JP2016081716A - Positive electrode active material for lithium ion secondary battery, method for manufacturing the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a lithium ion secondary battery, which enables both of the achievement of a lithium ion secondary battery of a high energy density and the improvement of cycle characteristics.SOLUTION: A positive electrode active material for a lithium ion secondary battery comprises particles 1 each having a particle core part 1A formed from a compound expressed by the following compositional formula (1) and having a lamellar structure, and a particle surface layer part 1B formed from a compound expressed by the following compositional formula (2) and having a lamellar structure: LiNiMnCoMO(1); and LiNiMnMO(2).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material used for a lithium ion secondary battery, a manufacturing method thereof, and a lithium ion secondary battery using the positive electrode active material.

  In recent years, due to concerns about the prevention of global warming and the depletion of fossil fuels, there are high expectations for electric vehicles that require less energy for driving and power generation systems that use natural energy such as sunlight and wind power. However, these technologies have the following technical problems, which are one factor that hinders their spread.

  For example, in an electric vehicle, there are problems in improving the energy density of the driving battery and increasing the cruising distance in one charge. In addition, for example, in a power generation system using natural energy, the amount of power generation varies greatly, so that an electric power storage system using a large-capacity battery with high cost is required for leveling the output. In order to solve these technical problems, an inexpensive secondary battery having a high energy density is required.

  Lithium ion secondary batteries have a higher energy density per weight than other secondary batteries such as nickel metal hydride batteries and lead batteries, and therefore are expected to be applied to electric vehicles and power storage systems. However, in order to solve the technical problems as described above, it is necessary to further increase the energy density of the lithium ion secondary battery. For that purpose, it is necessary to increase the energy density of the positive electrode and the negative electrode of the lithium ion secondary battery.

  A Li-containing composite oxide particle powder containing Ni, Mn and Co and / or Al is known as a Li-containing composite oxide particle powder having a large charge / discharge capacity and excellent thermal stability during charging ( See Patent Document 1 below). In the Li-containing composite oxide particle powder described in Patent Document 1, Co and Al are present inside the particle, the concentration of Mn has a concentration gradient with respect to the radial direction of the particle, and at the center of the particle On the other hand, the concentration of Mn on the particle surface is high.

JP 2009-137834 A

  In the Li—Ni based composite oxide particle powder for non-aqueous electrolyte secondary battery described in Patent Document 1, when the Ni composition ratio on the particle surface increases, the Li component tends to precipitate on the particle surface. Therefore, when the positive electrode active material is dispersed in pure water, the Li component on the particle surface is eluted, and the pH of the dispersion tends to increase and the alkalinity tends to increase. When a positive electrode of a lithium ion secondary battery is produced using such a positive electrode active material, decomposition of the electrolyte solution of the lithium ion secondary battery is promoted, and the positive electrode capacity decreases with the passage of the number of charge / discharge cycles, There is a possibility that the cycle characteristics may deteriorate.

  The present invention has been made in view of the problems described above, and provides a positive electrode active material for a lithium ion secondary battery and a method for producing the same capable of achieving both high energy density and improved cycle characteristics of the lithium ion secondary battery. An object of the present invention is to provide a lithium ion secondary battery using the positive electrode active material.

  In order to achieve the above object, the positive electrode active material for a lithium ion secondary battery of the present invention is represented by the inside of a particle comprising a compound having a layered structure represented by the following composition formula (1) and the following composition formula (2). And a particle surface layer portion made of a compound having a layered structure.

_{Li 1 + a Ni b Mn c} Co d M e O 2 ... (1)
Li _{1 + w} Ni _x Mn _y M _z O ₂ (2)

  However, in the composition formulas (1) and (2), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb. In the composition formula (1), a , B, c, d and e are 0.9 ≦ a + b + c + d + e ≦ 1.1, −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0 0.05 ≦ d ≦ 0.3 and 0 ≦ e ≦ 0.35. In the composition formula (2), w, x, y, and z are 0.9 ≦ w + x + y + z ≦ 1.1, 0. .01 ≦ w ≦ 0.3, 0.1 ≦ x ≦ 0.4, 0.35 ≦ y ≦ 0.6, and 0 ≦ z ≦ 0.2.

  By using the positive electrode active material of the present invention as a positive electrode material of a lithium ion secondary battery, it is possible to achieve both higher energy density and improved cycle characteristics of the lithium ion secondary battery.

FIG. 2 is a schematic cross-sectional view of positive electrode active material particles according to an embodiment of the present invention. The flowchart which shows the manufacturing process of the positive electrode active material shown in FIG. The conceptual diagram explaining the mixing process shown in FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery according to an embodiment of the present invention.

  Hereinafter, a positive electrode active material for a lithium ion secondary battery according to an embodiment of the present invention, a manufacturing method thereof, and a lithium ion secondary battery using the positive electrode active material according to embodiments of the present invention will be described with reference to the drawings.

(Positive electrode active material for lithium ion secondary battery)
FIG. 1 is a schematic cross-sectional view showing particles 1 of a positive electrode active material 10 for a lithium ion secondary battery according to an embodiment of the present invention.

  The positive electrode active material 10 for a lithium ion secondary battery of the present embodiment is a powdery material composed of a plurality of particles 1 and is used as a positive electrode material of a lithium ion secondary battery described later. The particles 1 of the positive electrode active material 10 include a particle interior 1A made of a compound having a layered structure with a high Ni content represented by the following composition formula (1), and an excess of Li represented by the following composition formula (2). And a particle surface layer portion 1B made of a compound having a layered structure.

_{Li 1 + a Ni b Mn c} Co d M e O 2 ... (1)
Li _{1 + w} Ni _x Mn _y M _z O ₂ (2)

  However, in the composition formulas (1) and (2), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb.

In the composition formula (1), a, b, c, d, and e are 0.9 ≦ a + b + c + d + e ≦ 1.1, −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0. 9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.3, and 0 ≦ e ≦ 0.35. Furthermore, in the composition formula (1), a and e are preferably numerical values satisfying −0.05 ≦ a ≦ 0.1 and 0 ≦ e ≦ 0.1. In this case, the particle interior 1A is constituted. For example, LiNi _0.8 Mn _0.1 Co _0.1 O ₂ can be used as the compound having a layered structure with a high Ni content.

In the composition formula (2), w, x, y and z are 0.9 ≦ w + x + y + z ≦ 1.1, 0.01 ≦ w ≦ 0.3, 0.1 ≦ x ≦ 0.4, It is a numerical value satisfying 35 ≦ y ≦ 0.6 and 0 ≦ z ≦ 0.2. Furthermore, in the composition formula (2), w, x, y, and z are 0.1 ≦ w ≦ 0.25, 0.1 ≦ x ≦ 0.3, 0.5 ≦ y ≦ 0.6, and 0, respectively. A numerical value satisfying ≦ z ≦ 0.1 is preferable. In this case, for example, Li _1.2 Ni _0.2 Mn _0.6 O ₂ can be used as the compound having a Li-excess layered structure constituting the particle surface layer portion 1B. In the composition formulas (1) and (2), oxygen may be deficient as long as the layered structure is maintained.

