JP2016095980A - Positive electrode active material, positive electrode, 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 with a high capacity and a superior capacity-keeping rate in cycle charge/discharge; a positive electrode arranged by use of such a positive electrode active material; and a lithium ion secondary battery including such a positive electrode.SOLUTION: A positive electrode active material comprises a layer structure for a lithium ion secondary battery, which is expressed by the following composition formula: LiNiMO. A lithium ion secondary battery comprises a positive electrode including the positive electrode active material. When the voltage to lithium metal of the positive electrode is represented by V, and the charging capacity of the lithium ion secondary battery is represented by Q in charging the lithium ion secondary battery with a current of 44 Ah/kg based on the weight of the positive electrode active material, a peak value of dQ/dV of a charge curve in a graph of which the horizontal axis shows V, and the vertical axis shows dQ/dV is 190-300 Ah(kg V).SELECTED DRAWING: Figure 4

Description

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

  One type of non-aqueous secondary battery in which a non-aqueous electrolyte mediates electrical conduction between electrodes is a lithium ion secondary battery. A lithium ion secondary battery is a secondary battery in which lithium ions are responsible for electrical conduction between electrodes in charge and discharge reactions. Compared to other secondary batteries such as nickel-hydrogen storage batteries and nickel-cadmium storage batteries, the energy density Is high and the memory effect is small. Therefore, lithium ion secondary batteries can be used for small power sources such as portable electronic devices and household electric appliances, stationary power sources such as power storage devices, uninterruptible power supply devices, power leveling devices, ships, railways, hybrid vehicles, Applications are expanding to medium and large power sources such as drive power sources for electric vehicles.

In particular, in applications where miniaturization of lithium ion secondary batteries is required, demands for improving the energy density of the positive electrode are increasing. LiM1O ₂ with alpha-NaFeO ₂ type layer structure (M1 is Ni, Mn, shows an element such as Co.) positive electrode active material, it has a high charge-discharge capacity as compared with other positive electrode active material. This positive electrode active material tends to increase in capacity as the proportion of Ni contained in M increases, and can improve the charge / discharge capacity when charged to a high charge potential of 4.1 V or higher. On the other hand, there is a problem that the capacity maintenance rate is likely to decrease due to cycle charge and discharge in which charge and discharge are repeated.

For such a problem, a positive electrode active material for a non-aqueous secondary battery having a uniform particle size distribution and good cycle characteristics and output characteristics when used in a battery is disclosed (for example, Patent Document 1 below). reference). The positive electrode active material described in Patent Document 1 has the general _{formula: Li 1 + S Ni x Co} y Mn z Ca t Mg u A v O 2 (-0.05 ≦ s ≦ 0.20, x + y + z + t + u + v = 1,0.3 ≦ x ≦ 0.7, 0.1 ≦ y ≦ 0.4, 0.1 ≦ z ≦ 0.4, 0.0002 ≦ t ≦ 0.01, 0 ≦ u ≦ 0.005, 0.0002 ≦ t + u + v ≦ 0.02, A is a hexagonal crystal represented by Na, Al, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) and a layered structure The lithium-containing composite oxide particles have an average particle diameter of 3 to 8 μm, and [(d90−d10) / average particle diameter], which is an index indicating the spread of the particle size distribution, is 0.60 or less.

JP 2014-252964 A

  The positive electrode active material described in Patent Document 1 can increase the charge / discharge capacity when the value of x indicating the ratio of Ni in the general formula is increased. However, when the ratio of Ni is increased, there is a concern that the capacity retention rate may be reduced due to the phase change of the positive electrode active material being advanced by cycle charge / discharge.

  The present invention has been made in view of the above problems, and has a high capacity and a positive electrode active material for a lithium ion secondary battery excellent in capacity maintenance rate by cycle charge and discharge, a positive electrode using the positive electrode active material, and the It aims at providing the lithium ion secondary battery provided with the positive electrode.

In order to achieve the above object, the positive electrode active material of the present invention has a composition formula Li _{1 + a} Ni _{b McO 2 + α} (where M is Mn, Co, Mg, Al, Ti, Zr, Mo, Nb, Fe, Sn). , V, Zn, W, Na, B are at least one element including at least Mn, and a, b, c, and α are −0.1 ≦ a ≦ 0.3. , B + c = 1, b / (b + c)> 0.7, and −0.1 ≦ α ≦ 0.1.) The layered structure for a lithium ion secondary battery Lithium of the positive electrode when charging a lithium ion secondary battery comprising a positive electrode containing the positive electrode active material with a current of 44 Ah / kg based on the weight of the positive electrode active material. The voltage with respect to the metal is V, the charge capacity of the lithium ion secondary battery is Q, The axis V, the peak value of dQ / dV of the charging curve in the graph of the vertical axis of the dQ / dV is that this is 190Ah · (kg · ^{V) -1} or more and 300Ah · (kg · ^{V) -1} or less Features.

  According to the positive electrode active material of the present invention, the positive electrode using the same, and the lithium ion secondary battery including the positive electrode, the stability of the crystal structure of the positive electrode active material having a layered structure is improved, and the capacity and cycle charge are improved. It is possible to provide a positive electrode active material for a lithium ion secondary battery having an excellent capacity retention rate by discharge, a positive electrode using the positive electrode active material, and a lithium ion secondary battery including the positive electrode.

The typical fragmentary sectional view which shows embodiment of the lithium ion secondary battery of this invention. The schematic cross section which shows one Embodiment of the particle | grains which comprise the positive electrode active material of this invention. The schematic cross section which shows other embodiment of the particle | grains which comprise the positive electrode active material of this invention. 4 is a graph in which the voltage with respect to lithium metal of the positive electrode including the positive electrode active material shown in FIG. 2 or FIG. 3 is V, the charge capacity of the lithium ion secondary battery is Q, the horizontal axis is V, and the vertical axis is dQ / dV.

  Hereinafter, embodiments of a positive electrode active material of the present invention, a positive electrode using the same, and a lithium ion secondary battery including the positive electrode will be described with reference to the drawings.

(Positive electrode and lithium ion secondary battery)
First, a positive electrode for a lithium ion secondary battery and an embodiment of a lithium ion secondary battery including the same will be described. FIG. 1 is a schematic partial cross-sectional view showing an embodiment of a positive electrode for a lithium ion secondary battery of the present invention and a lithium ion secondary battery including the positive electrode.

  The lithium ion secondary battery 100 of this embodiment has, for example, a cylindrical shape, a bottomed cylindrical battery can 101 that contains a non-aqueous electrolyte, and a wound electrode that is accommodated in the battery can 101. A group 110 and a disk-shaped battery lid 102 that seals the upper opening of the battery can 101 are provided. The battery can 101 and the battery lid 102 are made of, for example, a metal material such as aluminum, and the battery lid 102 is fixed to the battery can 101 by caulking or the like via a sealing material 106 made of an insulating resin material. The battery cans 101 are sealed by the battery lid 102 and are electrically insulated from each other. The shape of the lithium ion secondary battery 100 is not limited to a cylindrical shape, and other arbitrary shapes such as a square shape, a button shape, and a laminate sheet shape can be adopted.

