JP5610031B1 - Positive electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery exhibiting excellent cycle characteristics and excellent charge / discharge capacity when used at high temperature and high voltage. A current collector, a positive electrode active material layer that is disposed on a surface of the current collector and includes a positive electrode active material and a binder, and a coating layer that is disposed on at least a part of the surface of the positive electrode active material layer The coating layer has a plurality of insulating inorganic powders and a coating layer binder, and the coating layer has voids. [Selection] Figure 1

Description

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

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, secondary batteries with small and light weight and high capacity are required. Currently, lithium ion secondary batteries using lithium cobalt oxide (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material are commercialized as high capacity secondary batteries.

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

  As a cause of this, when driven at a high temperature and a high voltage, it is considered that the oxidative decomposition of the electrolyte occurs in the vicinity of the positive electrode during charging, and the metal components such as the positive electrode active material are eluted by the acid generated by the oxidative decomposition. Yes. When the metal component is eluted from the positive electrode active material, the capacity of the positive electrode decreases and the cycle characteristics deteriorate. Also, the electrolytic solution decomposition product and metal component elution product generated by the oxidative decomposition of the electrolytic solution and the 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, a decomposition product is deposited on the surface of the negative electrode active material, and the deposit inhibits lithium insertion 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 describes that a coating layer containing filler particles made of inorganic particles containing magnesia and a binder is formed on a positive electrode active material layer. In this technique, the coating 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 by having this coating layer, the lithium ion secondary battery is excellent in cycle characteristics and storage characteristics under high temperature and high voltage use environments.

  However, Patent Document 1 does not describe how much the charge / discharge capacity decreases due to the formation of the coating layer. Patent Document 1 describes that magnesia constituting the coating layer reacts with water and becomes alkaline, and it is assumed that magnesia, which is a constituent component of the coating layer, elutes. That is, it is estimated that the coating 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 under a high temperature and high voltage use environment.

  Patent Document 2 discloses a secondary battery in which the surface of the positive electrode facing the negative electrode is covered with a film selected from an insulator, a semiconductor, and a composite of an insulator and a semiconductor that can transmit ions involved in the battery reaction. Is described. Patent Document 2 describes that the surface of the positive electrode is covered with the above film, whereby 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 film is formed by sputtering a nitride 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 is a dense film without voids. Therefore, it is presumed that the lithium battery having the protective film made of nitride described in Patent Document 2 has a lower resistance to charge / discharge capacity than the lithium battery without the protective film. Patent Document 2 does not describe how much the charge / discharge capacity decreases due to 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 Laid-Open No. 6-168739

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

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

  As a result of intensive studies by the present inventors, a lithium ion secondary material is formed by disposing a coating layer having a plurality of insulating inorganic powders and a coating layer binder and having voids on the surface of the positive electrode active material layer. It was found that the battery can exhibit excellent cycle characteristics and charge / discharge capacity even under high temperature and high voltage use environment.

  That is, the positive electrode for a lithium ion secondary battery according to the present invention includes a current collector, a positive electrode active material layer disposed on the surface of the current collector and including a positive electrode active material and a binder, and a surface of the positive electrode active material layer. A coating layer disposed at least in part, the coating layer having a plurality of insulating inorganic powders and a binder for the coating layer, wherein the coating layer has voids.

  In an image obtained by observing the surface of the coating layer with a scanning electron microscope, the aperture ratio of the surface of the coating layer is preferably 5% or more and 50% or less.

  The thickness of the coating 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 this invention has the said positive electrode for lithium ion secondary batteries, a negative electrode, and a non-aqueous electrolyte, It is characterized by the above-mentioned.

  The positive electrode for lithium ion secondary batteries of this invention has a coating layer arrange | positioned in at least one part of the surface of a positive electrode active material layer. This coating layer has a plurality of insulating inorganic powders and a coating layer binder. And a coating layer has a space | gap.

  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 under high temperature and high voltage use environments.

It is a schematic diagram explaining the positive electrode for lithium ion secondary batteries of this embodiment. 4 is a graph showing cycle test results of laminated lithium ion secondary batteries of Example 1, Example 2, Example 3 and Comparative Example 1. FIG.