In the present embodiment, among the positive electrode materials having an α-NaFeO ₂ type layered structure, those having a Ni composition ratio larger than 0.5 are referred to as compounds having a layered state with a high Ni content. In general, the composition formula is Li _{1 + x} M ′ _1-x O ₂ (x> 0.1, M ′ is a metal containing at least Mn and Ni, and the composition ratio of Mn is larger than the composition ratio of Ni). The compound having a layer structure with an excess of Li formed can also be expressed as LiMO ₂ (M is a metal) -Li ₂ MnO ₃ .

  The particles 1 of the positive electrode active material 10 are composed of a compound having a layered structure with a high Ni content in the particle interior 1A serving as a nucleus, and an excess of Li so as to cover the compound having a layered structure with a high Ni content in the particle interior 1A. A particle surface layer portion 1B made of a compound having a layered structure is formed. In other words, the particle 1 of the positive electrode active material 10 includes a particle interior 1A as a core portion made of a compound having a layered structure with a high Ni content, and a coating layer made of a compound having a layer structure with excess Li covering the core portion. And the surface of the core portion is covered with a coating layer.

  High energy density is achieved in a lithium ion secondary battery using the positive electrode active material of the present invention as a positive electrode material by setting the Ni composition ratio of the compound inside the particles to 0.7 or more and 0.9 or less. be able to.

  Moreover, precipitation of the Li component on the particle surface can be suppressed by setting the Ni composition ratio of the compound in the particle surface layer portion to a low ratio of 0.1 or more and 0.4 or less. The degree of precipitation of the Li component can be evaluated by a pH value in which particles are dispersed in pure water. By precipitating the Li component on the particle surface, it is possible to suppress an increase in pH in which particles are dispersed in pure water, and weaken alkalinity. Therefore, by using the positive electrode active material of the present invention as a positive electrode material of a lithium ion secondary battery, the decomposition of the electrolyte of the lithium ion secondary battery is suppressed, and the positive electrode capacity decreases with the progress of the number of charge / discharge cycles. Can be suppressed, and the cycle characteristics can be improved.

  The mass ratio of the particle surface layer portion 1B to the particle interior 1A can be, for example, 2.5% by mass or more and 10% by mass or less. For example, if the mass ratio of the particle surface layer portion 1B to the particle interior 1A is smaller than 2.5% by mass, the particle surface layer portion 1B may not be able to sufficiently cover the particle interior 1A. Moreover, when the mass ratio of the particle surface layer portion 1B to the particle interior 1A is larger than 10% by mass, the mass ratio of the compound having a layered structure having a relatively high Ni content in the particle interior 1A is decreased, and the lithium ion secondary There is a possibility that it will be difficult to increase the capacity and energy density of the battery.

  Moreover, the diameter of the particle | grain inside 1A as a core part covered with the particle | grain surface layer part 1B as a coating layer can be 500 nm or more and 2 micrometers or less, for example. When the diameter of the particle interior 1A is smaller than 500 nm, the mass ratio of the compound having a layered structure with a high Ni content in the particle interior 1A decreases, making it difficult to increase the capacity and energy density of the lithium ion secondary battery. There is a risk of becoming. Further, when the diameter of the particle interior 1A is larger than 2 μm, the lithium ion secondary battery using the positive electrode active material 10 of the present embodiment for the positive electrode may not be able to exhibit a predetermined performance.

  The diameter of the particle interior 1A of the particles 1 of the positive electrode active material 10 is determined by, for example, observing a plurality of internal particles 1a (see FIG. 3) that become the particle interior 1A with an electron microscope before forming the particle surface layer portion 1B. It can be estimated by calculating the particle size. The diameter of the particle interior 1A is obtained by, for example, observing the cross section of the plurality of particles 1 of the positive electrode active material 10 with an electron microscope and measuring the average particle diameter of the particles 1 and the thickness of the particle surface layer portion 1B. Also good. The particle diameter of the internal particle 1a or the particle 1 is determined by measuring the major axis and the minor axis of the internal particle 1a or the particle 1 with an electron microscope, for example, and setting the average diameter as the particle diameter of the spherical inner particle 1a or the particle 1 Can be sought.

  Moreover, the average thickness of the particle | grain surface layer part 1B which covers the particle | grain inside 1A of the particle | grains 1 of the positive electrode active material 10 can be 20 nm or more and 50 nm or less, for example. When the average thickness of the particle surface layer portion 1B is smaller than 20 nm, the particle surface layer portion 1B may not be able to sufficiently cover the particle interior 1A. Further, when the average thickness of the particle surface layer portion 1B is larger than 50 nm, there is a possibility that the lithium ion secondary battery using the positive electrode active material 10 of the present embodiment for the positive electrode cannot exhibit predetermined performance.

Further, BET specific surface area of the positive electrode active material 10 is an aggregate of particles 1 is preferably 0.2 m ² / g or more and is 1.0 m ² / g or less. When the BET specific surface area is smaller than 0.2 m ² / g, the capacity of the lithium ion secondary battery using the positive electrode active material 10 as the positive electrode may be reduced. When the BET specific surface area is larger than 1.0 m ² / g, there is a possibility that the reduction in discharge capacity with the progress of the charge / discharge cycle of the lithium ion secondary battery using the positive electrode active material 10 as the positive electrode may increase. .

  When the Ni composition ratio is smaller than 0.7 in the particle interior 1A composed of a compound having a layered structure with a high Ni content represented by the composition formula (1), the specifications required for the lithium ion secondary battery are satisfied. It becomes difficult. More specifically, it is difficult to provide a small-sized, high capacity, high energy density lithium ion secondary battery. In addition, when the Ni composition ratio is larger than 0.9 in the particle interior 1A, the composition ratio of other elements of the compound having a layered structure with a high Ni content represented by the composition formula (1) is unnecessarily lowered. Resulting in.

In the particle surface layer portion 1B made of a compound having a layer structure with an excess of Li represented by the composition formula (2), when the Ni composition ratio becomes larger than 0.4, on the surface of the particle 1, for example, LiOH, Li ₂ CO ₃ Li components, such as, become easy to precipitate. Then, when the particle 1 is dispersed in pure water, the Li component on the surface of the particle 1 is eluted, and the pH when the particle is dispersed in pure water is increased to increase the alkalinity. That is, the degree of precipitation of the Li component can be evaluated by the pH value when the particles are dispersed in pure water. For example, when the pH when the particles 1 of the positive electrode active material 10 are dispersed in pure water at a ratio of 0.1% by mass exceeds 10, the lithium ion secondary battery using the positive electrode active material 10 as the positive electrode However, the decomposition of the electrolytic solution is promoted, and as the number of charge / discharge cycles progresses, the positive electrode capacity decreases, and the cycle characteristics may deteriorate.

