JP2017068958A - Positive electrode for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

A positive electrode for a lithium ion secondary battery is provided. A current collector, a conductive particle layer disposed on a surface of the current collector, and a positive electrode active material layer disposed on a surface of the conductive particle layer, the conductive particle layer comprising: The conductive particles have a conductive particle layer binder, and the conductive particles are at least one selected from a group of specific substances, and the positive electrode active material layer includes a first positive electrode active material and a first positive electrode. A second positive electrode active material having a lower charge / discharge potential and higher resistance than the active material, wherein the length of the longest portion of the second positive electrode active material is L1, and is orthogonal to the length direction of the length of the longest portion When the ratio of L1 / L2 when the longest length in the direction to be taken is L2 is the aspect ratio, the average value of the aspect ratio is 3 or more and 10 or less. [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 general, a positive electrode current collector of a lithium ion secondary battery uses a metal such as Al that forms a stable passive film on the surface in order to withstand corrosion by electrolytic salts. For example, when Al is used for the current collector, a passive film such as Al ₂ O ₃ or AlF ₃ is formed on the surface thereof. Since the above passive film is formed on the surface of the Al current collector, it is not easily corroded and the current collecting function is easily maintained.

  In recent years, it has been desired that lithium ion secondary batteries can be used satisfactorily even under high-voltage usage environments. Even under the high-voltage usage environment, even if the above passive film is formed, the Al current collector gradually corrodes, and the lithium ion secondary battery having the Al current collector deteriorates in storage characteristics and cycle characteristics. There are concerns.

  In order to maintain various battery characteristics of the lithium ion secondary battery under a high voltage use environment, studies have been made to form a protective layer on the current collector. For example, forming a protective layer by a dry process such as an ion sputtering method or a vacuum deposition method has been studied. In addition, the formation of a protective layer by a wet process using an organic solvent has been studied. Furthermore, it has been studied to form a protective layer using an environmentally friendly aqueous solvent in a wet process.

  For example, Patent Document 1 discloses a protective layer containing tin-doped indium oxide (ITO) or tin oxide, and specifically discloses a protective layer containing ITO fine particles and a polyester-based resin. Yes.

  Patent Document 2 discloses a current collector for a positive electrode of a nonaqueous electrolyte secondary battery in which a coating layer made of a conductive oxide or a conductive nitride is formed on the surface of the current collector main body by an ion sputtering method. ing.

On the other hand, in a lithium ion secondary battery, when an internal short circuit occurs, a large amount of current flows instantaneously and there is a risk that battery heat will be generated. Therefore, conventionally, as the positive electrode active material, a large amount of current is rapidly generated in the positive electrode by using the first positive electrode active material and the second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material. It is done to prevent it from flowing into the water. For example, in Patent Documents 3 and 4, a lithium metal composite oxide represented by LiNi _x Co _y Mn _z O ₂ (x + y + z = 1) is used as the first positive electrode active material, and the second positive electrode active material is used. It has been proposed to use LiFePO ₄ as the substance.

JP-A-10-308222 WO2013 / 172007 Publication Special table 2010-517238 WO2013 / 129182

  However, a current collector with a protective layer, and a positive electrode active material layer including a first positive electrode active material and a spherical second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material. It has been found that the electrode resistance of the positive electrode has significantly higher than the electrode resistance of the positive electrode having a current collector without a protective layer and the positive electrode active material layer, although the reason is unknown. If the electrode resistance is high, the output inherent in the lithium ion secondary battery may not be sufficiently exhibited.

  The present invention has been made in view of such circumstances, and suppresses an increase in electrode resistance, and a positive electrode for a lithium ion secondary battery that achieves both good safety and good output characteristics, and its It aims at providing the lithium ion secondary battery which has a positive electrode.

  As a result of earnest research, the inventors of the present invention have a positive electrode for a lithium ion secondary battery having a conductive particle layer having a conductive particle and a binder for a conductive particle layer as a protective layer, and a second positive electrode When the length of the longest portion of the active material is L1, and the ratio of L1 / L2 when the longest length in the direction orthogonal to the length direction of the longest portion is L2 is the aspect ratio, It has been found that an increase in electrode resistance of the positive electrode for a lithium ion secondary battery can be suppressed by having the second positive electrode active material having an average aspect ratio of 3 or more and 10 or less.

  That is, the positive electrode for a lithium ion secondary battery of the present invention includes a current collector, a conductive particle layer disposed on the surface of the current collector, a positive electrode active material layer disposed on the surface of the conductive particle layer, The conductive particle layer includes conductive particles and a binder for the conductive particle layer. The conductive particles include indium oxide, zinc oxide, zinc peroxide, tin (II) oxide, tin oxide ( IV), tin oxide (VI), germanium nitride, titanium nitride, zirconium nitride, hafnium nitride, tantalum nitride, niobium nitride, vanadium nitride, tungsten nitride, element X-doped indium oxide (element X is Zn, Mo, W, Ti, Element Y-doped tin oxide (IV) (element Y is at least one selected from F, W, Ta, Sb, P and B) and element Z-doped oxidation zinc The element Z is at least one selected from Ga, Al, and B), and the positive electrode active material layer has a lower charge / discharge potential than the first positive electrode active material and the first positive electrode active material and has a resistance. The longest length of the second positive electrode active material is L1, and the longest length in the direction perpendicular to the length direction of the longest portion is L2. When the ratio of L1 / L2 is the aspect ratio, the average aspect ratio is 3 or more and 10 or less.

  In addition, a lithium ion secondary battery of the present invention includes the above-described positive electrode for a lithium ion secondary battery.

  The positive electrode for lithium ion secondary batteries of this invention can suppress the increase in electrode resistance by having the said electroconductive particle layer and the 2nd positive electrode active material whose average value of an aspect-ratio is 3-10. The reason is considered to be that the contact area between the second positive electrode active material and the conductive particle layer is increased. It is presumed that when the contact area is increased, the interface resistance between the conductive particle layer and the positive electrode active material layer is reduced, and an increase in electrode resistance is suppressed. A lithium ion secondary battery having the positive electrode has good output characteristics. Moreover, since the electroconductive particle layer is arrange | positioned on the surface of the electrical power collector, the electrical power collector is protected favorably by the positive electrode for lithium ion secondary batteries of this invention. By using the first positive electrode active material and the second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material as the positive electrode active material, a large amount of current rapidly flows in the positive electrode during a short circuit. Can be prevented, and battery heat generation when the lithium ion secondary battery is short-circuited can be suppressed.

It is a cross-sectional schematic diagram explaining the positive electrode for lithium ion secondary batteries of this embodiment.

  Below, the form for implementing this invention is demonstrated. Unless otherwise specified, the numerical range “ab” described herein includes the lower limit “a” and the upper limit “b”. The numerical range can be configured by arbitrarily combining these upper limit value and lower limit value and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from the numerical value range can be used as upper and lower numerical values.

<Positive electrode for lithium ion secondary battery>
The positive electrode for a lithium ion secondary battery of the present invention has a current collector, a conductive particle layer disposed on the surface of the current collector, and a positive electrode active material layer disposed on the surface of the conductive particle layer.

(Current collector)
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 metals such as stainless steel, titanium, nickel, aluminum, and copper, or conductive resins. In particular, a current collector made of aluminum is preferable in terms of electrical conductivity, workability, and cost. Being made of aluminum refers to being made of pure aluminum or an aluminum alloy.