  The wound electrode group 110 is produced by winding a long strip-like positive electrode 111 and a negative electrode 112 facing each other with a long strip-like separator 113 around a winding center axis. In the wound electrode group 110, the positive electrode current collector 111 a is electrically connected to the battery lid 102 via the positive electrode lead piece 103, and the negative electrode current collector 112 a is electrically connected to the bottom of the battery can 101 via the negative electrode lead piece 104. Connected. An insulating plate 105 is disposed between the wound electrode group 110 and the battery lid 102 and between the wound electrode group 110 and the bottom of the battery can 101 to prevent a short circuit. The positive electrode lead piece 103 and the negative electrode lead piece 104 are current drawing members made of the same material as the positive electrode current collector 111a and the negative electrode current collector 112a, respectively. The positive electrode current collector 111a and the negative electrode current collector 104 are respectively 112a is joined by spot welding or ultrasonic pressure welding.

  The positive electrode 111 of this embodiment includes a positive electrode current collector 111a and a positive electrode mixture layer 111b formed on the surface of the positive electrode current collector 111a. As the positive electrode current collector 111a, for example, a metal foil such as aluminum or an aluminum alloy, an expanded metal, a punching metal, or the like can be used. The metal foil can have a thickness of, for example, about 8 μm to 20 μm. The positive electrode mixture layer 111b includes a positive electrode active material according to an embodiment described later. The positive electrode mixture layer 111b may include a conductive material, a binder, and the like.

  The negative electrode 112 includes a negative electrode current collector 112a and a negative electrode mixture layer 112b formed on the surface of the negative electrode current collector 112a. As the negative electrode current collector 112a, metal foil such as copper or copper alloy, nickel or nickel alloy, expanded metal, punching metal, or the like can be used. The metal foil can have a thickness of, for example, about 5 μm or more and 20 μm or less. The negative electrode mixture layer 112b contains a negative electrode active material used in a general lithium ion secondary battery. The negative electrode mixture layer 112b may include a conductive material, a binder, and the like.

  As the negative electrode active material, for example, one or more of a carbon material, a metal material, a metal oxide material, and the like can be used. As the carbon material, graphites such as natural graphite and artificial graphite, carbides such as coke and pitch, amorphous carbon, carbon fiber, and the like can be used. Further, as the metal material, lithium, silicon, tin, aluminum, indium, gallium, magnesium and alloys thereof, and as the metal oxide material, a metal oxide containing tin, silicon, lithium, titanium, or the like is used. it can.

  As the separator 113, for example, a polyolefin resin such as polyethylene, polypropylene, or a polyethylene-polypropylene copolymer, a microporous film such as a polyamide resin or an aramid resin, a nonwoven fabric, or the like can be used.

  The positive electrode 111 and the negative electrode 112 can be manufactured through, for example, a mixture preparation step, a mixture coating step, and a molding step. In the mixture preparation step, for example, using a stirring means such as a planetary mixer, a disper mixer, and a rotation / revolution mixer, the positive electrode active material or the negative electrode active material is stirred and mixed with a solution containing a conductive material and a binder, for example. Homogenize to prepare a mixture slurry.

  As the conductive material, a conductive material used in a general lithium ion secondary battery can be used. Specifically, for example, carbon particles such as graphite powder, acetylene black, furnace black, thermal black, and channel black, carbon fibers, and the like can be used as the conductive material. For example, the conductive material can be used in an amount of about 3% by mass to 10% by mass with respect to the total mass of the mixture.

  As the binder, a binder used in a general lithium ion secondary battery can be used. Specifically, for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyhexafluoropropylene, styrene-butadiene rubber, carboxymethylcellulose, polyacrylonitrile, modified polyacrylonitrile, and the like can be used as the binder. As the binder, for example, an amount of about 2% by mass to 10% by mass with respect to the mass of the entire mixture can be used.

  As the solvent of the solution, N-methylpyrrolidone, water, N, N-dimethylformamide, N, N-dimethylacetamide, methanol, ethanol, propanol, isopropanol, ethylene glycol, diethylene glycol, glycerin depending on the type of binder. , Dimethyl sulfoxide, tetrahydrofuran and the like.

  In the mixture coating process, first, the mixture slurry containing the positive electrode active material and the mixture slurry containing the negative electrode active material prepared in the mixture preparation process are applied to, for example, a bar coater, a doctor blade, a roll transfer machine, etc. By the means, it apply | coats on the surface of the positive electrode collector 111a and the negative electrode collector 112a, respectively. Next, the positive electrode current collector 111a and the negative electrode current collector 112a coated with the mixture slurry are each heat-treated to volatilize or evaporate the solvent of the solution contained in the mixture slurry, thereby removing the positive electrode current collector. A positive electrode mixture layer 111b and a negative electrode mixture layer 112b are formed on the surfaces of 111a and the negative electrode current collector 112a, respectively.

  In the molding step, first, the positive electrode mixture layer 111b formed on the surface of the positive electrode current collector 111a and the negative electrode mixture layer 112b formed on the surface of the negative electrode current collector 112a are subjected to, for example, a roll press or the like. Each is pressure-molded using a pressure means. Thereby, the positive electrode mixture layer 111b can be made to have a thickness of, for example, about 15 μm to 300 μm, and the negative electrode mixture layer 112b can be made to have a thickness of, for example, about 10 μm to 150 μm. Thereafter, the positive electrode current collector 111a and the positive electrode material mixture layer 111b, and the negative electrode current collector material 112a and the negative electrode material mixture layer 112b are each cut into long strips, whereby the positive electrode 111 and the negative electrode 112 can be manufactured. .

  The positive electrode 111 and the negative electrode 112 manufactured as described above are wound around the winding central axis in a state of being opposed to each other with the separator 113 interposed therebetween, so that a wound electrode group 110 is formed. In the wound electrode group 110, the negative electrode current collector 112a is connected to the bottom of the battery can 101 via the negative electrode lead piece 104, and the positive electrode current collector 111a is connected to the battery lid 102 via the positive electrode lead piece 103 for insulation. A short circuit with the battery can 101 and the battery lid 102 is prevented by the plate 105 and the like, and the battery can 101 is accommodated. Thereafter, a non-aqueous electrolyte is injected into the battery can 101, the battery lid 102 is fixed to the battery can 101 via the sealing material 106, and the battery can 101 is sealed to manufacture the lithium ion secondary battery 100. Can do.

