JP2016178070A - Positive electrode for lithium ion secondary battery, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode suitable for improvement of the life characteristic of a lithium ion secondary battery.SOLUTION: A positive electrode for a lithium ion secondary battery comprises: a positive electrode current collector 11; a first positive electrode active material layer 12 formed on the positive electrode current collector; and a second positive electrode active material layer 13 formed on the first positive electrode active material layer. The first positive electrode active material layer 12 includes: a positive electrode active material 14 such as a lithium manganese composite oxide having Spinel structure; and a positive electrode active material 15 such as a lithium nickel composite oxide or a lithium cobalt composite oxide, having lamellar crystal structure. The second positive electrode active material layer includes a positive electrode active material 16 such as a lithium transition metal composite phosphate compound having an olivine structure.SELECTED DRAWING: Figure 1

Description

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

  Non-aqueous electrolyte secondary batteries are widely used as small power sources for portable devices such as mobile phones, digital cameras, and laptop computers, and are also used as relatively large power sources for vehicles and homes. In particular, high-energy density and lightweight lithium-ion secondary batteries have become energy storage devices indispensable for daily life.

  When using a lithium ion secondary battery, a protection means such as a protection circuit fails for some reason, and an abnormal voltage is applied to the battery. When the temperature is abnormally high, heat generation and generation of gas species may occur due to electrolysis of the electrolyte solvent, resulting in an increase in battery temperature and an increase in internal pressure. Therefore, it is required to suppress the temperature rise and internal pressure rise due to such decomposition of the electrolyte solvent.

  On the other hand, studies are being made to improve the characteristics of lithium ion secondary batteries with the aim of increasing capacity, extending life and improving mass production stability.

Patent Document 1 aims to provide a non-aqueous chargeable battery having excellent stability even when a short circuit occurs under an abnormal condition in which an electrode is deformed, and a plurality of positive electrode active material layers are provided on a positive electrode current collector. A laminated non-aqueous electrolyte battery is described. Specifically, the positive electrode active material contained in the lower layer of the positive electrode active material layer adjacent to the positive electrode current collector is a Li-containing transition metal complex acid compound containing at least one of Mn or Fe (for example, LiMn ₂ O ₄ , LiFePO _{4 4} ), and the positive electrode active material contained in the upper layer of the positive electrode active material layer not adjacent to the positive electrode current collector is a Li-containing transition metal composite oxide containing at least one of Ni or Co (for example, LiNiO ₂ , LiCoO ₂ , LiNi _0.8 Co _0.2 O ₂ ).

  Patent Document 2 discloses a positive electrode current collector, a first positive electrode active material layer (including a lithium composite oxide having a layered crystal structure) formed on the positive electrode current collector, and the first positive electrode active material. A lithium secondary battery including a second positive electrode active material layer (including a lithium composite compound having a spinel structure or an olivine structure) formed on the layer is described. It is described that such a configuration can improve safety during overcharge.

  Patent Document 3 discloses a positive electrode in which an active material layer having a multilayer structure including different active materials is provided on a current collector, and the active material layer is a first layer including a first active material. A positive electrode having (a composite oxide containing Li and Ni) and a second layer containing a second active material (a phosphate compound containing Li and Fe) having higher thermal stability than the first active material. Have been described. It is described that the thermal stability can be improved by such a configuration, and as a result, deterioration of characteristics after continuous charging for a long time or after storage at high temperature can be suppressed.

JP 2001-338639 A JP 2008-198596 A JP 2006-134770 A

  However, in the techniques disclosed in Patent Documents 1 and 3, deterioration of the adhesion strength between the current collector and the lower layer of the positive electrode active material due to long-term continuous charging or long-term storage under high temperature, There has been a problem in that the positive electrode active material upper layer and the positive electrode active material lower layer are peeled from the expansion / contraction difference due to charge / discharge, resulting in an increase in resistance and a decrease in capacity of the positive electrode.

  In the technique disclosed in Patent Document 2, when a lithium composite compound (lithium manganate) having a spinel structure is used for the second positive electrode active material layer, Mn precipitation at the negative electrode due to Mn elution at a high temperature occurs. There is a problem that the battery capacity decreases. In addition, there is a problem in that separation occurs due to an expansion / contraction difference associated with charging / discharging between the first positive electrode active material and the second positive electrode active material layer, and resistance of the positive electrode is increased and capacity is decreased.