  Further, in the case where the pH when the particles 1 of the positive electrode active material 10 are dispersed in pure water at a ratio of 0.1% by mass exceeds 10, in the manufacturing process of the positive electrode of the lithium ion secondary battery, the positive electrode active material Production of a slurry containing ten particles 1 becomes difficult. More specifically, the slurry containing the particles 1 of the positive electrode active material 10 gels and becomes a non-uniform mixed state. When such a non-uniformly mixed slurry is applied to the surface of a metal foil as a positive electrode current collector to produce a positive electrode, the lithium ion secondary battery may not be able to exhibit predetermined performance.

  In addition, in the particle surface layer portion 1B made of a compound having a layer structure with an excess of Li represented by the composition formula (2), if the Li composition ratio is 1.01 or more, the Li component on the surface of the particle 1 is converted into the particle surface layer portion. It becomes easy to incorporate into 1B, and precipitation and elution of the Li component can be prevented. Thereby, the cycling characteristics of the lithium ion secondary battery which used the particle | grains 1 of the positive electrode active material 10 for the positive electrode can be improved. On the other hand, when the composition ratio of Li becomes smaller than 1.01 in the particle surface layer portion 1B, the Li component easily precipitates on the surface of the particle 1, and the lithium ion secondary battery using the particle 1 of the positive electrode active material 10 as the positive electrode There is a possibility that not only the cycle characteristics are deteriorated but also the capacity and energy density are lowered.

  Also, a particle surface layer portion 1B made of a compound having a layered structure with an excess of Li and represented by the composition formula (2) or a particle interior 1A made of a compound having a layered structure with a high Ni content represented by the composition formula (1). In this case, the crystal structure can be stabilized by making the composition ratio of Mn larger than 0.

  In the particle 1 of the positive electrode active material 10 of the present embodiment, it is desirable that the layered crystal structure of the particle interior 1A and the layered crystal structure of the particle surface layer portion 1B are continuous and integrated as a crystal structure. That is, the particle 1 of the positive electrode active material 10 has a layered structure in which the compound having the Li-excess layered structure in the particle surface layer portion 1B has a high Ni content in the particle inside 1A at the interface between the particle inside portion 1A and the particle surface layer portion 1B. It is preferable that the crystal structure of the particle surface layer portion 1B and the crystal structure of the particle interior 1A are continuous and integrated. Mn contained in the particle surface layer portion 1B or the particle interior 1A contributes to the integration of such a crystal structure. It can be confirmed by mapping elements in a transmission electron microscope (TEM) image, for example, that the layered crystal structure is continuous from the particle surface layer portion 1B to the particle interior 1A.

  Moreover, in the inside 1A of the particle composed of a compound having a layered structure with a high Ni content represented by the composition formula (1), the layered structure with a high Ni content is obtained by setting the Co composition ratio to 0.05 or more. It becomes easy to produce | generate the crystal | crystallization of the compound which has. If the Co composition ratio is smaller than 0.05, impurities may be easily mixed into the particle interior 1A.

Moreover, the particles 1 of the positive electrode active material 10 of the present embodiment may further include a coating layer that covers the particle surface layer portion 1B. Examples of the material for the covering layer that covers the particle surface layer portion 1B include electrochemical reaction such as Al ₂ O ₃ , SiO ₂ , MgO, TiO ₂ , SnO ₂ , B ₂ O ₃ , Fe ₂ O ₃ , ZrO ₂ , and AlF _3. Inert materials can be used. In this case, the particle 1 surface of the positive electrode active material 10 becomes the surface of the coating layer covering the particle surface layer portion 1B of the particle 1.

(Method for producing positive electrode active material for lithium secondary battery)
Hereinafter, the manufacturing method of the positive electrode active material 10 of this embodiment is demonstrated.
FIG. 2 is a flowchart showing the manufacturing process of the positive electrode active material 10 of the present embodiment. FIG. 3 is a conceptual diagram for explaining the mixing step S1 shown in FIG.

  When manufacturing the positive electrode active material 10 of this embodiment, the mixing step S1 and the heat treatment step S2 are performed. In the mixing step S1, the inner particles 1a, which are primary particles composed of a compound having a layered structure with a high Ni content represented by the composition formula (1), are smaller than the particle diameter of the inner particles 1a, and the composition formula (2) The surface layer particles 1b, which are primary particles made of a compound having a layer structure with an excess of Li represented by the following formula, are mixed in a solid phase state. In the mixing step S1, the inner particles 1a and the surface layer particles 1b may be dispersed and mixed in a dispersion medium such as pure water, or the inner particles 1a and the surface layer particles 1b may be stirred and mixed without using the dispersion medium. May be. Thereby, the surface layer particle | grains 1b are arrange | positioned on the surface of the internal particle 1a. In addition, in FIG. 3, illustration of the surface layer particle | grains 1b which exist other than the surface of the internal particle 1a is abbreviate | omitted.

  In the mixing step S1, the inner particles 1a can have a particle size of, for example, 500 nm or more and 2 μm or less, and the surface particle 1b can have a particle size of, for example, 10 nm or more and 100 nm or less. That is, the particle size of the surface layer particle 1b made of a compound having a layer structure with an excess of Li is, for example, 1/200 or more and 1/5 or less of the particle size of the internal particle 1a made of a compound having a layered structure with a high Ni content. It can be. Moreover, the mass ratio of the surface layer particle 1b with respect to the internal particle 1a can be 2.5 mass% or more and 10 mass% or less, for example.

  Next, a heat treatment step S2 is performed in which the mixture obtained in the mixing step S1 is heat treated at a temperature of 700 ° C. or higher and 850 ° C. or lower, for example. As a result, as shown in FIG. 1, particles 1 having a particle interior 1A made of a compound having a layered structure with a high Ni content and a particle surface layer portion 1B made of a compound having a Li-excess layered structure are formed. The positive electrode active material 10 of the embodiment is obtained. In addition, you may implement the post process which obtains the positive electrode active material 10 by performing air cooling, grinding | pulverization, disintegration, classification, etc. of the baked material obtained by heat processing process S2 as needed.

  When the temperature of the heat treatment in the heat treatment step S2 is lower than 700 ° C., the composition formula (2) is formed on the surface of the internal particles 1a made of the compound having a layered structure with a high Ni content represented by the composition formula (1). The surface layer particles 1b made of a compound having a Li-excess layered structure represented by Further, when the temperature of the heat treatment in the heat treatment step S2 is higher than 850 ° C., the composition of the particles 1 is uniformed, and the distinction between the particle interior 1A and the particle surface layer portion 1B in the particles 1 is lost.