The non-aqueous electrolyte injected to the battery can _{101, LiClO 4, LiPF 6,} LiBF 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 _{F 4 (SO 3) 2,} LiN (CF 3 SO 2) 2, LiC (CF 3 SO2) lithium salts such as ₃ can be used a solution prepared by dissolving in a nonaqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is preferably 0.7M or more and 1.5M or less.

  As the non-aqueous solvent, diethyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, ethyl methyl carbonate, methyl propyl carbonate, methyl acetate, dimethoxyethane and the like can be used. In addition, various additives should be added to the non-aqueous electrolyte for the purpose of suppressing oxidative decomposition and reductive decomposition of the electrolytic solution, preventing precipitation of metal elements, improving ion conductivity, and improving flame retardancy. Can do. Examples of such additives include 1,3-propane sultone and 1,4-butane sultone that suppress decomposition of the electrolytic solution, insoluble polyadipic anhydride that improves the storage stability of the electrolytic solution, and hexahydrophthalic anhydride. An acid or the like, or a fluorine-substituted alkyl boron that improves flame retardancy can be used.

  The lithium ion secondary battery 100 having the above configuration uses the battery lid 102 as a positive external terminal and the bottom of the battery can 101 as a negative external terminal. The electric power accumulated in the rotating electrode group 110 can be supplied to an external device or the like. As described above, the lithium ion secondary battery 100 according to the present embodiment includes, for example, small power sources such as portable electronic devices and household electric devices, stationary power sources such as uninterruptible power sources and power leveling devices, ships, railways, and hybrids. It can be used as a drive power source for automobiles, electric cars and the like.

(Positive electrode active material)
Hereinafter, the positive electrode active material of the present embodiment included in the positive electrode mixture layer 111b of the positive electrode 111 included in the lithium ion secondary battery 100 will be described in detail. 2 and 3 are schematic cross-sectional views showing particles constituting the positive electrode active materials 1A and 1B according to one embodiment of the present invention.

The positive electrode active materials 1A and 1B of the present embodiment are represented by the following composition formula (1).
Li _{1 + a} Ni _{b McO 2 + α} (1)

  However, in Formula (1), M is at least Mn selected from the group consisting of Mn, Co, Mg, Al, Ti, Zr, Mo, Nb, Fe, Sn, V, Zn, W, Na, and B. A, b, c, and α are −0.1 ≦ a ≦ 0.3, b + c = 1, b / (b + c)> 0.7, and −0. It is a number that satisfies 1 ≦ α ≦ 0.1.

The positive electrode active materials 1A and 1B represented by the composition formula (1) are layered oxides capable of inserting and desorbing lithium ions in accordance with electrode reactions, and are mainly α-NaFeO ₂ type layered oxides. A diffraction peak by a X-ray diffraction method having a crystal structure is a lithium metal composite oxide showing a pattern that can be assigned to the space group R3-m (“-” is a superscript bar of “3”).

In the composition formula (1), a indicating the excess or deficiency of Li is a number satisfying −0.1 ≦ a ≦ 0.3, and the composition ratio of Li is 0.9 or more and 1.3 or less. The That is, the positive electrode active materials 1A and 1B are not limited to oxides in which Li is arranged only at the 3a site in the α-NaFeO ₂ type crystal structure, and so-called layered solid solution oxidation in which lithium is in excess of the stoichiometric ratio. objects may be _{(Li (Li p M 1-} p) O 2 (0 <p <1), is expressed as Li ₂ MO 3 -LiMO 2 etc..). By setting the composition ratio of lithium in the positive electrode active materials 1A and 1B in such a range, a high discharge capacity can be ensured.

  In the composition formula (1), b indicating the ratio of Ni and c indicating the ratio of M which is a transition metal element other than Li and Ni satisfy b + c = 1 and b / (b + c)> 0.7. That is, the ratio of Ni in the transition metal element other than Li in the composition formula (1) exceeds 70%. Thereby, the capacity of the positive electrode active materials 1A and 1B represented by the composition formula (1) can be increased.

  M in the composition formula (1) is at least selected from the group consisting of Mn, Co, Mg, Al, Ti, Zr, Mo, Nb, Fe, Sn, V, Zn, W, Na, and B. One or more elements including Mn. For example, M may be Mn alone or may contain Mn and other metal elements. When M contains Mn, the layered structure of the positive electrode active materials 1A and 1B can be stabilized.

  When M in the composition formula (1) contains Co, not only the layered structure of the positive electrode active materials 1A and 1B is stabilized, but also the charge characteristics of the charge characteristics due to the improvement of the rate characteristics and the valence change accompanying the charge and discharge. Increase is possible. However, since Co is unstable in supply and expensive, the ratio of Co in M is preferably in the range of 0.05 to 0.3. It is particularly preferable that M contains two kinds of elements, Mn and Co.

  In the case where M in the composition formula (1) includes a metal element such as Mg, Al, Ti, Zr, Mo, Nb, Fe, Sn, V, Zn, W, Na, and B, the positive electrode active material 1A, The layered structure of 1B can be stabilized. Further, for example, from the viewpoint of stabilizing the layered structure of the Ni-rich layered compound having a layered structure in which the ratio of Ni in the metal element other than Li in the formula (1) exceeds 70%, M is Mg, Al and Ti are preferably included.

  When M in the composition formula (1) includes Al, the ratio of Al in the metal element other than Li, that is, the ratio of Al when b + c = 1 in the composition formula (1) is 0. Is preferably less than 0.04. In the positive electrode active materials 1A and 1B represented by the composition formula (1), when Al in the metal element other than Li is 0.04 or more, the 1C discharge capacity and the capacity maintenance rate of the lithium ion secondary battery are lowered. Because.

  When M in the composition formula (1) includes Mg, the ratio of Mg in the metal element other than Li, that is, the ratio of Mg when b + c = 1 in the formula (1) is Preferably it is less than 0.02. This is because when the ratio of Mg in the metal element other than Li is 0.02 or more, the 1C discharge capacity and the capacity retention rate of the lithium ion secondary battery are lowered.

  Α in the composition formula (1) is a numerical value indicating an excess or deficiency of oxygen and satisfies −0.1 ≦ α ≦ 0.1. That is, the ratio of oxygen in the composition formula (1) is 1.9 or more and 2.1 or less. It is known that the amount of oxygen deviates somewhat from the stoichiometric composition depending on analysis conditions, composition conditions, and the like. Therefore, the positive electrode active materials 1A and 1B represented by the composition formula (1) may have an oxygen amount that is within a range in which the layered structure can be maintained. In addition, when the layered structure of the positive electrode active materials 1A and 1B can be maintained, the amount of oxygen may be changed as long as it is within a range of about 5%. Moreover, the positive electrode active materials 1A and 1B represented by the composition formula (1) may have substitution between sites or defects on the crystal structure.