  This invention is made | formed in view of the said problem, and the objective of this invention is providing the technique which can improve the lifetime characteristic of a lithium secondary battery.

According to one aspect of the invention,
A positive electrode current collector;
A first positive electrode active material layer formed on the positive electrode current collector;
A second positive electrode active material layer formed on the first positive electrode active material layer,
The first positive electrode active material layer includes a positive electrode active material A having a spinel structure and a positive electrode active material B having a layered crystal structure,
The second positive electrode active material layer is provided with a positive electrode for a lithium ion secondary battery including the positive electrode active material C having an olivine structure.
According to another aspect of the invention,
A lithium ion secondary battery including a positive electrode capable of inserting / extracting lithium ions, a negative electrode capable of inserting / extracting lithium ions, and an electrolyte, wherein the positive electrode is the positive electrode described above. A secondary battery is provided.

  According to the embodiment of the present invention, it is possible to provide a lithium ion secondary battery with improved life characteristics and a positive electrode suitable for the lithium ion secondary battery.

It is typical sectional drawing for demonstrating the positive electrode by embodiment of this invention. It is typical sectional drawing for demonstrating an example of the lithium ion secondary battery by embodiment of this invention.

  A positive electrode for a lithium ion secondary battery according to an embodiment of the present invention includes a positive electrode current collector and a first positive electrode active material layer (hereinafter referred to as “first active material layer” as appropriate) formed on the positive electrode current collector. ) And a second positive electrode active material layer (hereinafter, appropriately referred to as “second active material layer”) formed on the first positive electrode active material layer.

  The first active material layer includes a positive electrode active material A having a spinel structure and a positive electrode active material B having a layered crystal structure. As the positive electrode active material A having a spinel structure, a lithium manganese composite oxide having a spinel structure can be used. As the positive electrode active material B having a layered crystal structure, a lithium nickel composite oxide having a layered crystal structure, a lithium cobalt composite oxide having a layered crystal structure, or a mixture thereof can be used.

  The second active material layer includes a positive electrode active material C having an olivine structure. As the positive electrode active material C having an olivine structure, a lithium transition metal composite phosphate can be used.

  Each of the positive electrode active materials A, B, and C may be obtained by substituting some of the elements constituting the positive electrode active material with other elements.

The positive electrode active material A has the following formula:
Li _{1 + x} Mn _2-xy Me1 _y O ₄
(Wherein Me1 includes at least one selected from the group consisting of Mg, Al, Fe, Co, Ni, and Cu, 0 <x <0.25, 0 <y <0.5)
It is preferable that it is a compound represented by these.

The positive electrode active material B has the following formula:
Li _{1 + a} (Ni _1-pqr Co _p Mn _q Me2 _r ) O ₂
(In the formula, Me2 includes at least one selected from the group consisting of Mg, Al, Fe, Cr, Ti, and In, and −0.5 ≦ a <0.1, 0 ≦ p <0.45, 0 ≦ q <0.45, 0 ≦ r <0.15)
It is preferable that it is a compound represented by these.

The positive electrode active material C has the following formula:
LizMPO ₄
(In the formula, M includes at least one selected from the group consisting of Fe, Co, Ni, Mn, and Cu, 0.95 ≦ z ≦ 1.1)
It is preferable that it is a compound represented by these.

The particles of the positive electrode active material C are desirably coated with a conductive filler.
This coating can be performed by a generally known coating method. For example, it can be coated by vapor deposition or spray drying. Specifically, when the spray drying method is used, the positive electrode active material particles and the carbon source material are dispersed in a solvent and sprayed in a high-temperature atmosphere, so that the solvent is instantaneously blown and the carbon material is coated. Active material particles can be formed.

  In the positive electrode active material having a layered crystal structure, instability of the crystal structure is likely to occur in part in the charged state, and decomposition of the electrolyte is induced at the local interface between the electrolyte and the unstable crystal structure, Decomposed gas may be generated. By covering the first active material layer including the positive electrode active material having a layered crystal structure with the second active material layer including a relatively stable positive electrode active material, the interface between the positive electrode active material having the layered crystal structure and the electrolytic solution is reduced. , Generation of cracked gas can be suppressed.