(Lithium ion secondary battery)
FIG. 4 is a schematic cross-sectional view of a lithium ion secondary battery 100 using the positive electrode active material 10 for a lithium ion secondary battery of the present embodiment as a positive electrode 104.
The lithium ion secondary battery 100 includes a bottomed cylindrical battery can 101 having an opening at the top, a gasket 102 and a sealing lid 103 for sealing the opening of the battery can 101, and a positive electrode 104 accommodated in the battery can 101. And a negative electrode 105, and a separator 106 interposed between the positive electrode 104 and the negative electrode 105. In addition, the lithium ion secondary battery 100 includes a positive electrode lead 107 that connects the positive electrode 104 to the sealing lid 103, a negative electrode lead 108 that connects the negative electrode 105 to the bottom 101 b of the battery can 101, and the positive electrode 104 and the negative electrode 105 and the sealing lid 103. And an insulating plate 110 disposed between the positive electrode 104 and the negative electrode 105 and the bottom 101b of the battery can 101.

  The battery can 101 has, for example, a cylindrical shape, and the sealing lid 103 has a circular shape when viewed from above. The battery can 101 and the sealing lid 103 are made of a metal material such as stainless steel (SUS), aluminum, or an aluminum alloy. For example, at least one of the battery can 101 and the sealing lid 103 is plastically deformed and caulked. The battery can 101 and the sealing lid 103 are integrally provided. The shape of the exterior of the lithium ion secondary battery 100 including the battery can 101 and the sealing lid 103 is not limited to a cylindrical shape, and may be, for example, a flat shape, a square shape, a coin shape, a button shape, or a sheet shape. Good. The battery can 101 and the sealing lid 103 may be a bag-like aluminum laminate sheet lined with an insulating sheet made of polyethylene or polypropylene, for example.

  The gasket 102 is made of an insulating resin material, and electrically insulates between the battery can 101 and the sealing lid 103 and hermetically seals between the battery can 101 and the sealing lid 103. The insulating plates 109 and 110 are made of, for example, an insulating resin material.

  Each of the positive electrode 104 and the negative electrode 105 is provided in a long band shape, and is laminated so as to face each other with a long band-shaped separator 106 interposed from one end to the other end in the longitudinal direction, and is wound in parallel with the width direction. It is wound around the axis. The positive electrode 104 includes, for example, a positive electrode current collector that is an aluminum foil, and a positive electrode mixture layer formed on the surface of the positive electrode current collector. The positive electrode mixture layer of the positive electrode 104 includes the positive electrode active material 10 of the present embodiment described above. The negative electrode 105 includes, for example, a negative electrode current collector that is a copper foil, and a negative electrode mixture layer formed on the surface of the negative electrode current collector.

As the separator 106, for example, a polyolefin such as polyethylene or polypropylene, a resin such as polyamide or aramid, a microporous thin film made of fibrous glass or the like can be used. The separator 106 may be covered with an insulating inorganic compound layer such as alumina or glass in order to improve heat resistance or flame retardancy. The separator 106 separates the positive electrode 104 and the negative electrode 105 to prevent a short circuit, and has ion conductivity through which lithium ions (Li ⁺ ) pass.

  In the positive electrode 104, the positive electrode current collector is electrically connected to the sealing lid 103 via the positive electrode lead 107, and a short circuit with the sealing lid 103 is prevented by the insulating plate 109. In the negative electrode 105, the negative electrode current collector is electrically connected to the bottom 101 b of the battery can 101 via the negative electrode lead 108, and a short circuit with the bottom 101 b of the battery can 101 is prevented by the insulating plate 110.

  When manufacturing the lithium ion secondary battery 100, first, the positive electrode 104 and the negative electrode 105 wound with the separator 106 interposed therebetween are accommodated in the battery can 101, and the positive electrode 104 and the negative electrode 105 are respectively sealed with the sealing lid 103 and the battery can. 101. Next, after injecting the electrolyte into the battery can 101 in dry air or under an inert gas atmosphere, the battery can 101 is hermetically sealed with the gasket 102 and the sealing lid 103, whereby the lithium ion secondary battery 100 is manufactured. Is done.

As the electrolytic solution, a nonaqueous electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent can be used. Examples of the lithium salt include lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), and six fluorine. Lithium phosphate (LiAsF ₆ ) or the like can be used singly or in combination.

  As the non-aqueous solvent, a chain or cyclic carbonate solvent such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate, or a fluorine solvent such as perfluoroalkyl ether can be used. These carbonates may be derivatives substituted with fluorine. In addition, vinylene carbonate, phenylcyclohexane, 1,3-propane sultone, diphenyl disulfide, etc. are added to the electrolyte to improve battery life, or phosphate esters are added to improve flame retardancy. You may do it.

  Hereinafter, the manufacturing method of the positive electrode 104 and the negative electrode 105 of the lithium ion secondary battery 100 of this embodiment is demonstrated.

  When producing the positive electrode 104 of the lithium ion secondary battery 100, first, the positive electrode current collector, the positive electrode active material 10 for the lithium ion secondary battery, the conductive agent, the binder, and the binder solvent are first prepared. prepare.

  As the positive electrode current collector, for example, an aluminum foil having a thickness of about 10 μm to about 30 μm can be used. Further, the positive electrode current collector may be in the form of expanded metal, punching metal, or the like.

  The conductive agent is not particularly limited as long as it is a conductive agent used for a positive electrode for a general lithium ion secondary battery. For example, natural graphite powder, carbon fiber, carbon black, metal powder, conductive polymer, etc. are used. Can do. More specifically, as carbon black, for example, acetylene black, furnace black, thermal black, channel black, and the like can be used. Moreover, as metal powder, aluminum, nickel, copper, silver, etc. can be used, for example. As the conductive polymer, for example, polyphenylene or the like can be used. It is preferable that content of the electrically conductive agent in a positive mix layer is 0.1 to 10 mass% with respect to the whole mass of a positive mix layer.

  Although it will not specifically limit if it is a binder used for the positive electrode for common lithium ion secondary batteries as a binder, For example, fluorine-type, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyhexafluoropropylene, etc. Resins, styrene resins such as styrene-butadiene rubber, olefin resins such as polyethylene and polypropylene, acrylic resins such as polyacrylic acid, polymethacrylic acid and polyacrylonitrile, and cellulose resins such as carboxymethylcellulose and hydroxyethylcellulose Resin or the like can be used.

  Examples of the solvent include amides such as N-methylpyrrolidone (NMP), N, N-dimethylformamide, N, N-dimethylacetamide, alcohols such as methanol, ethanol, propanol, and isopropanol, ethylene glycol, diethylene glycol, and glycerin. Polyhydric alcohols such as, ethers, dimethyl sulfoxide, tetrahydrofuran, water and the like can be used.

  Next, the positive electrode active material 10 of this embodiment, a conductive material, a binder, and a solvent are mixed to carry out a slurry preparation step for preparing a slurry. In the slurry preparation step, it is preferable to use a high-viscosity stirrer having a relatively strong shearing force as a mixing means. Specifically, a planetary mixer, a disper mixer, a rotation / revolution mixer, or the like can be used as the mixing means.