<Positive electrode for lithium ion secondary battery>
The positive electrode for a lithium ion secondary battery of the present invention includes a current collector, a positive electrode active material layer disposed on the surface of the current collector, a coating layer disposed on at least a part of the surface of the positive electrode active material layer, Have

  A current collector refers to a chemically inert electronic high conductor that keeps a current flowing through an electrode during discharge or charging of a lithium ion secondary battery. Examples of the current collector material include metal materials such as stainless steel, titanium, nickel, aluminum, and copper, or conductive resins. The current collector can take the form of a foil, a sheet, a film, or the like. Therefore, for example, a metal foil such as a copper foil, a nickel foil, an aluminum foil, and a stainless steel foil can be suitably used as the current collector.

  The current collector preferably has a thickness of 10 μm to 100 μm.

  The positive electrode active material layer includes a positive electrode active material and a binder. The positive electrode active material layer may further contain a conductive additive as necessary.

As the positive electrode active material, a lithium-containing compound or another metal compound can be used. Examples of the lithium-containing compound, for example, lithium-cobalt composite oxide having a layered structure, the lithium nickel composite oxide having a layered structure, the lithium manganese composite oxide having a spinel structure represented by the general _{formula: Li a Co p Ni q Mn} r D _s O _x (D is a doping component, which is a component composed of Al, Mg, Ti, Sn, Zn, W, Zr, Mo, Fe, Na, and is added as necessary, p + q + r + s = 1, 0 < p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0.2) Lithium cobalt-containing composite metal oxide having a layered structure, general formula: olivine-type lithium phosphate composite oxide represented by LiMPO ₄ (M is at least one of Mn, Fe, Co, and Ni), general formula: Fluorine olivine type lithium phosphate complex oxide represented by Li ₂ MPO ₄ F (M is at least one of Mn, Fe, Co and Ni), silicate type represented by general formula: Li ₂ MSiO ₄ A lithium composite oxide (M is at least one of Mn, Fe, Co, and Ni) can be used. Examples of other metal compounds include oxides such as titanium oxide, vanadium oxide, and manganese dioxide, and disulfides such as titanium sulfide and molybdenum sulfide.

The positive electrode active material is preferably composed of a lithium-containing oxide represented by the chemical formula: LiMO ₂ (M is at least one selected from Ni, Co, and Mn), and further has a general formula: Li _a Co _{p Ni q Mn r D s O x} (D is doped component, Al, Mg, Ti, Sn , Zn, W, Zr, Mo, Fe, and a component consisting of Na, are added if necessary, p + q + r + s = 1, 0 <p <1, 0 ≦ q <1, 0 ≦ r <1, 0 ≦ s <1, 0.8 ≦ a <2.0, −0.2 ≦ x− (a + p + q + r + s) ≦ 0. It is preferably made of a lithium cobalt-containing composite metal oxide having a layered structure represented by 2).

Examples of the lithium-containing oxide include 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 can be used. Among these, 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 in terms of thermal stability.

The positive electrode active material preferably has an average particle diameter _{D 50} is a powder shape is 1 m to 20 m. When the average particle diameter D ₅₀ of the positive electrode active material is small, when using a positive electrode active material of the same mass, increasing the number of the positive electrode active material, the total surface area of the positive active material is increased. When the surface area of the positive electrode active material is increased, the reaction area between the positive electrode active material and the electrolytic solution is increased, the decomposition of the electrolytic solution is promoted, and the cycle characteristics of the lithium ion secondary battery are deteriorated. 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. When the average particle diameter D ₅₀ of the positive electrode active material is larger than 20 μm, the resistance of the lithium ion secondary battery is increased, and the output characteristics of the lithium ion secondary battery are lowered. 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 connecting the positive electrode active material and the conductive additive to the current collector. As the binder, for example, 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, alkoxy Silyl group-containing resins and rubbers such as styrene butadiene rubber (SBR) can be used.