  In addition, the positive electrode active material having an olivine structure included in the second active material layer formed on the first active material layer has a higher DC resistance than the positive electrode active material having a spinel structure and the positive electrode active material having a layered crystal structure ( The rate of increase) is high. Therefore, by arranging the second active material layer on the outermost layer of the positive electrode (the side facing the negative electrode), it is possible to reduce the flowing current even if an internal short circuit occurs in an overcharged state or the like.

  Furthermore, a positive electrode active material having an olivine structure has a lower reaction potential (3.4 V) than a positive electrode active material having a spinel structure or a positive electrode active material having a layered crystal structure. Therefore, it is possible to easily detect the overdischarge of the battery.

  The positive electrode active material B having a layered crystal structure has a large particle expansion and contraction due to the charge / discharge cycle. Therefore, when used alone for the first active material layer, peeling between the first active material layer and the current collector and peeling between the first active material layer and the second active material layer are likely to occur. Capacity may be reduced. In order to suppress this capacity reduction, an active material A having a spinel structure opposite to that of the active material having a layered crystal structure in the behavior of expansion and contraction of the active material particles due to the charge / discharge cycle is mixed.

  Moreover, when the active material A having a spinel structure is used alone, Mn ions are eluted by charge / discharge cycles and high-temperature storage, and the capacity tends to deteriorate due to precipitation of the Mn ions on the opposing negative electrode surface. However, when the active material A having a spinel structure and the active material B having a layered crystal structure are mixed and used, the active material B having a layered crystal structure works as a proton scavenger and can suppress elution of Mn ions. . Mn ions that cannot be captured by the active material B having the layered crystal structure are inhibited from moving to the negative electrode by the active material C having the olivine structure of the second active material layer. In this manner, Mn ions eluted from the active material A having a spinel structure are prevented from precipitating on the negative electrode surface, and the battery life can be extended.

  According to the embodiment of the present invention, since such an improvement effect can be obtained, a long-life lithium ion secondary battery can be provided. In addition, it is possible to provide a secondary battery having a high energy density, a long lifetime, and good output characteristics.

  The mixing ratio (mass ratio A: B) of the positive electrode active material A and the positive electrode active material B is preferably 80:20 to 5:95 from the viewpoint of obtaining a sufficient mixing effect, and higher energy while obtaining a sufficient mixing effect. From the point of obtaining the density, 50:50 to 5:95 is preferable, and 20:80 to 5:95 is more preferable.

  The thickness of the first active material layer is not particularly limited, and can be appropriately set according to desired characteristics. For example, it can be set thick from the viewpoint of energy density, and from the viewpoint of output characteristics. Can be set thin. The thickness of the first active material layer can be appropriately set, for example, in the range of 10 to 200 μm.

  The thickness of the second active material layer is preferably 3 to 50 μm, more preferably 3 to 30 μm, more preferably 5 to 20 μm, and particularly preferably 5 to 15 μm. When the thickness of the second active material layer is increased, sufficient energy and long life characteristics of the lithium ion secondary battery can be obtained, while the energy density tends to decrease. Conversely, if the second active material layer is made thinner, it becomes difficult to obtain the effect of suppressing the current flowing when the second active material layer (active material having an olivine structure) flows through an internal short circuit in an overcharge state. In addition, the second active material layer (active material having an olivine structure) does not sufficiently exhibit the effect of inhibiting the migration of Mn ions eluted from the active material having a spinel structure to the negative electrode, making it difficult to obtain good long-life characteristics. .

Since the energy density of the first active material layer is greater than the energy density of the second active material layer, the thickness of the first active material layer may be greater than the thickness of the second active material layer from the viewpoint of obtaining a sufficient volume energy density. preferable.
The total thickness of the first active material layer and the second active material layer can be set, for example, in the range of 13 to 250 μm, and is preferably set in the range of 50 to 180 μm.

The average particle diameters of the positive electrode active materials A, B, and C are each preferably from 0.1 to 50 μm, more preferably from 1 to 30 μm, and more preferably from 2 to 25 μm from the viewpoints of reactivity with the electrolytic solution and rate characteristics. Further preferred. Here, the average particle diameter means the particle diameter (median diameter: D ₅₀ ) at an integrated value of 50% in the particle size distribution (volume basis) by the laser diffraction scattering method.