  Aluminum having a purity of 99.0% or more is referred to as pure aluminum, and an alloy obtained by adding various elements to aluminum is referred to as an aluminum alloy. Examples of the aluminum alloy include Al—Cu, Al—Mn, Al—Fe, Al—Si, Al—Mg, AL—Mg—Si, and Al—Zn—Mg. Examples of the aluminum alloy include A1000 series alloys (pure aluminum series) such as JIS A1085 and A1N30, A3000 series alloys (Al-Mn series) such as JIS A3003 and A3004, and A8000 series alloys such as JIS A8079 and A8021 (Al--). Fe-based).

  The shape of the current collector can be in the form of foil, sheet, film, linear shape, rod shape, mesh or the like. For example, a foil can be suitably used as the shape of the current collector.

  When the current collector is a foil, sheet, or film, the thickness of the current collector is preferably 10 μm to 100 μm, and more preferably 15 μm to 25 μm.

  It is preferable to remove impurities on the surface of the current collector in advance. If, for example, oils or fats adhere to the surface of the current collector, the adhesion of the conductive particle layer may be deteriorated. Therefore, the current collector is preferably a degreased current collector. Examples of the degreasing treatment include heat treatment, corona treatment, and plasma treatment.

(Conductive particle layer)
The conductive particle layer has conductive particles and a binder for conductive particle layers. If necessary, the conductive particle layer includes a dispersant for dispersing the conductive particles, a viscosity modifier for improving the handling of the coating film, an antifoaming material for managing foam generated in the coating liquid, and the like. May be included. Commercially available products may be used as appropriate for the dispersant, viscosity modifier, antifoaming agent, and the like.

  The conductive particles are indium oxide, zinc oxide, zinc peroxide, tin oxide (II), tin oxide (IV), tin oxide (VI), germanium nitride, titanium nitride, zirconium nitride, hafnium nitride, tantalum nitride, niobium nitride , Vanadium nitride, tungsten nitride, element X doped indium oxide (element X is at least one selected from Zn, Mo, W, Ti, Zr, Sn and H), element Y doped tin oxide (IV) (element Y is At least one selected from F, W, Ta, Sb, P and B) and at least one selected from element Z-doped zinc oxide (the element Z is at least one selected from Ga, Al and B). .

  The conductive particles are conductive, resistant to organic solvents, resistant to oxidation-reduction reactions, and have low reaction activity. Further, the conductive particles are less likely to burn themselves and hardly react with aluminum. Since the conductive particles are conductive, they are unlikely to become a resistance of a lithium ion secondary battery.

  Since the current collector has a conductive particle layer disposed on the surface thereof, direct contact between the electrolytic solution and the current collector is suppressed, and corrosion of the current collector by the electrolytic solution and / or electrolytic salt can be suppressed.

  The said electroconductive particle may be used independently and may use 2 or more types together.

As the conductive particles, antimony-doped tin oxide (IV) (abbreviated as ATO) in which Sb ₂ O ₃ is added to tin (IV) oxide is particularly preferable. ATO has high electrical conductivity and is resistant to oxygen, electrolytes and electrolyte salts in the atmosphere, and exhibits its resistance even at high voltages. The higher the electrical conductivity of the conductive particles, the easier it is to suppress the resistance increase of the positive electrode, and it is easier to suppress the decrease in the capacity of the lithium ion secondary battery. The ratio of the antimony doping amount of ATO is preferably more than 0% by mass and 20% by mass or less, and more preferably 5% by mass to 16% by mass. The higher the proportion of antimony doped, the higher the electrical conductivity of ATO. However, when the ratio of the antimony doping amount exceeds 20% by mass, the electric conductivity of ATO does not increase in proportion to the antimony amount.

  Examples of the shape of the conductive particles include a spherical shape, a flat shape, and a polygonal shape. The shape of the conductive particles is preferably a spherical shape. If the shape of the conductive particles is spherical, the thickness of the conductive particle layer can be easily made constant, and uneven coating of the conductive particle layer on the current collector can be easily reduced.

The average particle diameter D50 of the conductive particles is preferably 10 nm to 1000 nm, more preferably 20 nm to 100 nm, and further preferably ₅₀ nm to 80 nm. When the average particle diameter D ₅₀ of the conductive particles is too small, the increase in the resistance between the conductive particles, the conductive particle layer is likely to hardly have suitable conductivity. When the average particle diameter D ₅₀ of the conductive particles is too large, there is a risk that the conductive particle layer becomes too thick. The average particle diameter D ₅₀ can be measured by particle size distribution measurement method. The average particle diameter D ₅₀ is that the particle size cumulative value of the volume distribution in the particle size distribution measurement by laser diffraction method is equivalent to 50%. That is, the average particle diameter D ₅₀ means the median size measured by volume.

  The binder for conductive particle layers binds conductive particles to each other and binds the conductive particles and the current collector. The binder for electroconductive particle layer will not be specifically limited if it is a binder which can be used for a lithium ion secondary battery.

  Examples of the binder for the conductive particle layer include, for example, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene copolymer (abbreviation FEP), fluorine-containing resins such as fluorine rubber, and thermoplastic resins such as polypropylene and polyethylene. Examples thereof include imide resins such as polyimide and polyamideimide, acrylic resins such as poly (meth) acrylic acid, alkoxysilyl group-containing resins, styrene / butadiene rubber, carboxymethylcellulose, polyethylene glycol, and polyacrylonitrile. These binders for electroconductive particle layers can be used individually or in combination of 2 or more types.

  As the binder for the conductive particle layer, polyacrylic acid, polytetrafluoroethylene, and polyethylene glycol are preferably used. These binders for conductive particle layers are excellent in adhesion to the current collector and excellent in coating property to the current collector.

  The compounding ratio of the conductive particles and the conductive particle layer binder is preferably, as a mass ratio, conductive particles: conductive particle layer binder = 50: 50 to 99: 1. It is more preferable that the conductive particles: the binder for the conductive particle layer = 80: 20 to 95: 5. If the blending ratio is within this range, in the conductive particle layer, if the ratio of the binder for the conductive particle layer is too small, the conductive particles and the conductive particles and the current collector may not be bound easily. . Moreover, when there is too much ratio of the binder for electroconductive particle layers, there exists a possibility that the resistance of an electrode may raise with an electroconductive particle layer.

  The thickness of the conductive particle layer is preferably 10 nm to 1000 nm, more preferably 20 nm to 500 nm, and further preferably 50 nm to 200 nm. If the thickness of the conductive particle layer is too small, the effect of protecting the current collector by the conductive particle layer may be difficult to obtain. If the thickness of the conductive particle layer is too large, the volume occupied by the conductive particle layer becomes too large in the positive electrode, and the amount of the active material must be reduced, which may reduce the battery capacity.

  In the conductive particle layer, voids are formed between the conductive particles, between the conductive particles and the conductive particle layer binder, between the conductive particles and the current collector, and the like. The porosity of the conductive particle layer is preferably 5% to 50%, more preferably 10% to 45%, and further preferably 15% to 40%.

  If the porosity of the conductive particle layer is too small, the resistance of the electrode may be increased. In addition, as described below, one of the second positive electrode active materials having a needle shape that is considered to have a particularly high contact area between the second positive electrode active material and the conductive particle layer when the porosity of the conductive particle layer is too small. There is a possibility that it is difficult to take a form in which the portion enters the gap. If the porosity of the conductive particle layer is too large, the effect of protecting the current collector by the conductive particle layer may be difficult to obtain.