  In addition, the crystal structure of the particles of the positive electrode active materials 1A and 1B according to the present embodiment can be confirmed by X-ray diffraction (XRD) or the like. The average composition of the particles of the positive electrode active materials 1A and 1B according to this embodiment can be confirmed by high frequency inductively coupled plasma (ICP), atomic absorption spectrometry (AAS), or the like. In addition, the element distribution in the particles of the positive electrode active materials 1A and 1B according to the present embodiment includes time-of-flight secondary ion mass spectrometry (TOF-SIMS), Auger Electron Spectroscopy (Auger Electron Spectroscopy). Spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), transmission electron microscope-electron energy loss spectroscopy (TEM-EELS), and the like.

  The average particle diameter of primary particles, which are individual particles constituting the positive electrode active materials 1A and 1B according to the present embodiment, is preferably 0.1 μm or more and 2 μm or less. By setting the average particle size of the primary particles to 2 μm or less, the filling properties of the positive electrode active materials 1A and 1B in the positive electrode of the lithium ion secondary battery are improved, and a good energy density can be achieved. Further, the particles constituting the positive electrode active materials 1A and 1B may be secondary particles obtained by combining primary particles by granulating primary particles by dry granulation or wet granulation. As a granulation means, granulators, such as a spray dryer and a rolling fluidized bed apparatus, can be used, for example. The average particle size of the secondary particles is preferably 3 μm or more and 50 μm or less.

  The average particle size of the particles constituting the positive electrode active materials 1A and 1B of the present embodiment is, for example, the observation of particles by a scanning electron microscope (SEM), a transmission electron microscope (TEM), or the like. Can be measured based on By observing the particles, for example, 10 primary particles or secondary particles can be extracted in the order of the particle size being closer to the median value, and a weighted average of these particle sizes can be calculated to obtain an average particle size. In addition, a particle diameter can be calculated | required as an average value of the long diameter and short diameter of a particle | grain in an electron microscope image.

  The particles of the positive electrode active material 1B have a surface layer portion 12 and a core portion 11B. When M in the composition formula (1) includes Al, the molar mass concentration of Al in the surface layer portion 12 is preferably higher than the molar molar concentration of Al in the core portion 11B. Thereby, the change of the structure near the surface can be suppressed with a small amount of element substitution. When M in the composition formula (1) includes Mg, the Mg molar concentration of the surface layer portion 12 is preferably higher than the Mg molar concentration of the core portion 11B. Thereby, the change of the structure near the surface can be suppressed with a small amount of element substitution.

  Moreover, the particles of the positive electrode active materials 1A and 1B may be primary particles in which individual particles are separated or secondary particles in which a plurality of particles are bonded. In each case, the primary particles or secondary particles preferably have a surface layer portion 12 and a core portion 11B. In this case, the molar ratio Li / Mn of Li and Mn in the surface layer part 12 is preferably lower than the molar ratio Li / Mn of Li and Mn in the core part 11B. Thereby, the change of the structure near the surface can be suppressed.

  In FIG. 4, the lithium ion secondary battery 100 including the positive electrode 111 including the positive electrode active materials 1A and 1B of the present embodiment is charged with a current of 44 Ah / kg based on the weight of the positive electrode active materials 1A and 1B. In this case, the voltage with respect to the lithium metal of the positive electrode 111 is V, the charge capacity of the lithium ion secondary battery 100 is Q, the horizontal axis is V, and the vertical axis is dQ / dV.

  In FIG. 4, curves L1 to L3 are curves showing the lithium ion secondary battery 100 of the present embodiment using the cathode active materials 1A and 1B of the present embodiment, and the curve L0 uses a conventional cathode active material. 2 is a curve showing a conventional lithium ion secondary battery.

4, the positive electrode active materials 1A and 1B of the present embodiment indicated by the curves L1 to L3 have a peak value of dQ / dV of 190 Ah · (kg · V) ⁻¹ or more and 300 Ah · in the graph shown in FIG. (Kg · V) ⁻¹ or less. The operation will be described below using the positive electrode active materials 1A and 1B of the present embodiment as an example.

  When the lithium ion secondary battery 100 shown in FIG. 1 is employed in, for example, an electric vehicle or a consumer electric device, a high capacity can be obtained, and a decrease in discharge capacity is small even when the number of charge / discharge cycles increases. That is, it is required to suppress a decrease in the discharge cycle capacity maintenance rate.

A conventional lithium ion secondary battery using a conventional positive electrode active material composed of a layered compound represented by LiM1O ₂ , particularly when charged to a high potential of 4.3 V on the basis of Li metal, Structural changes starting from the vicinity of the surface occur, and the capacity maintenance rate of cycle charge / discharge decreases. The cause is at least destabilization of the crystal structure of the positive electrode active material accompanying charge / discharge. As the ratio of Ni in the M1 of lamellar compound represented by LiM1O ₂ increases, increased charge and discharge capacity of the lithium ion secondary battery, the proportion of Li is increased to intercalation and deintercalation. However, a Ni-rich layered compound in which the ratio of Ni in M1 is greater than 0.7 has a large change in lattice volume accompanying charge / discharge, and it is difficult to maintain a conventional layered structure.

  A lithium ion secondary battery using a positive electrode active material made of a Ni-rich layered compound is made of Li metal compared to a lithium ion secondary battery using a positive electrode active material made of a layered compound having a Ni ratio of 0.7 or less. A remarkable reaction peak appears at dQ / dV of the charging curve in the range from 4.1 V to 4.3 V at the reference potential. This peak suggests that an irreversible phase change occurs in the positive electrode active material as the charge capacity increases. Further, the intensity of this peak is related to the amount of phase change of the positive electrode active material, and it is considered that the higher the peak, the more advanced the phase change. As a result, the capacity maintenance rate of cycle charge / discharge of the lithium ion secondary battery is significantly reduced.

In particular, as shown by a curve L0 in FIG. 4, the voltage with respect to the lithium metal of the positive electrode when charging the lithium ion secondary battery at a current of 44 Ah / kg based on the weight of the positive electrode active material is V, the lithium ion secondary. When the peak value of dQ / dV in the graph in which the charge capacity of the battery is Q, the horizontal axis is V, and the vertical axis is dQ / dV exceeds 300 Ah · (kg · V) ⁻¹ , The decrease in the capacity of the secondary battery and the decrease in the capacity maintenance rate of cycle charge / discharge are remarkable. Further, when the peak value of dQ / dV is less than 190 Ah · (kg · V) ⁻¹ , the capacity of the lithium ion secondary battery is significantly reduced.

  In addition, the vicinity of the surface of the positive electrode active material particles made of the Ni-rich layered compound is a region where Li can be easily inserted and desorbed, so the crystal structure is considered to be the most unstable. Therefore, in order to improve the cycle characteristics of the positive electrode active material composed of the Ni-rich layered compound, it is necessary to suppress changes in the structure of the particles of the positive electrode active material, particularly the structure near the surface.

  The peak value of dQ / dV can be realized by, for example, element substitution or surface coating on the positive electrode active materials 1A and 1B.