  Next, a positive electrode according to an embodiment of the present invention will be described with reference to FIG.

  FIG. 1 is a schematic diagram for explaining a configuration of a positive electrode according to an embodiment of the present invention. 1 is formed on a positive electrode current collector 11, a first active material layer 12 including positive electrode active material A particles 14 and positive electrode active material B particles 15, and a first active material layer. The second active material layer 13 includes the positive electrode active material C particles 16. The positive electrode is disposed so that the second active material layer 13 faces the negative electrode active material layer on the negative electrode current collector 19 with the separator 17 interposed therebetween. For convenience of explanation, FIG. 1 is schematically expressed in an exaggerated manner, and the technical scope of the present invention is not limited to the form shown in the drawings.

  The positive electrode active material layer (first and second active material layers) can be formed as follows. First, a slurry containing a positive electrode active material, a binder, and a solvent (and optionally a conductivity imparting agent) is prepared, and this is applied onto the positive electrode current collector (or the first active material layer formed earlier) and dried. It can be formed by pressing as necessary. N-methyl-2-pyrrolidone (NMP) can be used as a slurry solvent used for producing the positive electrode.

  The first active material layer and the second active material layer can each include a conductivity imparting agent and a binder in addition to the positive electrode active material. There is no restriction | limiting in particular as an electroconductivity imparting agent, The electroconductive material used normally as electroconductivity imparting agents, such as carbonaceous materials, such as carbon black, acetylene black, natural graphite, artificial graphite, and carbon fiber, can be used. Moreover, as a binder, what is normally used as a binder for positive electrodes, such as polytetrafluoroethylene (PTFE) and a polyvinylidene fluoride (PVDF), can be used.

  1-10 mass% is preferable and, as for content in the active material layer of an electroconductivity imparting agent, 1-10 mass% is preferable.

  A higher proportion of the positive electrode active material in the positive electrode active material layer is preferable because the capacity per mass increases, but from the viewpoint of reducing the resistance of the electrode, it is preferable to add a conductivity imparting agent, It is preferable to add a binder. If the ratio of the conductivity imparting agent is too small, it becomes difficult to maintain sufficient conductivity, and the resistance of the electrode is likely to increase. When the ratio of the binder is too small, it becomes difficult to maintain the adhesive force with the current collector, the active material, and the conductivity-imparting agent, and electrode peeling may occur.

  Further, the porosity of the active material layer excluding the current collector constituting the formed positive electrode is preferably 10 to 30%, more preferably 20 to 25%. Further, the porosity of the first active material layer is preferably 10 to 30%, and more preferably 20 to 25%. Further, the porosity of the second active material layer is preferably 20 to 35%, more preferably 25 to 30%. When the porosity of the active material layer is the above value, the discharge capacity at the time of use at a high discharge rate is improved, which is preferable.

The porosity means the ratio of the remaining volume obtained by subtracting the volume occupied by particles such as the active material and conductivity imparting agent from the apparent volume of the active material layer as a whole. Therefore, it can be determined by calculation from the thickness of the active material layer, the weight per unit area, and the true density of particles such as the active material and conductivity imparting agent.
Porosity = (apparent volume of active material layer−volume of particle) / (apparent volume of active material layer)

The “particle volume” (volume occupied by particles contained in the active material layer) in the above formula can be calculated by the following formula.
Particle volume =
(Weight per unit area of active material layer × area of active material layer × content ratio of particles) ÷ true density of particles Here, “area of active material layer” is the side opposite to the current collector side (separator side) ) Plane area.

As the negative electrode active material, materials capable of inserting and extracting lithium, such as lithium metal, carbonaceous material, and Si-based material, can be used. Examples of the carbonaceous material include graphite, amorphous carbon, diamond-like carbon, fullerene, carbon nanotube, and carbon nanohorn. As the Si-based material, Si, SiO ₂ , SiOx (0 <x ≦ 2), a Si-containing composite material, or the like may be used, or a composite containing two or more of these may be used.