  The porosity of the conductive particle layer can be calculated as follows. A cross section in a predetermined range of the conductive particle layer is observed, and the area of the entire conductive particle layer and the area of the void portion are calculated. The porosity is obtained by dividing the area of the void by the area of the entire conductive particle layer and multiplying by 100. It is good also as the calculated | required porosity by averaging each porosity obtained by the cross section of the several predetermined range. For example, a total of 10 visual fields having a width of 1 μm as a cross-sectional observation result of the conductive particle layer may be observed, and an average value may be calculated as the porosity.

  The cross-sectional observation result is obtained by measuring the cross section of the positive electrode with a scanning electron microscope (SEM), a transmission electron microscope (TEM), electron beam backscatter diffraction (EBSD), or the like. The image of the cross-sectional observation result may be analyzed using image analysis software.

  In addition, the surface of the conductive particle layer is uneven by the arrangement of the conductive particles. The size of the unevenness is not particularly limited.

  As will be described in detail below, a part of the second positive electrode active material having an average aspect ratio of 3 or more and 10 or less due to the presence of voids in the conductive particle layer and irregularities on the surface of the conductive particle layer. May be able to enter the conductive particle layer. In that case, since the contact area between the conductive particle layer and the positive electrode active material layer is particularly large, the effect of reducing electrode resistance is great.

  The method for disposing the conductive particle layer on the current collector is not particularly limited, but the following method can be employed. A conductive particle layer slurry is prepared by mixing a conductive particle binder and conductive particles in a solvent.

  As the solvent, water or an organic solvent can be used. As the organic solvent, ethanol, methanol, benzene, dichloromethane and the like can be used. Water contains a small amount of inorganic salt or the like, and can be used even when the pH is in the range of pH 4 to pH 9. However, from the viewpoint of corrosion of the current collector to be used, the water preferably has a pH of pH 6 to pH 8 from which impurities such as distilled water and ion exchange water are removed. Moreover, you may use what mixed the organic solvent and water by arbitrary ratios as a solvent.

  Here, the conductive particles are preferably dispersed in the slurry for the conductive particle layer. When the conductive particles are dispersed, the conductive particles are easily arranged in the entire conductive particle layer in the completed conductive particle layer. In order to disperse the conductive particles in the slurry for the conductive particle layer, when the solvent is water, the addition amount of the binder for the conductive particle layer, which is an organic substance, may be appropriately adjusted so that the conductive particles do not aggregate.

  Next, the slurry for conductive particle layers is applied to the current collector. As a coating method, a conventionally known method such as a roll coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, or a gravure coating method may be used.

  Thereafter, the current collector coated with the conductive particle layer slurry is dried, and the conductive particle layer is disposed on the surface of the current collector.

(Positive electrode active material layer)
The positive electrode active material layer is disposed on the surface of the conductive particle layer.

  The positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material. The positive electrode active material layer may include a binder and a conductive additive as necessary.

(First positive electrode active material)
As the first positive electrode active material, a known material that functions as a positive electrode active material of a lithium ion secondary battery may be employed. Since the charge / discharge potential of the second positive electrode active material is lower than the charge / discharge potential of the first positive electrode active material, the first positive electrode active material substantially plays the role of charge / discharge of the positive electrode.

  A positive electrode active material that can occlude and release lithium ions can be used as the first positive electrode active material. Specific examples of the first positive electrode active material include a lithium metal composite oxide.

The lithium metal composite oxide has a layered rock salt structure lithium metal composite oxide, a spinel structure lithium metal composite oxide, a layered rock salt structure lithium metal composite oxide, and a spinel structure such as LiMn ₂ O _4. The solid solution comprised with a mixture with lithium metal complex oxide is mentioned.

  As the lithium metal composite oxide having a layered rock salt structure, a lithium nickel cobalt manganese composite oxide represented by the formula (1) is preferable. Hereinafter, the lithium nickel cobalt manganese composite oxide may be referred to as NCM.

Li _a Ni _b Co _c Mn _(1-b-c-d) M ¹ _d O _(2-e) (1)
(In the formula (1), M ¹ represents at least one selected from the group consisting of Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr and W. A, b, c, d and e are 0.8 ≦ a ≦ 1.2, 0 <b ≦ 0.5, 0 <c ≦ 0.5, 0 ≦ d ≦ 0.5, b + c + d <1 , -0.1 ≦ e ≦ 0.2.

Examples of the compound having a layered rock salt structure 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. Examples include Mn _0.3 O ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ , and LiCoMnO ₂ . 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.

As the lithium metal composite oxide having a spinel structure, a general formula: Li _x (A _y Mn _2-y ) O ₄ (A is Ca, Mg, S, Si, Na, K, Al, P, Ga, Examples thereof include at least one element selected from Ge and at least one metal element selected from transition metal elements, and compounds represented by 0 <x ≦ 2.2 and 0 ≦ y ≦ 1).

Specific examples of the lithium metal composite oxide having a spinel structure include LiMn ₂ O ₄ and LiNi _0.5 Mn _1.5 O ₄ .

It is preferred first cathode active material is 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 first cathode active material is small, the specific surface area of the first positive electrode active material is increased. Therefore, it becomes the first reaction area of the positive electrode active mean particle diameter D ₅₀ of the material is too small and the first cathode active material and an electrolytic solution is excessively increased that, as a result, are accelerated decomposition of the electrolytic solution, The cycle characteristics of the lithium ion secondary battery may be deteriorated. When the average particle diameter D ₅₀ of the first cathode active material is too large, there is a possibility that the resistance of the lithium ion secondary battery increases. When the resistance of the lithium ion secondary battery is increased, the output characteristics of the lithium ion secondary battery may be lowered.

(Second positive electrode active material)
The second positive electrode active material is a material that can function as a positive electrode active material of a lithium ion secondary battery, and has a lower charge / discharge potential and higher resistance than the first positive electrode active material. When the second positive electrode active material is present in the positive electrode for a lithium ion secondary battery, heat generation of the lithium ion secondary battery can be suppressed even when the positive electrode and the negative electrode of the lithium ion secondary battery are short-circuited.

  For example, when the first positive electrode active material is a lithium metal composite oxide, specific examples of the second positive electrode active material include a lithium phosphate material and a lithium silicate material. Among these, as the second positive electrode active material, a lithium phosphate material is preferable. Lithium phosphate materials have an olivine structure. Lithium phosphate materials having an olivine type structure have a strong bond between phosphorus and oxygen, so that oxygen is hardly desorbed even at high temperatures, and has high thermal stability. Moreover, the positive electrode containing a lithium phosphate material tends to return to a voltage after a voltage drop due to a short circuit in a lithium ion secondary battery.

As the lithium phosphate material, a general formula: LiM _h PO ₄ (M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo) And at least one element selected from the group consisting of materials represented by 0 <h <2).

  Moreover, as a lithium phosphate type material, the lithium iron phosphate compound represented by following formula (2) is preferable.