  FIG. 2 is a schematic cross-sectional view of particles of the positive electrode active material 1A subjected to element substitution, and FIG. 3 is a schematic cross-sectional view of particles of the positive electrode active material 1B subjected to surface coating.

  In the positive electrode active material 1A of the present embodiment, the element substitution is a Ni-rich layered compound in which the ratio of Ni in the metal element other than Li in the formula (1) exceeds 70% and has a layered structure. This means that the element of the part is substituted with a substitute element. In the element replacement, it is preferable to replace an element that does not contribute to charging / discharging in order to suppress a decrease in capacity caused by replacing a part of the positive electrode active material 1A with an inert element. For example, a part of Mn in the positive electrode active material 1A can be substituted with a substitution element. As the substitution element, for example, a metal element such as Al or Mg can be used. The concentration of the substitution element in the particles 11A of the positive electrode active material 1A may be different between the surface and the center.

  In FIG. 4, a curve L1 indicated by a dotted line connecting black triangle marks indicates a lithium ion secondary battery 100 using a positive electrode active material 1A in which Mn contained in M in the formula (1) is element-substituted by Al. ing. In addition, in FIG. 4, a curve L2 indicated by a one-dot chain line connecting the points indicated by black squares represents a lithium ion 2 using a positive electrode active material 1A in which Mn contained in M in the formula (1) is elementally substituted by Mg. The secondary battery 100 is shown.

The surface coating means that the Ni-rich layered compound is used as the core portion 11B and the core portion 11B is covered with the surface layer portion 12. From the viewpoint of suppressing the volume change of the core portion 11B accompanying charge / discharge, it is preferable to use a compound having a lower lattice volume change rate accompanying charge / discharge than the Ni-rich layered compound constituting the core portion 11B. . In order to suppress a decrease in capacity of the positive electrode active material 1 </ b> B composed of particles in which the core portion 11 </ b> B is covered with the surface layer portion 12, it is preferable to use a Li-containing composite compound having charge / discharge activity as much as possible. For example, Li ₂ MnO ₃ or Li _1.2 Ni _0.2 Mn _0.6 O ₂ can be used as the surface layer portion 12. Both surface coating and element substitution may be performed. By setting the surface layer portion to a Li-excess composition, cycle deterioration can be further suppressed at a high potential.

  When performing the surface coating, the entire core portion 11B may be covered with the surface layer portion 12, but a part of the core portion 11B may be exposed from the surface layer portion 12. That is, the surface layer portion 12 does not necessarily need to cover the entire core portion 11B. From the viewpoint of suppressing cycle deterioration of the lithium ion secondary battery 100, the formation rate, which is the ratio of the particles of the positive electrode active material 1B covered by the surface layer portion 12 on the surface of the core portion 11B, is 70% or more. Is preferred. In addition, from the viewpoint of suppressing cycle deterioration of the lithium ion secondary battery 100, the formation is a ratio of particles in which at least a part of the core portion 11B is covered with the surface layer portion 12 among the particles constituting the positive electrode active material 1B. The particle ratio is preferably 50%.

  Moreover, it is preferable that the average thickness of the surface layer part 12 is 20 nm or more and 200 nm or less. There exists a possibility that the effect which suppresses the cycle deterioration of a lithium ion secondary battery may fall that the average thickness of the surface layer part 12 is less than 20 nm. Moreover, when the average thickness of the surface layer part 12 is thicker than 200 nm, the ratio of the core part 11B which contributes to high capacity | capacitance falls, and there exists a possibility that the capacity | capacitance of the lithium ion secondary battery 100 may fall.

In FIG. 4, a curve L3 indicated by a broken line connecting dots with white circles indicates that the core portion 11B represented by the composition formula (1) is surface-coated by the surface layer portion 12 represented by the composition formula Li ₂ MnO _3. 1 shows a lithium ion secondary battery 100 using a positive electrode active material 1B made of fine particles. A curve L0 represented by a solid line connecting the black circles indicates the lithium ion secondary battery 100 using a positive electrode active material that is not subjected to element substitution and surface coating.

  Next, the manufacturing method of positive electrode active material 1A, 1B of this embodiment is demonstrated. The positive electrode active materials 1A and 1B of the present embodiment can be manufactured according to a general method for manufacturing a positive electrode active material, for example, by a solid phase method, a coprecipitation method, a sol-gel method, a hydrothermal method, or the like. be able to.

  When the positive electrode active materials 1A and 1B of the present embodiment are manufactured by the solid phase method, first, other than the raw material Li-containing compound, Ni-containing compound, Mn-containing compound, and Mn contained in M in the formula (1) A raw material powder preparation step of preparing a raw material powder by weighing, mixing, and pulverizing and mixing an M-containing compound containing the above elements at a ratio of a predetermined elemental composition can be performed. For the preparation of the raw material powder, either dry pulverization or wet pulverization can be used. As the pulverizing means, for example, a pulverizer such as a ball mill, a bead mill, a planetary ball mill, an attritor, or a jet mill can be used.

  As the Li-containing compound used in the raw material powder preparation step, for example, lithium acetate, lithium nitrate, lithium carbonate, lithium hydroxide, lithium chloride, lithium sulfate and the like can be used, and in particular, lithium carbonate and lithium hydroxide are used. Is preferred. As the Ni-containing compound and the Mn-containing compound, for example, oxides, hydroxides, carbonates, sulfates, acetates, and the like can be used. In particular, oxides, hydroxides, and carbonates are preferably used. . Further, as the M-containing compound containing an element other than Mn contained in M in the formula (1), for example, acetate, nitrate, carbonate, sulfate, oxide, hydroxide, or the like is used. In particular, it is preferable to use carbonates, oxides, and hydroxides.

  When the element substitution of the positive electrode active material 1A to be produced is performed like the particles of the positive electrode active material 1A shown in FIG. 2, the compound containing the substitution element is mixed with the raw material powder in the raw material powder preparation step. For example, in order to replace Mn in the positive electrode active material 1A with a substitution element such as Al or Mg, in the raw material powder preparation step, the raw material Li-containing compound, the Ni-containing compound, the Mn-containing compound, and the formula (1) Al), an oxide of Mg, an oxide of Mg, and the like can be mixed together with an M-containing compound containing an element other than Mn contained in M.

  Next, the raw material powder prepared in the raw material powder preparation step can be fired to perform a firing step in which the precursor particles of the positive electrode active material 1A or the core portion 11B are obtained. In the firing step, it is preferable to thermally sinter the raw material compound by pre-firing the raw material powder and to sinter the raw material powder after the pre-firing by main firing. Prior to the main firing, the raw powder subjected to the preliminary firing may be appropriately crushed and classified.