  When using lithium metal as the negative electrode active material, melt cooling method, liquid quenching method, atomization method, vacuum deposition method, sputtering method, plasma CVD method, photo CVD method, thermal CVD method, sol-gel method, etc. Thus, a negative electrode can be formed.

  When a carbonaceous material or Si-based material is used as the negative electrode active material, a carbonaceous material (or Si-based material) and a binder such as polyvinylidene fluoride (PVDF) are mixed and dispersed and kneaded in a solvent such as NMP. The negative electrode can be obtained by applying the obtained slurry onto a negative electrode current collector, drying, and pressing as necessary. In addition, after the negative electrode active material layer is formed in advance, a negative electrode can be obtained by forming a thin film to be a current collector by a method such as vapor deposition, CVD, or sputtering.

The average particle size of the negative electrode active material is preferably 1 μm or more, more preferably 2 μm or more, further preferably 5 μm or more, from the viewpoint of input / output characteristics, from the viewpoint of suppressing side reactions during charge / discharge and suppressing reduction in charge / discharge efficiency. And from the viewpoint of electrode production (smoothness of electrode surface, etc.), it is preferably 80 μm or less, more preferably 40 μm or less. Here, the average particle diameter means a particle diameter (median diameter: D ₅₀ ) at an integrated value of 50% in a particle size distribution (volume basis) by a laser diffraction scattering method.

  The negative electrode active material layer may contain a conductivity imparting agent as necessary. As the conductivity imparting agent, a conductive material generally used as a conductivity imparting agent for the negative electrode, such as carbonaceous materials such as carbon black, ketjen black, and acetylene black, can be used.

  As the positive electrode current collector, aluminum, stainless steel, nickel, titanium, or an alloy thereof can be used. As the negative electrode current collector, copper, stainless steel, nickel, titanium, or an alloy thereof can be used.

  As the electrolyte, a non-aqueous electrolyte solution in which a lithium salt is dissolved in one or two or more non-aqueous solvents can be used.

  Non-aqueous solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate; ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), etc. Chain carbonates; aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate; γ-lactones such as γ-butyrolactone; 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), etc. And cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. One of these non-aqueous solvents can be used alone, or a mixture of two or more can be used.

Examples of the lithium salt dissolved in the nonaqueous solvent, is not particularly limited, for example _{LiPF 6, LiAsF 6, LiAlCl 4} , LiClO 4, LiBF 4, LiSbF 6, LiCF 3 SO 3, LiCF 3 CO 2, Li (CF ₃ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , and lithium bisoxalatoborate are included. These lithium salts can be used individually by 1 type or in combination of 2 or more types. Moreover, a polymer component may be included as a non-aqueous electrolyte. The concentration of the lithium salt can be set in the range of 0.8 to 1.2 mol / L, preferably 0.9 to 1.1 mol / L.

As the separator, a porous resin film, a woven fabric, a non-woven fabric, or the like can be used. Examples of the resin constituting the porous film include polyolefin resins such as polypropylene and polyethylene, polyester resins, acrylic resins, styrene resins, and nylon resins. In particular, a polyolefin-based microporous membrane is preferable because of its excellent ion permeability and performance of physically separating the positive electrode and the negative electrode. If necessary, the separator may be formed with a layer containing inorganic particles. Examples of the inorganic particles include insulating oxides, nitrides, sulfides, carbides, etc. Among them, TiO. ₂ or Al ₂ O ₃ is preferably included.

  A case made of a flexible film, a can case, or the like can be used as the outer container, and a flexible film is preferably used from the viewpoint of reducing the weight of the battery.

  As the flexible film, a film in which a resin layer is provided on the front and back surfaces of a metal layer serving as a base material can be used. As the metal layer, a metal layer having a barrier property such as prevention of leakage of the electrolytic solution or entry of moisture from the outside can be selected, and aluminum, stainless steel, or the like can be used. On at least one surface of the metal layer, a heat-fusible resin layer such as a modified polyolefin is provided. An exterior container is formed by making the heat-fusible resin layers of the flexible film face each other and heat-sealing the periphery of the portion that houses the electrode laminate. A resin layer such as a nylon film or a polyester film can be provided on the surface of the exterior body that is the surface opposite to the surface on which the heat-fusible resin layer is formed.