Li _p Fe _q M ² _(1-q) PO ₄ (2)
(In the formula (2), M ² is at least one of the group consisting of Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W and Zr. (P is a value in the range of 0.9 ≦ p ≦ 1.1, q is a value in the range of 0 <q ≦ 1)

Specifically, as the lithium phosphate material, LiFePO ₄ , LiMnPO ₄ , LiVPO ₄ , LiNiPO ₄ , LiCoPO ₄ , LiTePO ₄ , LiV _2/3 PO ₄ , LiFe _2/3 PO ₄ , LiMn _7/8 Fe _{1 / 8} PO ₄ is mentioned.

As the second positive electrode active material, LiFePO ₄ which is a lithium iron phosphate compound is particularly preferable. The reason is as follows. LiFePO ₄ exhibits a relatively flat discharge curve during discharge. Then, even if the positive electrode and the negative electrode of the lithium ion secondary battery are short-circuited and a sudden discharge occurs, a sudden potential difference associated with the discharge does not occur at the location where LiFePO ₄ exists. Therefore, it is difficult to induce charge transfer from other parts in the electrode, and the occurrence of overcurrent can be suppressed. As a result, the heat generation of the lithium ion secondary battery can be suitably suppressed.

The lithium silicate-based material that can be used as the second positive electrode active material has a composition formula: Li _{2 + ab-} M Ab _{1-β M′β} Si _{1 + α} O _{4 + c} (where A is Na, K, Rb, and At least one element selected from the group consisting of Cs, M is at least one element selected from the group consisting of Fe and Mn, and M ′ is Mg, Ca, Co, Al, Ni, Nb And at least one element selected from the group consisting of Ti, Cr, Cu, Zn, Zr, V, Mo and W. Each subscript is as follows: 0 ≦ α ≦ 0.2, 0 ≦ β. ≦ 0.5, 0 ≦ a <1, 0 ≦ b <0.2, 0 <c <0.3). The above composition formula indicates the basic composition of the lithium silicate material. A part of Li, A, M, M ′, Si, and O in the above composition formula may be substituted with another element. The composition of the lithium silicate-based material may slightly deviate from the above composition formula due to unavoidable loss of Li, A, M, M ′, Si, or O or oxidation of the compound. Examples of the lithium silicate material include Li ₂ FeSiO ₄ , Li ₂ MnSiO _4, Li ₂ CoSiO ₄ , and Li ₂ NiSiO ₄ .

  As the second positive electrode active material, it is preferable to employ a material whose surface is coated with a carbon material. Examples of the carbon material include carbon black, acetylene black (AB), ketjen black (registered trademark) (KB), carbon nanotube, graphene, carbon fiber, and graphite.

  The ratio of L1 / L2 when the length of the longest portion of the second positive electrode active material is L1, and the longest length in the direction orthogonal to the length direction of the longest portion is L2, is the aspect ratio. In this case, the average aspect ratio is 3 or more and 10 or less. The average aspect ratio is preferably 4 or more and 8 or less, more preferably 4 or more and 6 or less.

  L1 and L2 of the second positive electrode active material can be measured by cross-sectional observation. L1 and L2 are measured by cross-sectional observation of each second positive electrode active material, and L1 / L2 is calculated. And the average value of the said aspect ratio is obtained by calculating an average value from L1 / L2 of a some 2nd positive electrode active material. The number of second positive electrode active materials to be observed may be appropriately selected depending on the size of the image obtained by observation of each cross section.

  Cross-sectional observation is an image obtained by measuring the cross section of the positive electrode for a lithium ion secondary battery of the present invention with a scanning electron microscope (SEM), a transmission electron microscope (TEM), an electron beam backscatter diffraction (EBSD), or the like. Based on this. You may analyze the said image using image analysis software.

  In the cross-sectional observation by SEM of the second positive electrode active material, the cross-sectional shape of the second positive electrode active material is observed in a shape such as a rod shape, a needle shape, an elliptical shape, for example. When the average aspect ratio is less than 3, the cross-sectional shape of the second positive electrode active material is a perfect circle shape or a shape close thereto, and when the aspect ratio is greater than 10, the cross-sectional shape of the second positive electrode active material is a shape close to the fiber shape. It becomes.

When the average value of the aspect ratio of the second positive electrode active material approaches 1, the shape of the second positive electrode active material becomes spherical. When the shape of the second positive electrode active material is spherical, the contact between the second positive electrode active material and the conductive particle layer is a point contact. Especially when the average particle diameter D ₅₀ of the second positive electrode active material than the size of the irregularities or voids in the surface of the conductive particle layer is large, it is difficult to the second positive electrode active material enters and voids of the conductive particle layer. Therefore, most second positive electrode active materials are in point contact with the surface of the conductive particle layer. In the case the average particle diameter D ₅₀ of the second positive electrode active material than the size of the irregularities or voids in the surface of the conductive particle layer is small, it is considered that spherical second positive electrode active material from entering the conductive particle layer. However, in that case, the contact of the second positive electrode active material that has entered the conductive particle layer with the other components of the positive electrode active material layer is considered to be point contact.

  On the other hand, when the average value of the aspect ratio of the second positive electrode active material is too large, the shape of the second positive electrode active material becomes a fiber shape. Therefore, the contact between the second positive electrode active material and the conductive particle layer is a line contact.

  On the other hand, when the cross-sectional shape of the second positive electrode active material is observed in a shape such as a rod shape, a needle shape, or an elliptical shape, the second positive electrode active material is considered to have a needle shape. . In this case, the second positive electrode active material is in surface contact with the surface of the conductive particle layer.

  In particular, when the size of a part of the second positive electrode active material that is needle-shaped is smaller than the size of the irregularities and voids on the surface of the conductive particle layer, the irregularities and voids of the conductive particle layer are formed on the second positive electrode active material. Can get in. In that case, the contact area between the conductive particle layer and the second positive electrode active material is particularly increased. Even if a part of the second positive electrode active material enters the conductive particle layer, the other part of the second positive electrode active material is contained in the positive electrode active material layer, and the conductive particle layer and the positive electrode active material layer The contact area with other components is also large.

  The contact area of the interface between the conductive particle layer and the positive electrode active material layer is increased through the second positive electrode active material having an average aspect ratio of 3 or more and 10 or less. It is estimated that the interface resistance becomes small. Further, since the contact area at the interface between the conductive particle layer and the positive electrode active material layer is increased, the binding property between the conductive particle layer and the positive electrode active material layer is also increased.

  FIG. 1 is a schematic cross-sectional view illustrating a positive electrode for a lithium ion secondary battery according to this embodiment. In FIG. 1, a conductive particle layer 2 is disposed on the surface of a current collector 1. A positive electrode active material layer 3 is disposed on the surface of the conductive particle layer 2. The conductive particle layer 2 includes conductive particles 21 and a conductive particle layer binder 22. The positive electrode active material layer 3 includes a first positive electrode active material 31 and a second positive electrode active material 32. In FIG. 1, the cross-sectional shape of the second positive electrode active material 32 is shown as an elongated oval shape. Concavities and convexities due to the conductive particles 21 are formed on the surface of the conductive particle layer 2. In FIG. 1, the end portion of the second positive electrode active material 32 enters the irregularities on the surface of the conductive particle layer 2. Therefore, the contact area at the interface between the conductive particle layer 2 and the positive electrode active material layer 3 is increased, and the contact resistance at the interface between the conductive particle layer 2 and the positive electrode active material layer 3 is decreased by the second positive electrode active material 32. It is estimated that Further, it is considered that the binding property at the interface between the conductive particle layer 2 and the positive electrode active material layer 3 is also increased.