  Here, the particle 1 of the positive electrode active material 10 of the present embodiment is composed of a compound in which the particle surface layer portion 1B has a Li-excess layered structure represented by the composition formula (2), and the Ni composition ratio of the particle surface layer portion 1B. Is 0.4 or less. Thereby, precipitation of the Li component on the surface of the particles 1 of the positive electrode active material 10 is suppressed, and the pH of the dispersion medium when the particles 1 of the positive electrode active material 10 are dispersed in pure water at a ratio of 0.1% by mass, that is, The pH of the solution in which the components deposited on the surfaces of the particles 1 are eluted in pure water can be set to 10 or less, for example. When the positive electrode active material 10 has such properties, it is possible to suppress the alkalinity of the slurry containing the positive electrode active material 10 from being increased, and it is possible to suppress the gelation of the slurry. Therefore, a slurry in which the positive electrode active material 10, the conductive material, and the binder are mixed more uniformly can be prepared.

  Next, a slurry coating step is performed in which the slurry containing the positive electrode active material 10 of the present embodiment prepared in the slurry preparation step is applied to the surface of the positive electrode current collector. In the slurry application step, the slurry prepared in the slurry preparation step is applied to the positive electrode current collector and dried to form a positive electrode mixture layer. In the slurry coating process, general coating means such as a die coater, a gravure coater, a doctor blade and the like can be used.

  Next, a compression / molding process for compressing the positive electrode mixture layer to form the positive electrode 104 is performed. In the compression / molding step, first, the positive electrode current collector on which the positive electrode mixture layer is formed is compressed by applying a predetermined pressure by a compression means such as a roll press to form the positive electrode mixture layer. The thickness of the positive electrode mixture layer after compression molding is, for example, about 100 μm or more and 300 μm or less. Then, the positive electrode 104 of the lithium ion secondary battery 100 can be produced by cutting or punching out the positive electrode current collector on which the positive electrode mixture layer is formed into a desired shape. As described above, the positive electrode 104 that realizes the lithium ion secondary battery 100 having a low DC internal resistance and a high discharge capacity can be suitably manufactured.

  When the negative electrode 105 of the lithium ion secondary battery 100 is manufactured, first, a negative electrode current collector, a negative electrode active material, a conductive agent, a binder, and a binder solvent are prepared. Note that the same conductive agent, binder, and solvent used for manufacturing the negative electrode 105 as those for the positive electrode 104 can be used. Note that the conductive material may be omitted in the manufacturing process of the negative electrode 105.

  As the negative electrode current collector, for example, a copper foil or nickel foil having a thickness of about 5 μm to about 20 μm can be used. The negative electrode current collector may be in the form of expanded metal, punching metal, or the like.

  The negative electrode active material is not particularly limited as long as it is a negative electrode active material used for a negative electrode of a general lithium ion secondary battery. For example, graphite, coke, pyrolytic carbon, carbon fiber, hard carbon, amorphous carbon, etc. Carbon materials, lithium metal oxides such as Si, Ti, Sn and the like typified by lithium titanate, elements such as Si, As, Al, Sn, Sb, Pb, In, Ga, alkaline earth metals and lithium Alloy, metallic lithium, or a composite material of these can be used.

  Next, also in the manufacturing process of the negative electrode 105, the slurry preparation process, the slurry coating process, and the compression / molding process are performed similarly to the manufacturing process of the positive electrode 104 described above. In addition, when making the negative electrode 105 contain a electrically conductive agent, what is necessary is just to weigh a desired quantity of electrically conductive material and mix it with a negative electrode active material and a binder in a slurry preparation process. The thickness of the negative electrode mixture layer of the negative electrode 105 that has undergone the compression / molding step is, for example, about 20 μm or more and 70 μm or less.

  The lithium ion secondary battery 100 of this embodiment including the positive electrode 104 and the negative electrode 105 manufactured as described above is supplied from an external generator or a power supply device via the sealing lid 103 and the bottom 101b of the battery can 101. The charged electric power is charged between the positive electrode 104 and the negative electrode 105. In addition, the lithium ion secondary battery 100 supplies power charged between the positive electrode 104 and the negative electrode 105 to an external device that consumes power through the sealing lid 103 and the bottom 101 b of the battery can 101.

  Here, the positive electrode 104 of the lithium ion secondary battery 100 of the present embodiment includes the positive electrode active material 10 of the present embodiment described above in the positive electrode mixture layer. And the positive electrode active material 10 of this embodiment is 1A of particle | grains consisting of the compound which has a layered structure with high Ni content represented by the said composition formula (1), and Li excess represented by the said composition formula (2) And a particle surface layer portion 1B made of a compound having a layered structure.

  Thus, the inside 1A of the particles of the positive electrode active material 10 is composed of a compound having a layered structure with a high Ni content, and the Ni composition ratio is a high ratio of 0.7 or more and 0.9 or less, so that lithium High energy density of the ion secondary battery 100 can be realized.

  Further, the particle surface layer portion 1B of the positive electrode active material 10 is made of a compound having a Li-excess layered structure with a Li composition ratio of 1.01 or more, and the Ni composition ratio has a low ratio of 0.1 or more and 0.4 or less. It has become. As a result, the precipitation of the Li component on the surface of the particle 1 is suppressed, the Li component is taken into the particle surface layer portion 1B, the elution of the Li component when the particle 1 is dispersed in pure water is suppressed, and the precipitated component is eluted. An increase in the pH of the solution can be suppressed and the alkalinity can be weakened.

  When the positive electrode active material 10 has such a property, decomposition of the electrolytic solution can be suppressed in the lithium ion secondary battery 100 including the positive electrode 104 including the positive electrode active material 10 in the positive electrode mixture layer. Therefore, according to the positive electrode active material 10 and the lithium ion secondary battery 100 of the present embodiment, it is possible to suppress a decrease in positive electrode capacity with the passage of the number of charge / discharge cycles, and to improve cycle characteristics.

  As described above, according to the positive electrode active material 10 and the lithium ion secondary battery 100 of the present embodiment, the inside 1A of the particles 1 of the positive electrode active material 10 is made of a compound having a layered structure with a high Ni content. The cycle characteristics of the lithium ion secondary battery 100 can be improved by realizing a high energy density of the lithium ion secondary battery 100 and the particle surface layer portion 1B being made of a compound having a layer structure with an excess of Li.

  In addition, the positive electrode active material 10 has a pH when particles 1 are dispersed in pure water at a ratio of 0.1% by mass, that is, a pH of a solution from which precipitates on the surface of the particles 1 are eluted is 10.0 or less. It has the property. Therefore, when the positive electrode 104 is manufactured, the alkalinity of the slurry containing the positive electrode active material 10 can be suppressed and gelation of the slurry can be suppressed. Therefore, a slurry in which the positive electrode active material 10, the conductive material, and the binder are more uniformly mixed can be prepared to form a more uniform positive electrode mixture layer, and the high energy of the lithium ion secondary battery 100 can be formed. It can contribute to densification and improvement of cycle characteristics. The pH when the particles 1 are dispersed in pure water at a ratio of 0.1% by mass is 9 or more.

  As described above, according to the positive electrode active material 10 and the lithium ion secondary battery 100 of the present embodiment, it is possible to achieve both high energy density and improved cycle characteristics of the lithium ion secondary battery. Moreover, the manufacturing method of the positive electrode active material 10 of this embodiment is suitable for manufacture of the positive electrode active material 10 of this embodiment.