  Various devices such as a doctor blade, die coater, gravure coater, transfer method, and vapor deposition method can be used as an apparatus for forming an active material layer having a multilayer structure (two-layer structure) on a current collector. It is possible to use a device that implements the above or a combination of these coating devices. In order to accurately form the coated end portion of the positive electrode active material, it is particularly preferable to use a die coater. The active material application method using a die coater is roughly divided into a continuous application method in which an active material is continuously formed along the longitudinal direction of a long current collector, and an active material application method along the longitudinal direction of the current collector. There are two types of intermittent application methods in which the application part and the non-application part are alternately and repeatedly formed, and these methods can be appropriately selected.

  FIG. 2 shows a cross-sectional view of an example (laminate type) lithium ion secondary battery according to an embodiment of the present invention. As shown in FIG. 2, the lithium ion secondary battery of this example includes a positive electrode current collector 3 made of a metal such as an aluminum foil, and a positive electrode active material layer 1 containing a positive electrode active material provided thereon. And a negative electrode current collector 4 made of a metal such as copper foil and a negative electrode active material layer 2 containing a negative electrode active material provided thereon. The positive electrode and the negative electrode are laminated via a separator 5 made of a nonwoven fabric or a polypropylene microporous film so that the positive electrode active material layer 1 and the negative electrode active material layer 2 face each other. This electrode pair is accommodated in a container formed by the outer casings 6 and 7 made of an aluminum laminate film. A positive electrode tab 9 is connected to the positive electrode current collector 3, and a negative electrode tab 8 is connected to the negative electrode current collector 4, and these tabs are drawn out of the container. An electrolytic solution is injected into the container and sealed. It can also be set as the structure where the electrode group by which the several electrode pair was laminated | stacked was accommodated in the container.

  Hereinafter, the present invention will be described using the following examples and comparative examples. The present invention is not limited to the following examples.

Example 1
As positive electrode active material A, spinel-type lithium manganate (Li _1.1 Mn _1.9 O ₄ ) having an average particle size of 9.8 μm and a BET specific surface area of 0.8 m ² / g, as positive electrode active material B, an average particle size of 6.5 μm, Lithium nickelate (LiNi _0.8 Co _0.15 Al _0.05 O ₄ ) having a layered crystal structure with a BET specific surface area of 0.5 m ² / g and carbon black as a conductivity-imparting agent were prepared. These were dry-mixed, and the resulting mixture was uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which vinylidene fluoride resin (PVDF) was dissolved as a binder to obtain a slurry (hereinafter referred to as “slurry A”). Produced. The mixing ratio (mass ratio) of the solid content in the slurry A is such that the mixing ratio (mass ratio) of the positive electrode active material A and the positive electrode active material B is A: B = 80: 20. Active material B: conductivity imparting agent: binder = 73.6: 18.4: 4: 4.

Subsequently, as the positive electrode active material C, a lithium transition metal composite phosphate compound (LiFePO ₄ ) having an olivine structure in which a surface having an average particle diameter of 4.5 μm and a specific surface area of 13.1 m ² / g is coated with a conductive filler. Prepared for. A slurry containing the positive electrode active material C (hereinafter “slurry B”) was prepared in the same manner as the slurry A, except that the positive electrode active material C was used as the positive electrode active material. The blending ratio (mass ratio) of the solid content in the slurry B at this time was positive electrode active material C: conductivity imparting agent: binder = 85: 5: 10.

  The slurry A is applied on an aluminum metal foil (thickness 20 μm) serving as a positive electrode current collector, and then NMP is evaporated, whereby a first active material layer (thickness 85 μm) containing the positive electrode active material A and the positive electrode active material B is obtained. ) To obtain a positive electrode sheet A formed on an aluminum metal foil.

  Next, after applying the slurry B onto the positive electrode sheet A, NMP is evaporated to form a second active material layer (film thickness 15 μm) containing the positive electrode active material B on the upper surface of the first active material layer (film thickness 85 μm). Thus, a positive electrode sheet B having a positive electrode active material layer having a thickness of 100 μm was obtained.

(Examples 2 to 7)
A positive electrode sheet was produced in the same manner as in Example 1 except that the mixing ratio of the positive electrode active material A and the positive electrode active material B was changed as shown in Table 1.