It is preferable that the average particle diameter D ₅₀ of the second positive electrode active material is smaller than the average particle diameter D ₅₀ of the first positive electrode active material.

The average particle diameter D ₅₀ of the second positive electrode active material is preferably 0.5 μm or more and 5 μm or less, more preferably 1 μm or more and 3 μm or less, and further preferably 1 μm or more and 2 μm or less.

  In the positive electrode for a lithium ion secondary battery of the present invention, when the total mass of the first positive electrode active material and the second positive electrode active material is 100% by mass, the blending ratio of the second positive electrode active material is 20% by mass or more and 40% by mass. % Or less is preferable. If the blending ratio of the second positive electrode active material is too large, the blending ratio of the first positive electrode active material may decrease and the battery capacity of the positive electrode may decrease. If the blending ratio of the second positive electrode active material is too small, the battery There is a risk that the effect of suppressing the heat generation will be reduced. The blending ratio of the second positive electrode active material is more preferably 22% by mass or more and 35% by mass or less, and further preferably 25% by mass or more and 30% by mass or less.

  In the positive electrode for a lithium ion secondary battery of the present invention, when the entire positive electrode active material layer is 100 mass%, the ratio of the total mass of the first positive electrode active material and the second positive electrode active material is 85 mass% or more and 99 mass%. It is desirable that the content be 90% by mass or more and 99% by mass or less. The positive electrode active material layer preferably contains as much positive electrode active material as possible in order to increase the capacity of the battery.

  The conductive assistant is added to increase the conductivity of the electrode. Therefore, the conductive auxiliary agent may be added arbitrarily when the electrode conductivity is insufficient, and may not be added when the electrode conductivity is sufficiently excellent. The conductive auxiliary agent may be any chemically inert electronic high conductor, such as carbon black, graphite, acetylene black, ketjen black (registered trademark), vapor grown carbon fiber (Vapor Grown Carbon). Fiber: VGCF) and various metal particles are exemplified. These conductive assistants can be added to the positive electrode active material layer alone or in combination of two or more.

The shape of the conductive auxiliary agent is not particularly limited, but it is preferable that the average particle diameter is small in view of its role. Conductive average particle diameter D ₅₀ of the auxiliary is preferably 10μm or less. The average particle diameter _{D 50} of the conductive auxiliary agent is more preferably a 0.01Myuemu～1myuemu.

  When the compounding quantity of the conductive support agent in a positive electrode active material layer is given, the inside of the range of 0.5 mass%-10 mass% is preferable, the inside of the range of 1 mass%-7 mass% is more preferable, 2 mass%-5 mass% % Is particularly preferable.

  The binder plays a role of connecting the positive electrode active material and the conductive additive to the surface of the current collector. As binders, polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene copolymer (abbreviation FEP), fluorine-containing resins such as fluoro rubber, thermoplastic resins such as polypropylene and polyethylene, polyimide, polyamide Examples thereof include imide resins such as imide, acrylic resins such as poly (meth) acrylic acid, alkoxysilyl group-containing resins, styrene / butadiene rubber, carboxymethyl cellulose, polyethylene glycol, and polyacrylonitrile.

  When the blending amount of the binder in the positive electrode active material layer is given, it is preferably in the range of 0.5% by mass to 10% by mass, more preferably in the range of 1% by mass to 7% by mass, and 2% by mass to 5% by mass. % Is particularly preferable. If the amount of the binder is too small, the formability of the positive electrode active material layer may be reduced when the composition is used as the positive electrode active material layer. In addition, when the amount of the binder is too large, the amount of the positive electrode active material in the positive electrode active material layer is decreased, so that the energy density of the electrode may be lowered.

  In order to dispose the positive electrode active material layer on the surface of the conductive particle layer, the following method is exemplified.

  First, a composition for forming a positive electrode active material layer containing a first positive electrode active material, a second positive electrode active material and, if necessary, a binder and a conductive additive is prepared, and an appropriate solvent is added to the composition to obtain a slurry. To do.

  In order to form a slurry, each component may be added simultaneously or sequentially and mixed with a mixing device. Examples of the mixing device include a mixing stirrer, a ball mill, a sand mill, a bead mill, a disperser, an ultrasonic disperser, a homogenizer, a homomixer, a planetary mixer, and a planetary stirring and defoaming device. What is necessary is just to set the mixing speed in a mixing apparatus suitably the speed | rate which each component of a composition can disperse | distribute or melt | dissolve suitably. The binder may be used as a solution in which the binder is dissolved in a solvent or a dispersed suspension in advance.

  Examples of the solvent include water, N-methyl-2-pyrrolidone (NMP), methanol, and methyl isobutyl ketone (MIBK).

  The slurry is applied to the surface of the conductive particle layer and then dried. Examples of the method for applying the slurry include a roll coating method, a dip coating method, a doctor blade method, a spray coating method, a curtain coating method, a lip coating method, a comma coating method, and a die coating method. Drying may be performed under normal pressure conditions, or under reduced pressure conditions using a vacuum dryer. What is necessary is just to set drying temperature suitably, and the temperature beyond the boiling point of the said solvent is preferable. What is necessary is just to set drying time suitably according to an application quantity and drying temperature.

  In order to increase the density of the positive electrode active material layer, compression may be applied to the current collector after the positive electrode active material layer is formed after drying. The compression may be performed by a compression device. Examples of the compression device include a roll press machine, a vacuum press machine, a hydraulic press machine, and a hydraulic press machine. Examples of the compression pressure in the compression device include a range of 1 kN to 5000 kN.

<Lithium ion secondary battery>
The lithium ion secondary battery of this invention has the positive electrode for lithium ion secondary batteries of this invention. The lithium ion secondary battery of this invention has the said positive electrode, a negative electrode, a separator, and electrolyte solution as a battery component.

(Negative electrode)
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, a compound that includes an element that can be alloyed with lithium, a polymer material, or the like can be used.

  Examples of the carbon-based material include non-graphitizable carbon, artificial graphite, natural graphite, cokes, graphites, glassy carbons, organic polymer compound fired bodies, carbon fibers, activated carbon, and carbon blacks. 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. Among them, the element that can be alloyed with lithium is preferably silicon (Si) or tin (Sn).

Examples of the compound having an element 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 , and 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 2 or LiSnO. As the compound having an element that can be alloyed with lithium, a silicon compound or a tin compound is preferable. As the silicon compound, SiO _x (0.5 ≦ x ≦ 1.6) is preferable. Examples of the tin compound include tin alloys (Cu—Sn alloy, Co—Sn alloy, etc.).

  Examples of the polymer material include polyacetylene and polypyrrole.

The negative electrode active material is preferably in powder form. When the negative electrode active material is in a powder form, the average particle diameter D ₅₀ of the negative electrode active material is preferably 0.5 μm or more and 30 μm or less, and more preferably 1 μm or more and 20 μm or less. When the average particle diameter D ₅₀ of the negative electrode active material is too small, the specific surface area of the powder of the negative electrode active material is increased, it increases the contact area of the powder of the anode active material and the electrolyte solution, proceed decomposition of the electrolyte solution Therefore, the cycle characteristics of the lithium ion secondary battery may be deteriorated. When the average particle diameter D ₅₀ of the negative electrode active material is too large, conductivity of the whole electrode becomes uneven, charging and discharging characteristics may deteriorate.

(Separator)
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.