  A conductive support agent is added to a positive electrode active material layer as needed, in order to improve the electroconductivity of an electrode. Carbon black, graphite, acetylene black (AB), ketjen black (registered trademark) (KB), vapor grown carbon fiber (VGCF), etc., which are carbonaceous fine particles, are used alone or in combination of two or more as conductive aids. Can be used. The amount of the conductive auxiliary agent used is not particularly limited, but can be, for example, about 1 part by mass 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 positive electrode active material layer-forming composition containing a positive electrode active material, a binder, and, if necessary, a conductive additive is prepared. An appropriate solvent is added to form a paste, which is then applied to the surface of the current collector and then dried. In addition, you may compress the collector which arrange | positioned the positive electrode active material layer so that an electrode density may be raised as needed.

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

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

  The coating 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 with the coating layer, the positive electrode active material is unlikely to be in direct contact with the electrolytic solution. Therefore, the decomposition reaction of the electrolyte solution by the positive electrode active material is suppressed. Therefore, it can suppress that cycling characteristics deteriorate.

  The coating layer has voids. The coating layer has a plurality of insulating inorganic powders and a coating layer binder. In the coating layer, the plurality of inorganic powders are arranged with some gaps, and the coating layer binder is arranged 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. Since there are voids in the coating layer, lithium ions easily pass through the coating layer and reach the positive electrode active material layer. That is, the covering layer is unlikely to become a resistance to lithium ion conduction, and even if the covering layer is disposed on the surface of the positive electrode active material layer, the charge / discharge capacity of the positive electrode is unlikely to decrease.

  Moreover, since the coating layer has a complicated shape formed of a plurality of inorganic powders, the surface area is large. Therefore, the coating layer is easy to physically trap the eluate of the metal component eluted from the positive electrode active material and the decomposition product of the electrolytic solution. Since the eluate of the metal component and the decomposition product of the electrolytic solution can be physically trapped on the surface of the positive electrode, it is possible to suppress the decomposition product from being deposited on the surface of the negative electrode active material. As a result, deterioration of the cycle characteristics of the lithium ion secondary battery can be suppressed.

  Moreover, the binder for coating layers binds inorganic powders and inorganic powder and a positive electrode active material layer. Therefore, the coating layer is difficult to peel from the positive electrode active material layer. Therefore, the effect of the coating layer is likely to persist even after repeated cycles.

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

  The void may be anywhere in the coating layer. The voids may be inside the coating layer as well as the surface of the coating layer. One gap may communicate with another gap.

  When the surface of the coating layer is observed with an electron microscope, it can be seen that the coating layer has an opening. This opening part points out the location where the positive electrode active material layer is observed from the surface through the space | gap contained in a coating layer. Therefore, the aperture ratio can be obtained by dividing the area of the opening by the area of the entire coating layer. At this time, the area of the entire coating layer includes the area of the opening.

  In an image obtained by observing the surface of the coating layer with a scanning electron microscope, the aperture ratio of the surface of the coating layer is preferably 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 opening ratio is 50% or less, the electrolyte solution can be less likely to be in direct contact with the positive electrode active material, and therefore, the decomposition of the electrolyte solution by the positive electrode active material can be satisfactorily 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 protected by the coating layer.

  The thickness of the coating layer is preferably 1 μm or more and 20% or less of the thickness of the positive electrode active material layer. If the thickness of the coating layer is 1 μm or more, the cycle characteristics of the lithium ion secondary battery can be further improved even under high temperature and high voltage use environment. If the thickness of the coating layer is 20% or less of the thickness of the positive electrode active material layer, it is not necessary to reduce the volume of the positive electrode active material layer, that is, the amount of the positive electrode active material in the battery, and the charge / discharge capacity of the lithium ion secondary battery It is not necessary to lower

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 to coat the positive electrode active material layer along the irregularities on the surface of the positive electrode active material layer I can do it. As a result, the surface of the positive electrode active material existing on the surface of the positive electrode active material layer is easily covered with the coating layer containing the inorganic powder. If at least a part of the surface of the positive electrode active material is coated with a coating layer containing an inorganic powder, the surface of the positive electrode active material can be prevented from coming into direct contact with the electrolytic solution, and the electrolytic solution is decomposed by the positive electrode active material. Can be suppressed. Inorganic powder preferably has an average particle size _{D 50} is 100nm or more 1μm or less. When the average particle diameter D ₅₀ of the inorganic powder is 100nm or 1μm or less, the coating layer tends to coat the surface of the positive electrode active material layer containing more inorganic powder.

