 μm thickness) made of polypropylene resin as a separator is sandwiched between the positive electrode B and the negative electrode to form an electrode plate group. The electrode plate group was covered with a pair of laminate films, the three sides were sealed, and then an electrolytic solution was injected into the bag-like laminate film. A solution in which LiPF ₆ was dissolved to 1 mol / l in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in EC: DEC = 3: 7 (volume ratio) was used as an electrolytic solution. Then, the remaining one side was sealed, and the four sides were airtightly sealed to obtain a laminate type lithium ion secondary battery in which the electrode plate group and the electrolytic solution were sealed. The positive electrode and the negative electrode are provided with a tab electrically connectable to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. Through the above steps, a laminate-type lithium ion secondary battery of Example 1 was produced.

(Example 2)
A laminated lithium ion secondary battery of Example 2 was produced in the same manner as in Example 1 except that the positive electrode B in Example 1 was changed to the positive electrode C.

(Example 3)
A laminated lithium ion secondary battery of Example 3 was produced in the same manner as in Example 1 except that the positive electrode B in Example 1 was changed to the positive electrode D.

(Comparative example 1)
A laminated lithium ion secondary battery of Comparative Example 1 was produced in the same manner as in Example 1 except that the positive electrode B in Example 1 was changed to the positive electrode A.

<Evaluation of cycle characteristics of laminated lithium ion secondary batteries of Example 1, Example 2, Example 3 and Comparative Example 1>
The cycle characteristics of the laminated lithium ion secondary batteries of Example 1, Example 2, Example 3 and Comparative Example 1 were evaluated. As evaluation of a cycle characteristic, the cycle test which repeated charging / discharging on condition of the following was performed, and the discharge capacity of each cycle was measured. During charging, CC charging (constant current charging) was performed at a temperature of 60 ° C. and a voltage of 4.5 V at a 1 C rate. At the time of discharge, CC discharge (constant current discharge) was performed at a rate of 3.0 V and 1 C. This charge and discharge was made into 1 cycle, and the cycle test was done to 200 cycles. The discharge capacity at this time was used by calculating the discharge capacity per mass of the positive electrode active material. Therefore, the unit of discharge capacity is mAh / g. FIG. 2 shows the relationship between the cycle number and the discharge capacity (mAh / g) of the laminate type lithium ion secondary batteries of Example 1, Example 2, Example 3 and Comparative Example 1.

From the results of FIG. 2, the first-order discharge capacity of the laminate-type lithium ion secondary batteries of Example 1, Example 2 and Example 3 is the same as or slightly higher than that of the laminate-type lithium ion secondary battery of Comparative Example 1 It was low. Specifically, when the initial discharge capacity of the laminate type lithium ion secondary battery of Comparative Example 1 is 191.5 mAh / g, the initial discharge capacity of the laminate type lithium ion secondary battery of Example 1 is 189. 4 mAh / g, initial discharge capacity of the laminate type lithium ion secondary battery of Example 2 is 187.9 mAh / g, and initial discharge capacity of the laminate type lithium ion secondary battery of Example 3 is 192.3 mAh / g there were.

From this, even when using the positive electrode in which the coating layer composed of the boron nitride powder and the binder for the coating layer is disposed on the surface of the positive electrode active material layer, the initial discharge capacity of the laminate type lithium ion secondary battery hardly changes. I understood it. There are voids in the covering layer. Therefore, even if the coating layer is disposed on the surface of the positive electrode active material layer, it is presumed that the coating layer does not have much resistance to lithium ion conduction. Further, from the results of the first discharge capacity of the laminate type lithium ion secondary batteries of Example 1, Example 2 and Example 3 in which the thickness of the cover layer is different, even if the thickness of the cover layer is thickened, the initial discharge capacity decreases. I found it difficult to do.

The discharge capacity of the laminate type lithium ion secondary battery of Example 1, Example 2 and Example 3 is almost equal to the discharge capacity of the laminate type lithium ion secondary battery of Comparative Example 1 up to 65 cycles. The From the 65th cycle onward, the discharge capacity of the laminate type lithium ion secondary batteries of Example 1, Example 2 and Example 3 became higher than the discharge capacity of the laminate type lithium ion secondary battery of Comparative Example 1.

Comparing the discharge capacity at the 200th cycle, the discharge capacity at 200th cycle of the laminate type lithium ion secondary battery of Comparative Example 1 is 95.9 mAh / g, compared with that of the laminate type lithium ion secondary battery of Example 1. The discharge capacity of the 200th cycle is 117.6 mAh / g, the discharge capacity of the 200th cycle of the laminate type lithium ion secondary battery of Example 2 is 127.3 mAh / g, and the laminate type lithium ion secondary battery of Example 3 The discharge capacity at the 200th cycle was 130.2 mAh / g. That is, the discharge capacity at the 200th cycle increased in the order of Comparative Example 1 <Example 1 <Example 2 <Example 3>. From this, it was found that when the thickness of the covering layer is larger, the cycle characteristics are higher. Further, from this result, a laminate type lithium ion secondary battery using a positive electrode in which a coating layer composed of boron nitride powder and a binder for coating layer is formed at 1 μm or more on the surface of the positive electrode active material layer at a high temperature of 60 ° C. It was found that the cycle characteristics were improved as compared with a laminate type lithium ion secondary battery using a positive electrode in which the covering layer was not formed.

From the above results, when the coating layer having the insulating inorganic powder and the coating layer binder and having the void is provided on the surface of the positive electrode active material layer, the lithium ion secondary battery is used at high temperature and high voltage It has been found that it has excellent cycle characteristics and charge / discharge capacity even under the environment.

1: current collector, 2: binder, 3: positive electrode active material, 4: positive electrode active material layer, 5: coating layer, 51: inorganic powder, 52: binder for coating layer, 6: void.

Claims

Current collector,
A positive electrode active material layer disposed on the surface of the current collector and containing a positive electrode active material and a binder;
A covering layer disposed on at least a part of the surface of the positive electrode active material layer;
Have
The coating layer comprises an insulating inorganic powder and a coating layer binder,
The covering layer has a void, and the positive electrode for a lithium ion secondary battery.
2. The lithium ion secondary battery according to claim 1, wherein an aperture ratio of the surface of the coating layer is 5% or more and 50% or less in an image obtained by observing the surface of the coating layer with a scanning electron microscope. Positive electrode.
The positive electrode for a lithium ion secondary battery according to claim 1, wherein a thickness of the covering layer is 1 μm or more and 20% or less of a thickness of the positive electrode active material layer.
The inorganic powder having an average particle diameter D 50 is the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3 is 100nm or more 1μm following.
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the inorganic powder is at least one selected from boron nitride, aluminum nitride and silicon nitride.
A positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5,
A negative electrode,
Non-aqueous electrolyte,
Having a lithium ion secondary battery.