(Examples 8 to 10)
The first positive electrode active material layer (film thickness: 85 μm) is adjusted so that the mixing ratio of the positive electrode active material A and the positive electrode active material B is 20:80 and the thickness of the second active material layer is the thickness shown in Table 1. A positive electrode sheet was produced in the same manner as in Example 1 except that the thickness of the slurry B applied on the positive electrode sheet A was changed.

(Comparative Example 1)
A positive electrode sheet having a thickness of 100 μm was prepared in the same manner as in Example 1 except that only the positive electrode active material B was used as the positive electrode active material.

(Comparative Example 2)
A positive electrode sheet having a thickness of 100 μm was prepared in the same manner as in Example 1 except that only the positive electrode active material A was used as the positive electrode active material.

(Comparative Examples 3 to 10)
A positive electrode sheet was produced in the same manner as in Example 1 except that the second positive electrode active material was not formed and the mixing ratio of the positive electrode active material A and the positive electrode active material B was as shown in Table 1.

(Production of lithium ion secondary battery)
A negative electrode active material (carbon) and a binder (PVDF) were mixed so that the mixing ratio (mass ratio) was carbon: PVDF = 90: 10 and dispersed in NMP to prepare a negative electrode slurry. This slurry was applied onto a copper foil (thickness 10 μm) serving as a negative electrode current collector to prepare a negative electrode sheet.

As an electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) as a solvent are mixed at a volume ratio of 3: 7 (EC: DEC), and a lithium salt (LiPF ₆ ) as an electrolyte has a concentration of 1 mol / L. What was dissolved in this way was used.

  The above negative electrode sheet and positive electrode sheet were laminated via a separator made of polypropylene. After providing a lead-out electrode for the positive electrode and a lead-out electrode for the negative electrode, this laminate was wrapped with a laminate film, injected with an electrolytic solution, and sealed to obtain a laminate type secondary battery shown in FIG.

(High temperature cycle test)
The laminated secondary batteries produced using the positive electrodes of Examples and Comparative Examples and the negative electrode were repeatedly charged and discharged under the following conditions to evaluate the high-temperature cycle life characteristics. The results are shown in Table 1.
Charge / discharge conditions: temperature 60 ° C., charge rate 1.0C, discharge rate 1.0C, charge end voltage 4.2V, discharge end voltage 2.5V.
The capacity retention rate (%) is a value obtained by dividing the discharge capacity (mAh) after 300 cycles by the discharge capacity (mAh) at the 10th cycle.

(Peel strength test)
The electrode of the battery after 300 cycles was subjected to a 180 degree peel test according to JIS K6854-2, and the peel strength normalized by the tape width used in the peel test was calculated. In each example and comparative example, five electrode samples were prepared, the peel strength was measured for each electrode sample, and the average value was taken as the peel strength. The results are shown in Table 1.

(Measurement of internal DC resistance and rate of increase in resistance)
The internal DC resistance of the obtained secondary battery was measured as follows.
At a temperature of 20 ° C., the battery was charged to 4.3 V at a charge rate of 0.5 C. After reaching 4.3 V, constant voltage charging was performed for 2 hours for charging. The battery after being charged was discharged at 0.1 C to a depth of discharge (DOD) of 50%, and then the voltage when discharged at a current of 1 C for 10 seconds was measured. Subsequently, after standing for 10 minutes, the voltage when charged for 10 seconds at a current of 1 C was measured. Further, the voltage was measured when the battery was discharged for 10 seconds at a current of 3 C after being left for 10 minutes, and the voltage when charged for 10 seconds at a current of 3 C after being left again for 10 minutes was measured. Subsequently, the voltage was measured when the battery was discharged for 10 seconds at a current of 5 C after being left for 10 minutes, and the voltage when charged for 10 seconds at a current of 5 C after being left again for 10 minutes was measured. Thereafter, the voltage was measured when the battery was discharged for 10 seconds at a current of 7 C after being left for 10 minutes, and the voltage when charged for 10 seconds at a current of 7 C after being left again for 10 minutes was measured. Finally, the voltage was measured when the battery was discharged for 10 seconds at a current of 10 C after being left for 10 minutes, and the voltage when charged for 10 seconds at a current of 10 C after being left again for 10 minutes was measured. A V-I straight line was obtained from the measurement results of these voltages, and the slope of the straight line at this time was defined as an internal DC resistance (DC resistance).