  Since the inorganic powder is insulative, it is possible to suppress a short circuit between the positive electrode and the negative electrode when a coating layer containing the insulating inorganic powder is disposed on the surface of the positive electrode active material layer.

  As the insulating inorganic powder, for example, a nitride, a carbide, or a metal oxide can be used. Examples of the nitride include boron nitride, aluminum nitride, silicon nitride, and carbon nitride. Examples of the carbide include silicon carbide and boron carbide. Examples of the metal oxide include silica, alumina, magnesium oxide, zirconium oxide, and hafnium oxide.

  In particular, the inorganic powder is preferably thermally stable. For this reason, the inorganic powder is preferably a nitride. In general, nitride is thermally stable up to about 1500 ° C. 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 extra side reactions associated with battery reactions. The inorganic powder is preferably composed of at least one selected from boron nitride, aluminum nitride, and silicon nitride because the substance has high chemical stability.

  The coating layer is disposed on the surface of the positive electrode active material layer. In order to prevent the coating layer and the positive electrode active material layer from being mixed, the coating layer is desirably disposed after the positive electrode active material layer is formed.

  The method for disposing 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. Dissolve the material of the coating layer in an organic solvent or water to create a solution, spray it on the application surface of the positive electrode active material layer using a sprayer, and volatilize and remove the organic solvent or water to coat the coating layer on the positive electrode active material layer Can be arranged. In this case, ethanol, N-methyl-2-pyrrolidone (NMP), methanol, butanol, methyl isobutyl ketone (MIBK), or the like can be used as the organic solvent. The water is preferably water from which impurities have been removed, such as distilled water or ion exchange water.

Also, the material of the coating layer is dissolved in an organic solvent or water for viscosity adjustment to create a paste-like mixture, the paste-like mixture is applied on the positive electrode active material layer, and dried after the application, whereby the positive electrode active material A coating layer can be disposed on the layer. As the coating method, it may be used SPRAY coating method. As the organic solvent for adjusting the viscosity, ethanol, NMP, methanol, butanol, MIBK and the like can be used. The water is preferably water from which impurities have been removed, such as distilled water or ion exchange water.

  In the above-described coating layer arrangement method, the aperture ratio in the coating layer can be adjusted by adjusting the coating amount of the coating layer. The larger the coating amount of the coating layer, the smaller the aperture ratio.

  As the binder for the coating layer, an inorganic binder containing an element such as silicon element, aluminum element, 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 a binder that can be dissolved in an organic solvent and solidifies at room temperature when the organic solvent volatilizes. When such a binder is used, it is not necessary to heat the electrode in order to form the coating layer. As the binder for the coating layer, for example, an inorganic binder that can be solidified at room temperature is preferably used. Since the inorganic binder is chemically stable, the decomposition reaction accompanying the battery reaction can be suppressed. As the inorganic binder, an inorganic binder containing zirconium element, particularly zirconium oxide, can be preferably used.

  FIG. 1 is a schematic diagram illustrating a positive electrode for a lithium ion secondary battery according to this embodiment. In FIG. 1, a positive electrode active material 3 is bound on a current collector 1 by a binder 2. The positive electrode active material layer 4 includes a positive electrode active material 3 and a binder 2. The coating layer 5 is disposed on the positive electrode active material layer 4.