(Electrolyte)
The electrolytic solution includes a solvent and an electrolyte dissolved in the solvent.

  As the solvent, for example, cyclic esters, chain esters, and 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 the 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, and 1,2-dibutoxyethane.

Moreover, as an 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.

As the electrolytic solution, for example, 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, dimethyl carbonate, or the like is from 0.5 mol / l to 1.7 mol / l. A solution dissolved at a certain concentration can be used.

  A separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. The electrode body may be either a stacked type in which the positive electrode, the separator and the negative electrode are stacked, or a wound type in which the positive electrode, the separator and the negative electrode are sandwiched. After connecting the positive electrode tab portion leading to the outside from the positive electrode current collector and the negative electrode tab portion leading to the outside from the negative electrode current collector using a current collecting lead, etc., an electrolytic solution was added to the electrode body A lithium ion secondary battery is preferable. Moreover, the lithium ion secondary battery of this invention should just be charged / discharged in the voltage range suitable for the kind of active material contained in an electrode.

  The lithium ion secondary battery can be mounted on a vehicle.

  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, an electric assist Bicycles and electric motorcycles are examples.

  As mentioned above, although embodiment of the positive electrode for lithium ion secondary batteries of this invention and the 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.

<Preparation for formation of conductive particle layer>
A 15 μm thick aluminum foil (manufactured by UACJ Co., Ltd.) was prepared. The prepared aluminum foil was heated in the atmosphere at 120 ° C. for 12 hours to reduce oils and fats on the surface of the aluminum foil.

As the conductive particles, the average particle diameter _{D 50} was prepared 30nm of antimony-doped tin oxide (hereinafter referred to as ATO). This ATO was SnO ₂ / Sb ₂ O ₅ = 90/10 (mass ratio), and the doping amount of antimony in ATO was 7.5% by mass. The powder shape of ATO was spherical.

  Polyacrylic acid (hereinafter referred to as PAA) (weight average molecular weight 5000) manufactured by Wako Pure Chemical Industries, Ltd. was prepared as a binder for the conductive particle layer. Moreover, ion-exchange water was prepared as a viscosity adjusting solvent.

<Preparation of slurry for conductive particle layer>
ATO, PAA, and ion-exchanged water were mixed so that the mass ratio of ATO: PAA was 90:10 to prepare a slurry for a conductive particle layer having a solid content of 4%.

<Preparation of current collector>
(Current collector A)
The aluminum foil itself having a thickness of 15 μm after the degreasing treatment was used as a current collector A.

(Current collector B)
The slurry for electroconductive particle layer was apply | coated to the 15-micrometer-thick aluminum foil after the degreasing process using the micro gravure coater. The aluminum foil after the application of the slurry for the conductive particle layer was dried at 100 ° C. The thickness of the obtained conductive particle layer was approximately 100 nm. The current collector on which the conductive particle layer was disposed was designated as current collector B.

  The porosity of the conductive particle layer obtained from SEM cross-sectional observation of current collector B was 30%. A total of 10 fields of view having a width of 1 μm as a result of the cross-sectional observation were observed, and the averaged value was calculated as the porosity.

<Preparation of positive electrode>
As the first positive electrode active material, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (hereinafter referred to as NCM) having an average particle diameter D ₅₀ of 6 μm was prepared. As the second positive electrode active material, needle-shaped LiFePO ₄ and spherical LiFePO ₄ were prepared. The needle-shaped LiFePO _{4 is referred} to as a needle-shaped LFP, and the spherical LiFePO ₄ is referred to as a spherical LFP. Both the needle-shaped LFP and the spherical LFP had a surface coated with a carbon material. Both the needle-shaped LFP and the spherical LFP had an average particle diameter D ₅₀ of 1.5 μm.

(Positive electrode of Example 1)
The positive electrode of Example 1 was produced as follows. NCM as the first positive electrode active material, needle-shaped LFP as the second positive electrode active material, acetylene black as the conductive additive, and polyvinylidene fluoride (PVDF) as the binder, 69 parts by mass, 25 parts by mass, 3 parts by mass and 3 parts by mass were mixed, and this mixture was dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode active material layer slurry.

The positive electrode active material layer slurry was placed on the current collector B, and applied using a comma coater so that the slurry became a film. The current collector B coated with the slurry was dried at 90 ° C. for 5 minutes to volatilize and remove NMP. The current collector and the coated material on the current collector were firmly and closely joined by a roll press. At this time, the density of the positive electrode active material layer was set to 2.9 g / cm ³ . Here, the density of the positive electrode active material layer was defined as mass of positive electrode active material layer (g) ÷ positive electrode active material layer thickness (cm) ÷ positive electrode active material layer area (cm ² ). The joined product was heated with a vacuum dryer at 120 ° C. for 6 hours, and then the positive electrode active material layer had a thickness of about 90 μm. This was used as the positive electrode of Example 1.

(Positive electrode of Comparative Example 1)
A positive electrode of Comparative Example 1 was obtained in the same manner as the positive electrode of Example 1 except that Current Collector A was used in place of Current Collector B.

(Positive electrode of Comparative Example 2)
A positive electrode of Comparative Example 2 was obtained in the same manner as the positive electrode of Example 1 except that spherical LFP was used instead of the needle-shaped LFP as the second positive electrode active material.

(Positive electrode of Comparative Example 3)
A positive electrode of Comparative Example 3 was obtained in the same manner as the positive electrode of Comparative Example 2 except that the current collector A was used in place of the current collector B.

<Measurement of electrode resistance>
The electrode resistance of the positive electrode of Example 1 and Comparative Examples 1 to 3 was measured.

  The electrode resistance in the thickness direction of each positive electrode was measured using an electrode resistance measuring instrument IM-02240 type (measurement electrode: φ16 mm, electrode load: 5 kg) manufactured by Imoto Seisakusho Co., Ltd. The electrode resistance was measured at n = 5, and the average value was calculated. The results are shown in Table 1.

<Measurement of peel strength>
The peel strengths of the positive electrodes of Example 1 and Comparative Examples 1 to 3 were measured. The test method conformed to JIS Z 0237. The test method was described in detail. The peel strength was measured by adhering the active material layer side to the pedestal with an adhesive tape and pulling the positive electrode upward with a tensile tester in the direction of 90 degrees. The peel strength was measured at n = 1. The results are shown in Table 1. Both the peeling surfaces of the positive electrodes of Example 1 and Comparative Example 2 were the interface between the positive electrode active material layer and the conductive particle layer. Both peeled surfaces of the positive electrodes of Comparative Examples 1 and 3 were the interface between the positive electrode active material layer and the current collector A.

<Measurement of aspect ratio of second positive electrode active material>
The cross section of the positive electrode of Example 1 and Comparative Example 2 was observed with a scanning electron microscope (hereinafter referred to as SEM), and the aspect ratio of the needle-shaped LFP and the spherical LFP was measured from the SEM cross-sectional photograph. The aspect ratio of the needle-shaped LFP was 1.5 to 10, and the average value of the aspect ratio was 5. The aspect ratio of the spherical LFP was 1 to 3, and the average value of the aspect ratio was 2.