  The heating temperature in the pre-baking can be, for example, about 400 ° C. to 700 ° C., and the heating temperature in the main baking can be, for example, about 700 ° C. to 900 ° C., preferably 720 ° C. to 820 ° C. . If it is such a temperature range, crystallinity can be improved, avoiding decomposition | disassembly of the particle | grains of the positive electrode active material 1A or the precursor particle | grains of the core part 11B, and volatilization of a component.

  The calcining time in the preliminary calcination is 2 hours or more and 24 hours or less, preferably 4 hours or more and 16 hours or less, and the calcination time in the main calcination is 2 hours or more and 24 hours or less, preferably 4 hours or more and 16 hours or less. The firing process may be repeated a plurality of times.

  The atmosphere in the firing step may be either an inert gas atmosphere or an oxidizing gas atmosphere, but is preferably an oxidizing gas atmosphere such as oxygen or air. By performing firing in an oxidizing gas atmosphere, it is possible to avoid contamination by impurities due to incomplete thermal decomposition of the raw material compound and to improve crystallinity. Note that the fired positive electrode active material 1A particles or core portion 11B precursor particles may be cooled or air-cooled, or may be rapidly cooled using liquid nitrogen or the like.

When the surface coating of the positive electrode active material 1B to be manufactured is performed like the particles of the positive electrode active material 1B shown in FIG. 3 described above, the precursor particles of the core portion 11B and the precursor of the surface layer portion 12 that have undergone the firing process. The coating step of mixing the particles to obtain the positive electrode active material 1 </ b> B composed of particles in which the core portion 11 </ b> B is covered with the surface layer portion 12 can be performed. The composition of the precursor particles of the surface layer portion 12 can be, for example, Li ₂ MnO ₃ or Li _1.2 Ni _0.2 Mn _0.6 O ₂ .

  In the coating step, it is preferable to perform heat treatment after the precursor particles of the core portion 11B and the precursor particles of the surface layer portion 12 are mixed. By performing the heat treatment, a solid solution phase is formed in the vicinity of the surface of the particles of the positive electrode active material 1B. The heating temperature of the heat treatment in the coating process may be equal to or lower than the heating temperature of the main baking in the baking process. Within such a temperature range, the coated particles can be dissolved in the vicinity of the surface while avoiding the decomposition of the particles of the positive electrode active material 1B and the volatilization of the components. The heat treatment time in the coating step is 10 minutes or more and 12 hours or less, preferably 30 minutes or more and 6 hours or less. The heat treatment in the coating process may be repeated a plurality of times.

  As described above, according to the present embodiment, the positive electrode active materials 1A and 1B for a lithium ion secondary battery and the positive electrode active materials 1A and 1B, which have a high capacity and an excellent capacity retention rate by cycle charge and discharge, are used. The positive electrode 111 and the lithium ion secondary battery 100 including the positive electrode 111 can be provided.

(Examples and Comparative Examples)
Examples of the positive electrode active material, the positive electrode, and the lithium ion secondary battery of the present invention and comparative examples thereof will be described below.

(Element substitution)
Table 1 shows positive electrode active materials of Example 1 and Example 2 in which element substitution was performed, positive electrode active materials in Comparative Example 1 in which element substitution was not performed, and Comparative Examples 2 and 3 in which element substitution was performed. The molar concentration ratio of the composition in the raw material of the positive electrode active material and the composition ratio of the positive electrode active material are shown.

  The positive electrode active material of Comparative Example 1 in which element substitution was not performed was prepared by the following procedure. First, the raw material lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate have a molar ratio of Li: Ni: Co: Mn of 1.03: 0.80: 0.10: 0.10. And weighed and mixed them to prepare a raw material powder. Next, after drying the obtained raw material powder, it was put into a high-purity alumina container and pre-baked at 650 ° C. for 12 hours in an oxygen stream. Next, after the obtained temporary fired body was air-cooled and crushed, it was again put into a high-purity alumina container and subjected to main firing at 770 ° C. for 8 hours under an oxygen stream. And the obtained sintered body was air-cooled, crushed and classified, and the positive electrode active material of Comparative Example 1 was obtained.

When the elemental analysis of the particle | grains of the obtained positive electrode active material of the comparative example 1 was conducted, Li: Ni: Co: Mn was 1.00: 0.80: 0.10: 0.10. Next, the crystal structure of the positive electrode active material was analyzed. As a result of measuring the crystal structure of the positive electrode active material using CuKα rays using an X-ray diffractometer (RINTIII manufactured by Rigaku), a peak of a layered structure belonging to R3-m was confirmed. Therefore, the elemental composition of the positive electrode active material of Comparative Example 1 was estimated to be Li _1.0 Ni _0.8 Co _0.1 Mn _0.1 O _2.0 .

  Moreover, the positive electrode active material of Example 1 and Example 2 which performed element substitution, and Comparative Example 2 and Comparative Example 3 was produced in the following procedures. First, in addition to the raw material lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate, the substitution element Al or Mg oxide is weighed so as to have a molar concentration ratio in the raw material shown in Table 1, These were wet pulverized and mixed to prepare a raw material powder. Thereafter, as in Comparative Example 1, the obtained raw material powder was dried and temporarily fired, followed by main firing, and the obtained fired body was air-cooled, crushed and classified, and Example 1 and Example 2, and positive electrode active materials of Comparative Examples 2 and 3 were obtained.

  Elemental analysis of the obtained positive electrode active material particles of Example 1 and Example 2 and Comparative Example 2 and Comparative Example 3 was performed. The composition ratio of Li: Ni: Co: Mn: Al: Mg was As shown in FIG. Next, as in Comparative Example 1, as a result of analyzing the crystal structure of the positive electrode active material, a layered structure peak attributable to R3-m was confirmed. Therefore, the element composition of the positive electrode active materials of the positive electrode active materials of Example 1 and Example 2 and Comparative Example 2 and Comparative Example 3 is as follows: Li: Ni: Co: Mn: Al: Mg is the composition ratio shown in Table 1. Presumed to be.

(Surface coating)
Table 2 shows the composition ratio of the core portions of the positive electrode active materials of Examples 3 to 7 and Comparative Examples 4 and 5 subjected to surface coating, the weight of the core portion in 100 g of the positive electrode active material particles, and the surface layer. The composition ratio of the parts and the weight of the surface layer part in 100 g of the positive electrode active material particles are shown. In Table 2, the positive electrode active material of Comparative Example 1 which is not subjected to surface coating is shown as a positive electrode active material having only a core portion.

  The positive electrode active materials of Examples 3 to 7 and Comparative Examples 4 and 5 subjected to surface coating were prepared by the following procedure. First, similarly to the positive electrode active material of Comparative Example 1, the raw material lithium carbonate, nickel hydroxide, cobalt carbonate, and manganese carbonate were mixed at a molar concentration ratio of Li: Ni: Co: Mn of 1.03: 0. The raw material powder was prepared by weighing to 80: 0.10: 0.10 and wet crushing and mixing them. Next, similarly to Comparative Example 1, the obtained raw material powder was dried and temporarily fired, followed by main firing, and the obtained fired body was air-cooled, crushed and classified, and core part precursor particles. Was made.