  In the coating 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. Yes. A plurality of inorganic powders 51 or the inorganic powder 51 and the positive electrode active material layer 4 are bound together by the coating layer binder 52. As seen in FIG. 1, voids 6 are formed between the inorganic powders 51, between the inorganic powder 51 and the coating layer binder 52, or between the inorganic powder 51 and the positive electrode active material layer 4. The inorganic powder 51 is arranged along the unevenness of the surfaces of the positive electrode active material 3 and the binder 2.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention has the said positive electrode for lithium ion secondary batteries, It is characterized by the above-mentioned. The lithium ion secondary battery having the positive electrode for a lithium ion secondary battery has excellent cycle performance even in a high temperature and high voltage use environment.

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

  The negative electrode has a current collector and a negative electrode active material layer bound to the surface of the current collector. A negative electrode active material layer contains a negative electrode active material and a binder, and contains a conductive support agent as needed. The current collector, binder and conductive additive are the same as those described for the positive electrode.

  As the negative electrode active material, a carbon-based material that can occlude and release lithium, an element that can be alloyed with lithium, an elemental compound that has an element that can be alloyed with lithium, or a polymer material can be used.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, coke, graphite, glassy carbon, organic polymer compound fired body, carbon fiber, activated carbon, or carbon black. Here, the organic polymer compound fired body refers to a material obtained by firing and carbonizing a polymer material such as phenols and furans at an appropriate temperature.

  Elements that can be alloyed with lithium are Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Ge, Sn. , Pb, Sb, Bi may be included. Among them, the element that can be alloyed with lithium is preferably silicon (Si) or tin (Sn).

Examples of elemental compounds having elements that can be alloyed with lithium include 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 or LiSnO can be used. The elemental compound having an element that can be alloyed with lithium is preferably a silicon compound or a tin compound. The silicon compound is preferably SiO _x (0.5 ≦ x ≦ 1.5). As the tin compound, for example, a tin alloy (Cu-Sn alloy, Co-Sn alloy, or the like) can be used.

  As the polymer material, polyacetylene, polypyrrole, or the like can be used.

  The separator separates the positive electrode and the negative electrode and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two 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 ceramics can be used.

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

  For example, cyclic esters, chain esters, and ethers can be used as the solvent. 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. Examples of ethers that can be used include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, and the like.

As the electrolyte dissolved in the electrolytic solution, for example, a lithium salt such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ can be used.

For example, as an electrolytic solution, 0.5 mol / l to 1.7 mol / l of a lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO _{3 in} a solvent such as ethylene carbonate, dimethyl carbonate, propylene carbonate, and dimethyl carbonate. A solution dissolved at a certain concentration can be used.

  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.

  The vehicle may be a vehicle that uses electric energy from a battery as a whole or a part of a power source. For example, an electric vehicle, a hybrid vehicle, a plug-in hybrid vehicle, a hybrid railway vehicle, an electric forklift, an electric wheelchair, and an electric assist. Bicycles and electric motorcycles are examples.

  As mentioned above, although embodiment of the positive electrode for lithium ion secondary batteries and lithium ion secondary battery of this invention was described, this invention is not limited to the said embodiment. The present invention can be implemented in various forms without departing from the gist of the present invention, with modifications and improvements that can be made by those skilled in the art.

  Hereinafter, the present invention will be described in more detail with reference to examples.

<Creation of positive electrode for lithium ion secondary battery>
(Positive electrode A)
First, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ having an average particle diameter D ₅₀ of 5 μm as a positive electrode active material, acetylene black as a conductive additive, and polyvinylidene fluoride (PVDF) as a binder, The mixture was mixed as 94 parts by mass, 3 parts by mass, and 3 parts by mass to obtain a mixture. This mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare 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 applied to the current collector using a doctor blade so that the slurry became a film. The obtained sheet was dried at 80 ° C. for 20 minutes to volatilize and remove NMP. Thereafter, the current collector and the coated material on the current collector were firmly and closely joined by a roll press. At this time, the electrode density was set to 3.2 g / cm ² . The bonded product was heated in a vacuum dryer at 120 ° C. for 6 hours. The bonded product after heating was cut into a predetermined shape (rectangular shape of 25 mm × 30 mm) 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. Tetrabutoxyzirconium was prepared as a binder for the coating layer. Boron nitride powder and tetrabutoxyzirconium were weighed and mixed so as to have a mass ratio of 3: 2 to obtain a mixture. The mixture was dispersed in butanol. The mixture dispersed in butanol as described above was sprayed on the surface of the positive electrode A using the sprayer as the number of sprays was one. The spraying time for one spraying was 5 seconds per unit area in the spraying range. By volatilizing and removing butanol, a coating layer made of boron nitride powder having a thickness of 1 μm and a coating layer binder was formed on the surface of the positive electrode A. This is designated as 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 so as to follow the surface of the positive electrode active material layer.