<Section observation>
From the SEM cross-sectional photograph of the positive electrode of Example 1, in the cross section of the positive electrode of Example 1, the tip portion of the second positive electrode active material enters the voids and irregularities formed between the conductive particles of the conductive particle layer, and the conductivity A state in which the second positive electrode active material bites into the particle layer was observed. In the cross-sectional observation of the positive electrode of Comparative Example 2, no appearance of the second positive electrode active material invading into the conductive particle layer was observed. It is considered that the second positive electrode active material could not penetrate into the conductive particle layer because the particle size of the second positive electrode active material was larger than the size of the irregularities on the surface of the conductive particle layer.

  From the results shown in Table 1, when the electrode resistances of the positive electrodes of Comparative Example 2 and Comparative Example 3 were compared, the electrode resistance of the positive electrode of Comparative Example 2 was large. That is, when the current collector using the spherical second positive electrode active material and the conductive particle layer is used, the electrode resistance is greatly increased. Since the conductive particle layer has a higher resistance than the current collector itself, the contact area between the spherical second positive electrode active material and the conductive particle layer, the spherical second positive electrode active material, and the current collector Therefore, it is estimated that the electrode resistance of the positive electrode of Comparative Example 2 has increased.

  On the other hand, when the electrode resistance of the positive electrode of Comparative Example 1 and Example 1 was compared, the electrode resistance of the positive electrode of Example 1 was significantly smaller than the electrode resistance of the positive electrode of Comparative Example 1. In the positive electrode of Example 1, it is presumed that a good conductive path was formed and the electrode resistance was reduced by increasing the contact area between the conductive particle layer and the needle-shaped second positive electrode active material. .

  Moreover, when seeing peeling strength, the peeling strength of the positive electrode of Example 1 became large significantly compared with the peeling strength of the positive electrode of Comparative Examples 1-3. As described above, according to the SEM cross-sectional photograph of the positive electrode of Example 1, it was observed that the second positive electrode active material bite into the conductive particle layer in the positive electrode of Example 1. From this, it is considered that the adhesion area increased between the conductive particle layer and the needle-shaped second positive electrode active material, and the peel strength was greatly increased. On the other hand, in the positive electrodes of Comparative Example 2 and Comparative Example 3, the peel strength showed an equivalent value. Therefore, in the positive electrodes of Comparative Example 2 and Comparative Example 3, it is considered that the adhesion area between the positive electrode active material layer and the conductive particle layer and the adhesion area between the positive electrode active material layer and the current collector A are almost the same.

  This peel strength result is a result that supports the result of the contact area in electrode resistance. When the electrode resistance of the positive electrode of Example 1 is compared with the electrode resistance of the positive electrode of Comparative Example 2, the electrode resistance of the positive electrode of Example 1 is significantly low. Since the peel strength of the positive electrode of Example 1 is significantly larger than the peel strength of the positive electrode of Comparative Example 2, the positive electrode of Example 1 has a large contact area between the conductive particle layer and the needle-shaped second positive electrode active material. In the positive electrode of Comparative Example 2, since the contact area between the conductive particle layer and the spherical second positive electrode active material is small, it is confirmed that the difference in electrode resistance has occurred. Since the peel strength of the positive electrode of Example 1 was high, it is presumed that the lithium ion secondary battery using the positive electrode of Example 1 has a long life characteristic.

<Production of laminated lithium-ion secondary battery>
(Positive electrode)
(Positive electrode of Example 1A)
A coating film is formed on both sides of the current collector with a comma coater, cut into a predetermined shape (rectangular shape with a positive electrode active material layer area of 40 mm × 80 mm), and the positive electrode active material layer has a thickness of about 90 μm on one side. Obtained the positive electrode of Example 1A under the same conditions as those of the positive electrode of Example 1.

(Positive electrode of Comparative Example 1A)
A positive electrode of Comparative Example 1A was obtained in the same manner as the positive electrode of Example 1A, except that Current Collector A was used instead of Current Collector B.

(Positive electrode of Comparative Example 2A)
A positive electrode of Comparative Example 2A was obtained in the same manner as the positive electrode of Example 1A, except that spherical LFP was used instead of the needle-shaped LFP as the second positive electrode active material.

(Positive electrode of Comparative Example 3A)
A positive electrode of Comparative Example 3A was obtained in the same manner as the positive electrode of Comparative Example 2A, except that Current Collector A was used instead of Current Collector B.

(Preparation of negative electrode)
As the negative electrode active material, SiO having an average particle diameter D ₅₀ of 4 μm and natural graphite having an average particle diameter D ₅₀ of 20 μm were prepared. Polyamideimide was prepared as a binder resin. Acetylene black was prepared as a conductive aid.

  The negative electrode active material, the conductive auxiliary agent, and the binder resin were mixed at a mass ratio of SiO: graphite: conductive auxiliary agent: binder resin = 32: 50: 8: 10. An appropriate amount of NMP was added as a solvent to the above mixture to prepare a negative electrode active material layer slurry.

A 20 μm copper foil was prepared as a negative electrode current collector, and the slurry for negative electrode active material layer was applied in a film form on both sides of the copper foil using a comma coater. The copper foil coated with the negative electrode active material layer slurry was dried at 80 ° C. for 5 minutes to volatilize and remove NMP, and then pressed by a roll press to obtain a bonded product. At this time, the density of the negative electrode active material layer on one side was set to 1.6 g / cm ³ . Here, the density of the negative electrode active material layer was defined as mass of the negative electrode active material layer (g) ÷ thickness of the negative electrode active material layer (cm) ÷ area of the negative electrode active material layer (cm ² ). The bonded product was heated in a vacuum dryer at 200 ° C. for 2 hours, and then cut into a predetermined shape (a rectangular shape having a negative electrode active material layer area of 44 mm × 84 mm) to form a negative electrode having a negative electrode active material layer thickness of 40 μm on one side. .

(Preparation of laminated lithium ion secondary battery of Example 1)
A laminate type lithium ion secondary battery was manufactured using 30 positive electrodes 1A and 31 negative electrodes. Specifically, a rectangular sheet (48 mm × 88 mm, thickness 25 μm) made of polyethylene as a separator was sandwiched between each positive electrode and each negative electrode, and 30 sets were laminated 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. 1 mol / liter of LiPF ₆ was added to a solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) as an electrolytic solution in EC: EMC: DMC = 3: 3: 4 (volume ratio). A solution dissolved so as to be 1 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. The positive electrode and the negative electrode have a tab portion that can be electrically connected to the outside, and a part of the tab portion extends to the outside of the laminated lithium ion secondary battery. The lithium ion secondary battery of Example 1 was fabricated through the above steps.

(Lithium ion secondary battery of Comparative Example 1)
A lithium ion secondary battery of Comparative Example 1 was produced in the same manner as the lithium ion secondary battery of Example 1 except that the positive electrode of Comparative Example 1A was used instead of the positive electrode of Example 1A.

(Lithium ion secondary battery of Comparative Example 2)
A lithium ion secondary battery of Comparative Example 2 was produced in the same manner as the lithium ion secondary battery of Example 1 except that the positive electrode of Comparative Example 2A was used instead of the positive electrode of Example 1A.

(Lithium ion secondary battery of Comparative Example 3)
A lithium ion secondary battery of Comparative Example 3 was produced in the same manner as the lithium ion secondary battery of Example 1 except that the positive electrode of Comparative Example 3A was used instead of the positive electrode of Example 1A.