  The embodiment of the present invention has been described in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

[Example]
Examples of the positive electrode active material for a lithium ion secondary battery, a method for producing the same, and a lithium ion secondary battery of the present invention will be described below in comparison with comparative examples different from the present invention.

Example 1
The internal particle 1a shown in FIG. 3 described in the above embodiment was manufactured by the following procedure. First, the raw material lithium carbonate, nickel carbonate, manganese carbonate, and cobalt carbonate are mixed so that the molar ratio of Li: Ni: Mn: Co is 1.0: 0.8: 0.1: 0.1. Weighed, wet pulverized and mixed to prepare raw material powder. The obtained raw material powder was vacuum-dried using an evaporator, then charged into a high-purity alumina container, and pre-baked at 600 ° C. for 12 hours in an oxygen stream.

Next, after the obtained calcined product was air-cooled and crushed, it was again put into a high-purity alumina container and subjected to main firing at 850 ° C. for 8 hours under an oxygen stream. And the obtained sintered body was air-cooled, crushed and classified to obtain internal particles 1a. When the obtained internal particle 1a was analyzed, a composition formula satisfying the following composition formula (1): Li _1.0 Ni _0.8 Mn _0.1 Co _0.1 O _2.0 was obtained. It was confirmed that 1a consists of a compound having a layered structure with a high Ni content. Further, the particle diameter of the internal particles 1a was measured with an electron microscope and found to be about 1 μm.

_{Li 1 + a Ni b Mn c} Co d M e O 2 ... (1)

  However, in the composition formula (1), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and a, b, c, d, and e are 0 .9 ≦ a + b + c + d + e ≦ 1.1, −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.3, and 0 It is a numerical value satisfying ≦ e ≦ 0.35.

  Next, the surface layer particles 1b shown in FIG. 3 described in the above embodiment were manufactured by the following procedure. First, raw material lithium carbonate, nickel carbonate, and manganese carbonate are weighed so that Li: Ni: Mn is a molar concentration ratio of 1.2: 0.2: 0.6, and these are wet-ground and mixed. Thus, raw material powder was prepared. The obtained raw material powder was vacuum-dried using an evaporator, then charged into a high-purity alumina container, and calcined at 700 ° C. for 12 hours in an air stream.

And the obtained sintered body was air-cooled, crushed and classified to obtain surface layer particles 1b. The obtained was analyzed for surface particle 1b, composition formula satisfy the following formula _{(2): Li 1.2 Ni 0.2} Mn 0.6 O 2.0 is obtained, the surface layer particles 1b excess Li It was confirmed to be composed of a compound having a layered structure of Moreover, when the particle diameter of the surface layer particle | grains 1b was measured with the electron microscope, it was about 50 nm.

Li _{1 + w} Ni _x Mn _y M _z O ₂ (2)

  However, in composition formula (2), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb, and w, x, y, and z are 0.9. ≦ w + x + y + z = 1.1, 0.01 ≦ w ≦ 0.3, 0.1 ≦ x ≦ 0.4, 0.35 ≦ y ≦ 0.6, and 0 ≦ z ≦ 0.2 .

  Next, the inner particles 1a and the surface layer particles 1b are weighed so that the mass ratio is 0.95: 0.05, and mixed in a state of being dispersed in water in a planetary ball mill. The indicated mixing step S1 was carried out. The mixing speed was 100 rpm and the mixing time was 3 hours. The mixed dispersion medium was vacuum dried using an evaporator, then charged into a high-purity alumina container, and subjected to heat treatment at 800 ° C. for 1 hour under an oxygen stream, thereby carrying out heat treatment step S2 shown in FIG. . As a result, a positive electrode active material 10 composed of the particles 1 shown in FIG. 1 was obtained. The particle size of the particle 1 was measured by an electron microscope and found to be about 1 μm.

  Next, the crystal structure of the particles 1 of the obtained positive electrode active material 10 was analyzed. As a result of measurement using an X-ray diffractometer (RINTIII, manufactured by Rigaku Corporation) using CuKα rays, a peak of a layered structure belonging to R3-m could be confirmed. Further, the particles 1 of the positive electrode active material 10 were dispersed at a ratio of 0.01 g with respect to 10 g (10 ml) of pure water, and 5 minutes after the dispersion, the pH of the dispersion medium was measured using a pH meter. As a result, the pH of the dispersion medium was 9.5.

Next, the BET specific surface area of the positive electrode active material 10 was measured. Using an automatic specific surface area / pore distribution measuring device (BELSORP-mini manufactured by Nippon Bell Co., Ltd.), the specific surface area was calculated from the adsorption / desorption isotherm by the Langmuir method. As a result, the BET specific surface area of the positive electrode active material 10 was 0.6 m ² / g.

  Next, a lithium ion secondary battery was produced using the obtained positive electrode active material 10. First, 90 parts by mass of the positive electrode active material 10, 6 parts by mass of a conductive material, and 4 parts by mass of a binder were mixed in a solvent, and stirred for 3 hours using a planetary mixer to prepare a slurry. . The conductive material was carbon particle powder, the binder was polyvinylidene fluoride, and the solvent was N-methylpyrrolidone.

Next, the obtained slurry is applied to one side of a positive electrode current collector, which is an aluminum foil having a thickness of 15 μm, by a blade coater, dried, and using a roll press, the density is 2.70 g / cm ^3. The positive electrode mixture layer was formed by pressurization and molding. Thereafter, the positive electrode current collector on which the positive electrode mixture layer was formed was punched into a disk shape having a diameter of 15 mm to obtain a positive electrode for a lithium ion secondary battery.

The negative electrode was produced using metallic lithium. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate having a volume ratio of 1: 2 was used. A secondary battery was produced.

  Next, the produced lithium ion secondary battery was subjected to a charge / discharge test to evaluate charge / discharge cycle characteristics. The charge / discharge test was performed at an environmental temperature of 25 ° C. The charge / discharge cycle characteristics of the lithium ion secondary battery were determined by the following procedure. The lithium ion secondary battery was charged at a constant current and low voltage up to an upper limit voltage of 4.3 V at a current corresponding to 1 C, and after a pause of 10 minutes, was discharged to a lower limit voltage of 3.3 V at a constant current equivalent to 1 C. The initial discharge capacity of the lithium ion secondary battery was 185 Ah / kg. This charge / discharge cycle was repeated 50 times in total, and the ratio of the discharge capacity at the 50th cycle to the initial discharge capacity of the lithium ion secondary battery was evaluated. As a result, the capacity retention rate after 50 cycles of the lithium ion secondary battery according to Example 1 was 90%. The results of Example 1 are shown in Table 1.