(Positive electrode C)
A coating layer composed of a boron nitride powder having a thickness of 2 μm 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 sprays was two. This is designated as positive electrode C. Specifically, the number of times of spraying was set to twice, after spraying once over the entire spraying range, the second spraying was performed again at the location where the first spraying was performed in the same manner as the first spraying. It was. When the surface of the positive electrode C was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed so as to follow the surface of the positive electrode active material layer.

(Positive electrode D)
A coating layer comprising a boron nitride powder having a thickness of 5 μm 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 sprays was changed to 5. This is a positive electrode D. When the surface of the positive electrode D was observed with a scanning electron microscope (SEM), it was confirmed that the coating layer was disposed so as to follow the surface of the positive electrode active material layer.

<Image Observation of Covering Layer by Scanning Electron Microscope (SEM)>
Each coating layer of the positive electrodes B, C, and D was observed with a scanning electron microscope, and the aperture ratio on the surface of the coating layer was measured. The focal length of the SEM was set so that a surface depth of 1 μm to 5 μm could be observed depending on the thickness of the coating layer. An opening part points out the location where the positive electrode active material layer is observed from the surface through the space | gap contained in a coating layer. Aperture ratio (%) = area of opening portion / area of entire coating layer × 100. The opening ratio of the coating layer of the positive electrode B was 50%, the opening ratio of the coating layer of the positive electrode C was 25%, and the opening ratio of the coating layer of the positive electrode D was 5%.

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

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

A laminate type lithium ion secondary battery was manufactured using the positive electrode B and the negative electrode. Specifically, a rectangular sheet (27 × 32 mm, thickness 25 μm) made of polypropylene resin as a separator was sandwiched between the positive electrode B and the negative electrode to form an electrode plate group. The electrode plate group was covered with a set of two laminated films, and the three sides were sealed, and then an electrolyte solution was injected into the bag-like laminated film. As an electrolytic solution, a solution obtained by dissolving LiPF ₆ at 1 mol / l in a solvent obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at EC: DEC = 3: 7 (volume ratio) was used. Thereafter, the remaining one side was sealed to obtain a laminate type lithium ion secondary battery in which the four sides were hermetically sealed and the electrode plate group and the electrolyte were sealed. Note that the positive electrode and the negative electrode each have a tab that can be electrically connected to the outside, and a part of the tab extends to the outside of the laminated lithium ion secondary battery. The laminated lithium ion secondary battery of Example 1 was produced through the above steps.

(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 an evaluation of cycle characteristics, a cycle test in which charging and discharging were repeated under the following conditions was performed, and the discharge capacity of each cycle was measured. When charging, CC charging (constant current charging) was performed at 60 ° C. at a 1C rate and a voltage of 4.5V. When discharging, CC discharge (constant current discharge) was performed at 3.0 V and 1 C rate. This charging / discharging 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 number of cycles and the discharge capacity (mAh / g) of the laminated lithium ion secondary batteries of Example 1, Example 2, Example 3 and Comparative Example 1.

  The results of FIG. 2 show that the initial discharge capacity of the laminated lithium ion secondary batteries of Example 1, Example 2 and Example 3 is equal to or slightly the same as the laminated lithium ion secondary battery of Comparative Example 1. It was low. Specifically, the initial discharge capacity of the laminated lithium ion secondary battery of Comparative Example 1 is 191.5 mAh / g, whereas the initial discharge capacity of the laminated lithium ion secondary battery of Example 1 is 189.mA. 4 mAh / g, the initial discharge capacity of the laminate type lithium ion secondary battery of Example 2 is 187.9 mAh / g, the initial discharge capacity of the laminate type lithium ion secondary battery of Example 3 is 192.3 mAh / g there were.