<Capacity measurement of lithium ion secondary battery>
The battery capacities of the lithium ion secondary batteries of Example 1 and Comparative Examples 1 to 3 were measured. Charging was performed at a rate of 0.33 C at 25 ° C., CCCV charging at a voltage of 4.5 V, and CV time 3 hours (constant current constant voltage charging). When discharging, CC discharge was performed at a 0.33 C rate up to 2.5V. The discharge capacity at this time was measured and used as the battery capacity. The results are shown in Table 2.

<Cell resistance evaluation>
The cell resistances of the lithium ion secondary batteries of Example 1 and Comparative Examples 1 to 3 were measured. The cell resistance (mΩ) was measured at a voltage of 3.6 V and a 1 C rate of 6.5 A for 10 seconds. In Examples and Comparative Examples, two batteries each having the same configuration were prepared, the resistance of each battery was measured, and the average value was calculated.

  Table 2 shows the average cell resistance results of the lithium ion secondary batteries of Example 1 and Comparative Examples 1 to 3.

<Nail penetration test>
About the lithium ion secondary battery of Example 1 and Comparative Examples 1-3, the safety | security evaluation by the nail penetration test was performed. Specifically, each battery was charged at a constant current (CC) until it reached 4.5 V at a current value of 3.0 A. Thereafter, charging was continued so as to maintain the voltage within 4.5 V ± 0.02 V, and the charging was stopped when the total charging time reached 5 hours.

  Each lithium ion secondary battery subjected to the above charging treatment was placed on a restraining plate having a hole with a diameter of 20 mm. A restraint plate was placed on a press machine with a nail attached to the top. The nail was moved from the top to the bottom at a speed of 20 mm / sec until the nail penetrated the lithium ion secondary battery on the restraint plate and the tip of the nail was positioned inside the hole of the restraint plate. A temperature measuring device capable of measuring the surface temperature was attached to the lithium ion secondary battery. The nail was made of stainless steel (S45C defined by JIS G 4051), had a diameter of φ8 mm, and a nail tip angle of 60 °. The nail penetration test was conducted while measuring the surface temperature of the lithium ion secondary battery at room temperature and in the air. By this nail penetration test, the positive electrode and the negative electrode of the lithium ion secondary battery were short-circuited.

  The surface temperature of each lithium ion secondary battery at the time of an internal short circuit was measured, and the state of the battery was observed. The surface temperature of each lithium ion secondary battery after penetrating the nail once increased and then gradually decreased. Table 2 shows the maximum temperature among the observed surface temperatures.

  From the battery capacity of Table 2, the battery capacity of the lithium ion secondary battery of Example 1 was slightly higher than the battery capacity of the lithium ion secondary batteries of Comparative Examples 1 to 3. Moreover, the cell resistance of each lithium ion secondary battery was the result of the same tendency as the tendency of the electrode resistance of each positive electrode shown in Table 1. The cell resistance of the lithium ion secondary battery of Example 1 was smaller than the cell resistance of the lithium ion secondary batteries of Comparative Examples 1 to 3. Since the cell resistance of the lithium ion secondary battery of Example 1 was small, it is considered that the battery capacity of the lithium ion secondary battery of Example 1 was slightly increased.

  Also, from the nail penetration test results, it was confirmed that the lithium ion secondary batteries of Example 1 and Comparative Examples 1 to 3 did not generate heat at a high temperature even if they were internally short-circuited during the nail penetration test. The positive electrode active material layer contains the first positive electrode active material and the second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material, so that a large amount of current suddenly flows in the positive electrode during a short circuit. It is thought that the flow was suppressed. When the ultimate temperature of the nail penetration test of the lithium ion secondary batteries of Example 1 and Comparative Examples 1 to 3 is compared, the lithium ion secondary batteries of Example 1 and Comparative Example 1 having the needle-shaped second positive electrode active material are compared. This was lower than the lithium ion secondary batteries of Comparative Example 2 and Comparative Example 3 having a spherical second positive electrode active material. From this, it was found that the lithium ion secondary battery having the needle-shaped second positive electrode active material has higher safety.

  From the above results, the lithium ion secondary battery of Example 1 having the needle-shaped second positive electrode active material and the positive electrode having the conductive particle layer is compared with the lithium ion secondary battery of the comparative example. It has been demonstrated that the battery capacity is high and the safety is high.

  1: current collector, 2: conductive particle layer, 3: positive electrode active material layer, 21: conductive particle, 22: binder for conductive particle layer, 31: first positive electrode active material, 32: second positive electrode active material .

Claims (8)

A current collector,
A conductive particle layer disposed on a surface of the current collector;
A positive electrode active material layer disposed on a surface of the conductive particle layer;
Have
The conductive particle layer has conductive particles and a binder for conductive particle layer,
The conductive particles are indium oxide, zinc oxide, zinc peroxide, tin oxide (II), tin oxide (IV), tin oxide (VI), germanium nitride, titanium nitride, zirconium nitride, hafnium nitride, tantalum nitride, nitride Niobium, vanadium nitride, tungsten nitride, element X-doped indium oxide (element X is at least one selected from Zn, Mo, W, Ti, Zr, Sn, and H), element Y-doped tin oxide (IV) (element Y Is at least one selected from F, W, Ta, Sb, P and B) and at least one selected from element Z-doped zinc oxide (the element Z is at least one selected from Ga, Al and B). Yes,
The positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material having a lower charge / discharge potential and higher resistance than the first positive electrode active material,
The ratio of L1 / L2 when the length of the longest portion of the second positive electrode active material is L1, and the longest length in the direction orthogonal to the length direction of the longest portion is L2, is the aspect ratio. When the ratio is a ratio, the average value of the aspect ratio is 3 or more and 10 or less, and the positive electrode for a lithium ion secondary battery. 2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the conductive particles have an average particle diameter D ₅₀ of 10 nm or more and 1000 nm or less.   The positive electrode for a lithium ion secondary battery according to claim 1, wherein the conductive particle layer has a thickness of 10 nm to 1000 nm. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein the average particle diameter D ₅₀ of the second positive electrode active material is 0.5 μm or more and 5 μm or less.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 4, wherein the porosity of the conductive particle layer is 5% or more and 50% or less.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein at least a part of the surface of the second positive electrode active material is coated with a conductive material. The first positive electrode active material is a lithium nickel cobalt manganese composite oxide represented by the following formula (1):
Li _a Ni _b Co _c Mn _(1-b-c-d) M ¹ _d O _(2-e) (1)
(In the formula (1), M ¹ represents at least one selected from the group consisting of Mg, Al, B, Ti, V, Cr, Fe, Cu, Zn, Zr, Mo, Sn, Ca, Sr and W. A, b, c, d and e are 0.8 ≦ a ≦ 1.2, 0 <b ≦ 0.5, 0 <c ≦ 0.5, 0 ≦ d ≦ 0.5, b + c + d <1 , -0.1 ≦ e ≦ 0.2.
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6, wherein the second positive electrode active material is a lithium iron phosphate compound represented by the following formula (2).
Li _p Fe _q M ² _(1-q) PO ₄ (2)
(In the formula (2), M ² is at least one of the group consisting of Co, Mn, Ni, Mg, Al, B, Ti, V, Nb, Cu, Zn, Mo, Ca, Sr, W and Zr. (P is a value in the range of 0.9 ≦ p ≦ 1.1, q is a value in the range of 0 <q ≦ 1)   The lithium ion secondary battery provided with the positive electrode for lithium ion secondary batteries as described in any one of Claims 1-7.

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