(Example 2)
The internal particles 1a prepared in Example 1 were prepared. Next, surface layer particles 1b were produced by the following procedure. Raw material lithium carbonate, nickel carbonate, and manganese carbonate were weighed so that Li: Mn was a molar concentration ratio of 1: 2, and these were wet pulverized and mixed to prepare a raw material powder. The obtained raw material powder was vacuum-dried using an evaporator, then charged into a high-purity alumina container, and calcined at 700 ° C. for 12 hours in an air stream.

And the obtained sintered body was air-cooled, crushed and classified to obtain surface layer particles 1b. As in Example 1, was subjected to analysis of the obtained surface layer particles 1b, the composition formula satisfy the composition formula of _{(2): Li 1.33 Mn 0.67} O 2.0 is obtained, the surface layer particles 1b It was confirmed that consists of a compound having a layer structure with an excess of Li. Moreover, when the particle diameter of the surface layer particle | grains 1b was measured with the electron microscope, it was about 50 nm.

Next, the internal particles 1a and the surface layer particles 1b are weighed in the same manner as in Example 1, and the mixing step S1 and the heat treatment step S2 are performed, so that the positive electrode active material of Example 2 including the particles 1 shown in FIG. 10 was obtained. It was about 1 micrometer when the particle size of the particle | grains 1 of the obtained positive electrode active material 10 was measured with the electron microscope. Next, when the crystal structure of the obtained particles 1 of the positive electrode active material 10 of Example 2 was analyzed in the same manner as in Example 1, the peak of the layered structure attributed to R3-m could be confirmed. In addition, when the pH of the dispersion medium was measured by dispersing the particles 1 of the positive electrode active material 10 of Example 2 in pure water under the same conditions as in Example 1, the pH of the dispersion medium was 9.5. . Moreover, when the BET specific surface area of the positive electrode active material 10 of Example 2 was measured in the same manner as in Example 1, the BET specific surface area was 0.6 m ² / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Example 2 was produced using the positive electrode active material 10 of Example 2. And similarly to Example 1, the charging / discharging test of the lithium ion secondary battery of Example 2 was done, and the charging / discharging cycle characteristic was evaluated. The initial discharge capacity of the lithium ion secondary battery of Example 2 was 182 Ah / kg, and the capacity retention rate after 50 cycles was 89%. The results of Example 2 are shown in Table 1.

(Example 3)
Internal particles 1a were produced in the same manner as in Example 1 except that only the raw material ratio was changed. Lithium carbonate, nickel carbonate, cobalt carbonate, and manganese carbonate as raw materials were weighed so that Li: Ni: Mn: Co was 1.03: 0.8: 0.05: 0.15 in terms of molar concentration ratio. . When the obtained internal particles 1a were analyzed in the same manner as in Example 1, the composition formula satisfying the composition formula (1): Li _1.00 Ni _0.80 Mn _0.05 Co _0.15 O2 _{. 00} was obtained, and it was confirmed that the internal particles 1a consisted of a compound having a layered structure with a high Ni content. Further, the particle diameter of the internal particles 1a was measured with an electron microscope and found to be about 1 μm. As the surface layer particles 1b, those prepared in Example 1 were prepared.

Next, the internal particles 1a and the surface layer particles 1b are weighed in the same manner as in Example 1, and the mixing step S1 and the heat treatment step S2 are performed, so that the positive electrode active material of Example 3 including the particles 1 shown in FIG. 10 was obtained. It was about 1 micrometer when the particle size of the particle | grains 1 of the obtained positive electrode active material 10 was measured with the electron microscope. Next, when the crystal structure of the particles 1 of the positive electrode active material 10 of Example 3 was analyzed in the same manner as in Example 1, the peak of the layered structure belonging to R3-m could be confirmed. Further, when the pH of the dispersion medium was measured by dispersing the particles 1 of the positive electrode active material 10 of Example 3 in pure water under the same conditions as in Example 1, the pH of the dispersion medium was 9.6. . Moreover, when the BET specific surface area of the positive electrode active material 10 of Example 3 was measured in the same manner as in Example 1, the BET specific surface area was 0.7 m ² / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Example 3 was fabricated using the positive electrode active material 10 of Example 3. And similarly to Example 1, the charging / discharging test of the lithium ion secondary battery of Example 3 was performed, and the charging / discharging cycle characteristic was evaluated. The initial discharge capacity of the lithium ion secondary battery of Example 3 was 184 Ah / kg, and the capacity retention rate after 50 cycles was 92%. The results of Example 3 are shown in Table 1.

Example 4
The internal particles 1a prepared in Example 3 and the surface layer particles 1b prepared in Example 2 were prepared, the internal particles 1a and the surface layer particles 1b were weighed in the same manner as in Example 1, and the mixing step S1 and the heat treatment step By implementing S2, the positive electrode active material 10 of Example 4 which consists of the particle | grains 1 shown in FIG. 1 was obtained. It was about 1 micrometer when the particle size of the particle | grains 1 of the obtained positive electrode active material 10 was measured with the electron microscope. Next, when the crystal structure of the particles 1 of the positive electrode active material 10 of Example 4 was analyzed in the same manner as in Example 1, the peak of the layered structure attributed to R3-m was confirmed. Moreover, when the particles 1 of the positive electrode active material 10 of Example 4 were dispersed in pure water under the same conditions as in Example 1, the pH of the dispersion medium was measured. The pH of the dispersion medium was 9.5. . Moreover, when the BET specific surface area of the positive electrode active material 10 of Example 4 was measured in the same manner as in Example 1, the BET specific surface area was 0.6 m ² / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Example 4 was produced using the positive electrode active material 10 of Example 4. And similarly to Example 1, the charge / discharge test of the lithium ion secondary battery of Example 4 was performed, and the charge / discharge cycle characteristics were evaluated. The initial discharge capacity of the lithium ion secondary battery of Example 4 was 185 Ah / kg, and the capacity retention rate after 50 cycles was 91%. The results of Example 4 are shown in Table 1.

(Comparative Example 1)
Only the internal particles 1 a made of the compound having a layered structure with a high Ni content produced in Example 1 were used as the positive electrode active material of Comparative Example 1. Under the same conditions as in Example 1, the positive electrode active material particles (inner particles 1a) of Comparative Example 1 were dispersed in pure water and the pH of the dispersion medium was measured. The pH of the dispersion medium was 10.2. It was. Moreover, when the BET specific surface area of the positive electrode active material of the comparative example 1 which consists only of the internal particle 1a was measured similarly to Example 1, the BET specific surface area was 0.4 m < ² > / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Comparative Example 1 was produced using the positive electrode active material of Comparative Example 1. And similarly to Example 1, the charging / discharging test of the lithium ion secondary battery of the comparative example 1 was done, and the charging / discharging cycling characteristics were evaluated. The initial discharge capacity of the lithium ion secondary battery of Comparative Example 1 was 184 Ah / kg, and the capacity retention rate after 50 cycles was 77%. The results of Comparative Example 1 are shown in Table 1.