  Therefore, even when a positive electrode in which a coating layer composed of boron nitride powder and a binder for the coating layer is disposed on the surface of the positive electrode active material layer is used, the initial discharge capacity of the laminated lithium ion secondary battery is hardly changed. I understood it. There are voids in the coating layer. Therefore, even if the coating layer is disposed on the surface of the positive electrode active material layer, it is assumed that the coating layer did not have much resistance to lithium ion conduction. Further, from the results of the initial discharge capacities of the laminated lithium ion secondary batteries of Examples 1, 2 and 3 having different coating layer thicknesses, the initial discharge capacity is reduced even if the coating layer thickness is increased. I found out that there was nothing to do.

  Up to 65 cycles, the discharge capacities of the laminated lithium ion secondary batteries of Examples 1, 2 and 3 were almost the same as the discharge capacity of the laminated lithium ion secondary battery of Comparative Example 1. It was. After the 65th cycle, the discharge capacities of the laminated lithium ion secondary batteries of Example 1, Example 2 and Example 3 were higher than the discharge capacity of the laminated lithium ion secondary battery of Comparative Example 1.

  Comparing the discharge capacity at the 200th cycle, the discharge capacity at the 200th cycle of the laminate type lithium ion secondary battery of Comparative Example 1 was 95.9 mAh / g, whereas the laminate type lithium ion secondary battery of Example 1 was The discharge capacity at the 200th cycle was 117.6 mAh / g, the discharge capacity at the 200th cycle of the laminate type lithium ion secondary battery of Example 2 was 127.3 mAh / g, and the laminate type lithium ion secondary battery of Example 3 was 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 the thicker the coating layer, the higher the cycle characteristics. Further, from this result, the laminate type lithium ion secondary battery using the positive electrode in which the coating layer composed of the boron nitride powder and the coating layer binder is formed on the surface of the positive electrode active material layer at 1 μm or more at a high temperature of 60 ° C. It was found that the cycle characteristics were improved as compared with a laminated lithium ion secondary battery using a positive electrode on which no coating layer was formed.

  From the above results, when the surface of the positive electrode active material layer has a plurality of insulating inorganic powders and a binder for the coating layer and has a void on the surface of the positive electrode active material layer, the lithium ion secondary battery has a high temperature and a high voltage. It was found that it has excellent cycle characteristics and charge / discharge capacity even under the usage 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 (4)

A current collector,
A positive electrode active material layer disposed on a surface of the current collector and including a positive electrode active material and a binder;
A coating layer disposed on at least a part of the surface of the positive electrode active material layer;
Have
The coating layer has a plurality of insulating inorganic powders and a binder for the coating layer,
The coating layer have a void,
In an image obtained by observing the surface of the coating layer with a scanning electron microscope (Scanning Electron Microscope), the aperture ratio of the surface of the coating layer is 5% or more and 50% or less,
The opening ratio is calculated by the area of the opening divided by the area of the entire coating layer × 100,
The opening refers to a location where the positive electrode active material layer is observed from the surface of the coating layer,
The coating layer has a thickness of 1 μm or more and 5 μm or less,
The average particle diameter D ₅₀ of the inorganic powder is 100 nm or more and 1 μm or less,
The coating layer is formed by spraying the material of the coating layer on the application surface of the positive electrode active material layer using a sprayer or using the spray coating method on the application surface of the positive electrode active material layer. A positive electrode for a lithium ion secondary battery, wherein the positive electrode is manufactured by coating . 2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the inorganic powder comprises at least one selected from boron nitride, aluminum nitride, and silicon nitride. 3. The coating layer binder is at least one selected from an inorganic binder containing at least one element selected from silicon element, aluminum element and zirconium element, phenol resin, polyimide resin, epoxy resin and melamine resin. The positive electrode for lithium ion secondary batteries as described in 2. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3 ,
A negative electrode,
A non-aqueous electrolyte,
A lithium ion secondary battery.

Images