  Next, precursor particles of the surface layer portion of the particles constituting the positive electrode active material of Example 3 to Example 6 and Comparative Example 4, and the surface layer portion of the particles constituting the positive electrode active material of Example 7 and Comparative Example 5 Two kinds of precursor particles of the surface layer portion of the precursor particles were produced.

When preparing precursor particles for the surface layer portion of the particles constituting the positive electrode active materials of Example 3 to Example 6 and Comparative Example 4, first, lithium carbonate and manganese carbonate as raw materials were mixed in a molar concentration of Li: Mn. Weighed so that the ratio was 2.02: 1.0, and wet pulverized and mixed them to prepare raw material powder. After drying the obtained raw material powder, it was put into a high-purity alumina container and pre-baked at 700 ° C. for 12 hours in the air. The obtained calcined product was air-cooled and crushed to obtain precursor particles of the surface layer portion. When elemental analysis of the obtained precursor particles of the surface layer portion was performed, Li: Mn was 2.00: 1.0. Next, as in Comparative Example 1, as a result of analyzing the crystal structure of the precursor particles in the surface layer portion, a monoclinic structure peak attributed to C2 / m could be confirmed. Therefore, the elemental composition of the precursor particles in the surface layer portion was estimated to be Li ₂ MnO ₃ .

In preparing the precursor particles of the surface layer portion of the particles constituting the positive electrode active material of Example 7 and Comparative Example 5, first, lithium carbonate, nickel carbonate, and manganese carbonate as raw materials were mixed in a molar ratio of Li: Ni: Mn. Weighed so that the concentration ratio was 1.21: 0.2: 0.6, and wet pulverized and mixed them to prepare raw material powder. After drying the obtained raw material powder, it was put into a high-purity alumina container and heat-treated at 700 ° C. for 12 hours in the atmosphere. The obtained calcined product was air-cooled and crushed to obtain precursor particles of the surface layer portion. When the elemental analysis of the precursor particle | grains of the obtained surface layer part was performed, Li: Ni: Mn was 1.2: 0.2: 0.6. Next, as in Comparative Example 1, as a result of analyzing the crystal structure of the precursor particles in the surface layer portion, the peak of the layered structure belonging to R3-m could be confirmed. Therefore, the elemental composition of the precursor particles in the surface layer portion was estimated to be Li _1.2 Ni _0.2 Mn _0.6 O _2.0 .

  Next, in order to produce the positive electrode active materials of Example 3 to Example 7 and Comparative Example 4 and Comparative Example 5, the weight of the core part precursor particles and the surface layer part precursor particles shown in Table 2, respectively. These were weighed in a ratio and wet mixed, and then the mixture was dried to attach the precursor particles in the surface layer portion to the surface of the precursor particles in the core portion. Subsequently, these were put into a high-purity alumina container and subjected to heat treatment at 770 ° C. for 1 hour under an oxygen stream, and Examples 3 to 7 and Comparative Example 4 consisting of particles having a core part and a surface layer part. And the positive electrode active material of the comparative example 5 was obtained.

  Next, using the positive electrode active materials of Example 1 to Example 7 and Comparative Example 1 to Comparative Example 5, the lithium ion secondary of Examples 1 to 7 and Comparative Example 1 to Comparative Example 5, respectively. A battery was prototyped according to the following procedure.

First, the positive electrode active material of each Example and each comparative example, the electrically conductive agent, and the binder were mixed uniformly, and the positive electrode slurry was produced. Next, the positive electrode slurry was applied onto an aluminum current collector foil having a thickness of 20 μm, dried at 120 ° C., and compression-molded so as to have an electrode density of 2.60 g / cm ³ by pressing to obtain an electrode plate. It was. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm, and positive electrodes of the examples and comparative examples were produced. The negative electrode was produced using metallic lithium, and the lithium ion secondary battery of each example and each comparative example was prototyped using the produced positive electrode and negative electrode of each example, each comparative example, and a non-aqueous electrolyte. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ to a concentration of 1.0 mol / L in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 2 was used.

  Next, a charge / discharge test was performed on the manufactured lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 5, and the discharge capacity characteristics and the charge / discharge cycle characteristics were evaluated. The charge / discharge test was performed at an environmental temperature of 25 ° C.

The discharge capacity characteristics were determined by the following procedure with 1C = 220 Ah / kg based on the weight of the positive electrode active material. The charging and discharging conditions are a constant current and low voltage charge up to an upper limit voltage of 4.6 V at a current equivalent to 0.2 C for charging, and a constant current equivalent to 0.2 C for a discharge after resting for 30 minutes after charging. The discharge was set to a lower limit voltage of 3.3V. This initial charge / discharge was repeated for a total of 3 cycles. The dQ / dV value was calculated from the charge curve of the third cycle at 0.2C. DQ / dV peak value in the range of 4.1V at a voltage of the lithium metal reference to 4.3V (hereinafter referred to as _{dQ / dV 4.1~4.3V),} shown in Tables 3 and 4 below.

  Table 3 shows Example 1 and Example 2 in which the element substitution of the positive electrode active material was performed, Comparative Example 2 and Comparative Example 3, and Comparative Example 1 in which the element substitution of the positive electrode active material was not performed. Results are shown. Table 4 shows Examples 3 to 7, and Comparative Examples 4 and 5 in which the surface coating of the positive electrode active material particles was performed, and Comparative Example 1 in which the surface coating of the positive electrode active material particles was not performed. The result of the lithium ion secondary battery is shown.

  The charge / discharge cycle characteristics of the lithium ion secondary batteries of Examples 1 to 7 and Comparative Examples 1 to 5 were obtained by the following procedure. After the initial charge / discharge, the battery was charged at a constant current and a constant voltage with a current corresponding to 1 C to a maximum voltage of 4.6 V, and after a pause of 10 minutes, the battery was discharged with a constant current equivalent to 1.0 C to a lower limit voltage of 3.3 V. This charge / discharge cycle was repeated 90 times in total. The ratio between the discharge capacity at the 90th cycle of the 90 cycle charge / discharge and the discharge capacity at the first time was calculated as the cycle capacity retention rate, and the charge / discharge cycle characteristics were evaluated. Table 1 and Table 4 show the 1C discharge capacity and the 90th cycle charge / discharge capacity retention ratio in the first cycle of the cycle characteristics evaluation of the secondary batteries of each Example and each Comparative Example.