(Comparative Example 2)
Comparison was made by dispersing only the internal particles 1a made of the compound having a layered structure with a high Ni content prepared in Example 1 at a ratio of 1 g with respect to 10 g (10 ml) of pure water, and then vacuum drying. Used as the positive electrode active material of Example 2. Under the same conditions as in Example 1, the particles of the positive electrode active material of Comparative Example 2 (inner particles 1a) were dispersed in pure water and the pH of the dispersion medium was measured. The pH of the dispersion medium was 10.7. It was. Moreover, when the BET specific surface area of the positive electrode active material of the comparative example 2 which consists only of the internal particle 1a was measured like Example 1, the BET specific surface area was 0.3 m < ² > / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Comparative Example 2 was produced using the positive electrode active material of Comparative Example 2. And similarly to Example 1, the charging / discharging test of the lithium ion secondary battery of the comparative example 2 was done, and the charging / discharging cycling characteristics were evaluated. The initial discharge capacity of the lithium ion secondary battery of Comparative Example 2 was 178 Ah / kg, and the capacity retention rate after 50 cycles was 60%. The results of Comparative Example 2 are shown in Table 1.

(Comparative Example 3)
Only the internal particles 1 a made of the compound having a layered structure with a high Ni content prepared in Example 3 were used as the positive electrode active material of Comparative Example 3. Under the same conditions as in Example 1, the positive electrode active material particles (internal particles 1a) of Comparative Example 3 were dispersed in pure water and the pH of the dispersion medium was measured. The pH of the dispersion medium was 10.2. It was. Moreover, when the BET specific surface area of the positive electrode active material of the comparative example 3 which consists only of the internal particle 1a was measured similarly to Example 1, the BET specific surface area was 0.4 m < ² > / g.

  Next, similarly to Example 1, a lithium ion secondary battery of Comparative Example 3 was fabricated using the positive electrode active material of Comparative Example 3. And similarly to Example 1, the charging / discharging test of the lithium ion secondary battery of the comparative example 3 was done, and the charging / discharging cycling characteristics were evaluated. The initial discharge capacity of the lithium ion secondary battery of Comparative Example 3 was 183 Ah / kg, and the capacity retention rate after 50 cycles was 70%. The results of Comparative Example 3 are shown in Table 1.

  Referring to Table 1 showing the results of Examples 1 to 4 and Comparative Examples 1 to 3 described above, particles 1A composed of a compound having a layered structure with a high Ni content and particles composed of a compound having a layered structure containing excess Li In Examples 1 to 4 using the positive electrode active material 10 composed of the particles 1 having the surface layer portion 1B, the pH of the dispersion medium in which the particles 1 of the positive electrode active material 10 are dispersed in pure water is 10 or less, and the alkalinity is sufficient. We were able to weaken. In contrast, in Comparative Examples 1 to 3 using only the internal particles 1a made of a compound having a layered structure with a high Ni content as the positive electrode active material, the pH of the dispersion medium in which the particles of the positive electrode active material are dispersed in pure water. However, the alkalinity could not be lowered sufficiently.

Moreover, in Examples 1 to 4, the BET specific surface area of the positive electrode active material 10 could be 0.6 m ² / kg or more, whereas in Comparative Examples 1 to 3, the BET specific surface area was 0.4 m ^2. / Kg or less. Further, in Examples 1 to 4 and Comparative Examples 1 to 3, there was no significant difference in the initial charge / discharge capacity, but in Examples 1 to 4, the capacity retention rate after 50 cycles was at a high rate of 89% or more. In contrast, in Comparative Examples 1 to 3, the capacity retention after 50 cycles was reduced to 77% or less.

  From the above results, it was confirmed that the positive electrode active material and the lithium ion secondary battery according to Examples 1 to 3 can achieve both high energy density and improved cycle characteristics of the lithium ion secondary battery. .

1 particle, 1A particle inside, 1a inside particle, 1B particle surface layer part, 1b surface layer particle, 10 positive electrode active material, 100 lithium ion secondary battery, 104 positive electrode, S1 mixing step, S2 heat treatment step

Claims (5)

A positive electrode active material for a lithium ion secondary battery,
It is characterized by comprising particles having a particle interior comprising a compound having a layered structure represented by the composition formula (1) and a particle surface layer portion comprising a compound having a layered structure represented by the composition formula (2). Positive electrode active material for lithium ion secondary batteries.
_{Li 1 + a Ni b Mn c} Co d M e O 2 ... (1)
Li _{1 + w} Ni _x Mn _y M _z O ₂ (2)
However, in the composition formulas (1) and (2), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb.
In the composition formula (1), a, b, c, d and e are 0.9 ≦ a + b + c + d + e ≦ 1.1, −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.3, and a numerical value satisfying 0 ≦ e ≦ 0.35,
In the composition formula (2), w, x, y and z are 0.9 ≦ w + x + y + z ≦ 1.1, 0.01 ≦ w ≦ 0.3, 0.1 ≦ x ≦ 0.4, 0.35 ≦. It is a numerical value satisfying y ≦ 0.6 and 0 ≦ z ≦ 0.2.   2. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the dispersion medium has a pH of 10.0 or less when the particles are dispersed in pure water at a ratio of 0.1 mass%. . 3. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the BET specific surface area is 0.2 m ² / g or more and 1.0 m ² / g or less. A method for producing a positive electrode active material for a lithium ion secondary battery, comprising:
Internal particles composed of a compound having a layered structure represented by the composition formula (1), and surface layer particles composed of a compound having a particle size smaller than the inner particles and represented by the composition formula (2). A mixing step of mixing;
By heat-treating the mixture obtained in the mixing step at a temperature of 700 ° C. or higher and 850 ° C. or lower, the inside of the particle composed of the compound having a layered structure represented by the composition formula (1), and the composition formula (2 And a particle surface layer portion composed of a compound having a layered structure represented by:
The manufacturing method of the positive electrode active material for lithium ion secondary batteries characterized by having.
_{Li 1 + a Ni b Mn c} Co d M e O 2 ... (1)
Li _{1 + w} Ni _x Mn _y M _z O ₂ (2)
However, in the composition formulas (1) and (2), M is at least one element selected from the group consisting of Mg, Al, Ti, Zr, Mo, and Nb.
In the composition formula (1), a, b, c, d and e are 0.9 ≦ a + b + c + d + e ≦ 1.1, −0.1 ≦ a ≦ 0.2, 0.7 ≦ b ≦ 0.9, 0 ≦ c ≦ 0.35, 0.05 ≦ d ≦ 0.3, and a numerical value satisfying 0 ≦ e ≦ 0.35,
In the composition formula (2), w, x, y and z are 0.9 ≦ w + x + y + z ≦ 1.1, 0.01 ≦ w ≦ 0.3, 0.1 ≦ x ≦ 0.4, 0.35 ≦. It is a numerical value satisfying y ≦ 0.6 and 0 ≦ z ≦ 0.2. A lithium ion secondary battery comprising a positive electrode having a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector,
The said positive mix layer contains the positive electrode active material for lithium ion secondary batteries as described in any one of Claim 1 to 3, The lithium ion secondary battery characterized by the above-mentioned.

Images