  In addition, after performing the above cycle test, the above internal DC resistance is measured, and the rate of increase in internal DC resistance (= [internal DC resistance after cycle] / [internal DC resistance before cycle]) is calculated. did.

Table 1 summarizes the capacity retention rate, energy density (Wh / kg), peel strength, and resistance increase rate of the obtained secondary battery.

  As is clear from the comparison between each example and the comparative example, the first active material layer contains lithium manganese composite oxide A having a spinel crystal structure and lithium nickel composite oxide B having a layered crystal structure. By using a positive electrode containing a lithium transition metal composite phosphate compound C having an olivine structure for the second active material layer formed on the first active material layer, the long life of the lithium ion secondary battery is achieved. Characteristics (cycle life characteristics) can be improved.

  Furthermore, when the mixing ratio (mass ratio) of the positive electrode active material A and the positive electrode active material B is 50:50 to 5:95, it can be seen that both high energy density and long-term life characteristics are achieved. Therefore, the positive electrode according to the embodiment of the present invention can effectively contribute to an increase in capacity and long-term life characteristics of the lithium ion secondary battery.

DESCRIPTION OF SYMBOLS 1 Positive electrode active material layer 2 Negative electrode active material layer 3 Positive electrode collector 4 Negative electrode collector 5 Separator 6 Laminate exterior 7 Laminate exterior 8 Negative electrode tab 9 Positive electrode tab 11 Positive electrode collector 12 First active material layer 13 2nd Active material layer 14 Particles of positive electrode active material A 15 Particles of positive electrode active material B 16 Particles of positive electrode active material C 17 Separator 18 Negative electrode active material layer 19 Negative electrode current collector

Claims (9)

A positive electrode current collector;
A first positive electrode active material layer formed on the positive electrode current collector;
A second positive electrode active material layer formed on the first positive electrode active material layer,
The first positive electrode active material layer includes a positive electrode active material A having a spinel structure and a positive electrode active material B having a layered crystal structure,
A 2nd positive electrode active material layer is a positive electrode for lithium ion secondary batteries containing the positive electrode active material C which has an olivine structure. The positive electrode active material A has the following formula:
Li _{1 + x} Mn _2-xy Me1 _y O ₄
(Wherein Me1 includes at least one selected from the group consisting of Mg, Al, Fe, Co, Ni, and Cu, 0 <x <0.25, 0 <y <0.5)
The positive electrode for lithium ion secondary batteries of Claim 1 which is a compound represented by these. The positive electrode active material B has the following formula:
Li _{1 + a} (Ni _1-pqr Co _p Mn _q Me2 _r ) O ₂
(In the formula, Me2 includes at least one selected from the group consisting of Mg, Al, Fe, Cr, Ti, and In, -0.5 ≦ a <0.1, 0 ≦ p <0.45, 0 ≦ q <0.45, 0 ≦ r <0.15)
The positive electrode for lithium ion secondary batteries of Claim 1 or 2 which is a compound represented by these. The positive electrode active material C has the following formula:
Li _z MPO ₄
(In the formula, M includes at least one selected from the group consisting of Fe, Co, Ni, Mn, and Cu, 0.95 ≦ z ≦ 1.1)
The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the positive electrode is a compound represented by the formula:   The positive electrode for a lithium ion secondary battery according to claim 4, wherein the particles of the positive electrode active material C are coated with a conductive filler.   The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein a mixing ratio (mass ratio) of the positive electrode active material A and the positive electrode active material B is 50:50 to 5:95. .   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 layer has a thickness of 3 to 50 µm. In a lithium ion secondary battery including a positive electrode capable of inserting / extracting lithium ions, a negative electrode capable of inserting / extracting lithium ions, and an electrolyte,
The said positive electrode is a positive electrode as described in any one of Claim 1 to 7, The lithium ion secondary battery characterized by the above-mentioned.   The lithium ion battery according to claim 8, further comprising a separator, wherein the positive electrode and the negative electrode are arranged to face each other via the separator, and the positive electrode, the negative electrode, and the separator are accommodated in a container made of a laminate outer film. Next battery.

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