As shown in Table 1, in the positive electrode active material of Example 1, the whole metal element other than Li is set to 1, and a part of Mn is element-substituted with 0.02 Al. In the positive electrode active material of Example 2, the whole metal element other than Li is set to 1, and a part of Mn is element-substituted with 0.01 Al. As shown in Table 3, in the lithium ion secondary batteries of Example 1 and Example 2, the value of dQ / dV _{4.1 to 4.3} V is suppressed to 300 Ah · (kg · V) ⁻¹ or less. Thus, the layered structure of the positive electrode active material was maintained, and it was possible to obtain good cycle characteristics by setting the capacity maintenance rate to 68.8% or more while increasing the 1C discharge capacity to a high capacity of 174.3 Ah / kg or more.

In contrast, in the lithium ion secondary battery of Comparative Example 1 using the positive electrode active material of Comparative Example 1 in which element substitution was not performed, the value of dQ / dV _{4.1 to 4.3} V was 300 Ah · (kg · V ) Since it exceeded ^-1 , it is considered that the phase change of the positive electrode active material progressed and the capacity retention rate decreased.

Moreover, as shown in Table 1, the positive electrode active materials of Comparative Example 2 and Comparative Example 3 were subjected to element substitution, but the ratio b + c = 1 of the total metal elements other than Li in the composition formula (1) , Mn is partially replaced with 0.1 Al and 0.08 Mg, respectively. That is, in the positive electrode active materials of Comparative Example 2 and Comparative Example 3, the ratio of Al or Mg as the substitution element is the ratio of Al, where the ratio of the whole metal element other than Li in the composition formula (1) is b + c = 1. Is as high as 0.04 or more and the ratio of Mg is 0.02 or more. In this case, as shown in Table 3, in the lithium ion secondary batteries of Comparative Example 2 and Comparative Example 3, the value of dQ / dV _4.1 to 4.3 V was significantly reduced to 190 Ah · (kg · V). ^{By being} less than ⁻¹ , the 1C discharge capacity was significantly reduced, and the cycle capacity retention rate was also significantly reduced.

As shown in Table 2, the positive electrode active materials of Example 3 to Example 7 have particles having a core part and a surface part by surface coating, and the core part has a composition of Li _1.0 Ni _0.8 Co. It is a Ni-rich layered compound represented by _0.1 Mn _0.1 O ₂ , and the composition of the surface layer part is represented by Li ₂ MnO ₃ or Li _1.2 Ni _0.2 Mn _0.6 O _2.0. The The weight of the core part per 100 g of the positive electrode active material is in the range of 85 g to 97.5 g, and the weight of the surface layer part per 100 g of the positive electrode active material is in the range of 2.5 g to 15 g.

As shown in Table 4, in the lithium ion secondary batteries of Examples 3 to 7 using the positive electrode active materials of Examples 3 to 7, the values of dQ / dV _{4.1 to 4.3 V} were , 300 Ah · (kg · V) ⁻¹ or less, the layered structure of the positive electrode active material is maintained, and the capacity retention rate is 63.2 while the 1C discharge capacity is set to a high capacity of 170.1 Ah / kg or more. As a result, good cycle characteristics could be obtained.

On the other hand, in the lithium ion secondary battery of Comparative Example 1 using the positive electrode active material of Comparative Example 1 without surface coating, the value of dQ / dV _{4.1 to 4.3} V is 300 Ah · (kg · V ) Since it exceeded ^-1 , it is considered that the phase change of the positive electrode active material progressed and the capacity retention rate decreased.

In addition, as shown in Table 2, the positive electrode active materials of Comparative Example 4 and Comparative Example 5 have a core portion and a surface layer portion by performing surface coating, but the weight ratio of the surface layer portion is excessive. is there. Therefore, as shown in Table 4, in the lithium ion secondary batteries of Comparative Example 4 and Comparative Example 5 using the positive electrode active materials of Comparative Example 4 and Comparative Example 5, the cycle capacity retention rate was relatively good. However, the value of dQ / dV _4.1 to 4.3 V was significantly reduced to be less than 190 Ah · (kg · V) ⁻¹ , thereby greatly reducing the 1C discharge capacity.

  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.

1A, 1B Positive electrode active material (particles), 11B Core portion, 12 Surface layer portion, 100 Lithium ion secondary battery, 111 Positive electrode, 111a Positive electrode current collector, 111b Positive electrode mixture layer

Claims (10)

It has a layered structure and has a composition formula: Li _{1 + a} Ni _b M _c O _{2 + α} (where M is Mn, Co, Mg, Al, Ti, Zr, Mo, Nb, Fe, Sn, V, Zn, W, Na And at least one element selected from the group consisting of B, a, b, c, and α are −0.1 ≦ a ≦ 0.3, b + c = 1, b / ( b + c)> 0.7 and −0.1 ≦ α ≦ 0.1)), a positive electrode active material for a lithium ion secondary battery,
The voltage with respect to the lithium metal of the positive electrode when charging the lithium ion secondary battery including the positive electrode containing the positive electrode active material with a current of 44 Ah / kg based on the weight of the positive electrode active material is V, The peak value of dQ / dV of the charging curve in a graph in which the charging capacity of the lithium ion secondary battery is Q, the horizontal axis is V, and the vertical axis is dQ / dV is 190 Ah · (kg · V) ⁻¹ or more and 300 Ah · (Kg · V) ⁻¹ or less A positive electrode active material for a lithium ion secondary battery.   The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein M in the composition formula includes Al.   3. The positive electrode active material for a lithium ion secondary battery according to claim 2, wherein M in the composition formula includes Al at a ratio of less than 0.04 where b + c = 1. The positive electrode active material particles have a surface layer portion and a core portion,
4. The positive electrode active material for a lithium ion secondary battery according to claim 3, wherein the molar mass concentration of Al in the surface layer portion is higher than the molar molar concentration of Al in the core portion.   The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein M in the composition formula contains Mg.   The positive electrode active material for a lithium ion secondary battery according to claim 5, wherein M in the composition formula includes Mg at a ratio of less than 0.02 where b + c = 1. The positive electrode active material particles have a surface layer portion and a core portion,
The positive electrode active material for a lithium ion secondary battery according to claim 6, wherein the molar mass concentration of Mg in the surface layer portion is higher than the molar molar concentration of Mg in the core portion. The positive electrode active material particles are primary particles in which individual particles are separated or secondary particles in which a plurality of particles are bonded,
The primary particles or the secondary particles have a surface layer portion and a core portion,
2. The positive electrode for a lithium ion secondary battery according to claim 1, wherein a molar ratio Li / Mn of Li and Mn in the surface layer portion is lower than a molar ratio Li / Mn of Li and Mn in the core portion. Active material. A positive electrode for a lithium ion secondary battery comprising a positive electrode current collector and a positive electrode mixture layer formed on a surface of the positive electrode current collector,
The said positive mix layer contains the positive electrode active material as described in any one of Claim 1-8, The positive electrode for lithium ion secondary batteries characterized by the above-mentioned.   A lithium ion secondary battery comprising the positive electrode according to claim 9.

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