JP2016033848A - Positive electrode active material for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery arranged by use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery high in capacity and superior in charge and discharge cycle characteristics.SOLUTION: A positive electrode active material is in the form of particles and to be used for a nonaqueous electrolyte secondary battery. The positive electrode active material comprises: a LiMnO-LiMOsolid solution (M represents at least one selected from Co, Ni, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr, La, Ce, Sm and Mo); and a Ni-containing oxide. The ratio of Ni to Mn (Ni/Mn) on the particle surface is higher than that in the particle.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the same.

In Patent Document 1, Li _a Mn _x Ni _y Co _z O ₂ (0 <a ≦ 1.2, 0 ≦ x ≦ 1, 0 ≦ y ≦) for the purpose of increasing the capacity and reducing the resistance of a lithium secondary battery. The secondary particles in which the primary particles of the layered composite oxide represented by 1,0 ≦ z ≦ 1, x + y + z = 1) are aggregated, and the Co content in the outer peripheral portion of the secondary particles is defined as the internal Co content. A higher positive electrode active material is disclosed.

  Patent Documents 2 and 3 disclose a positive electrode active material that is provided on at least a part of the composite oxide particles and includes a coating layer made of an oxide containing at least Li and Ni, and a surface layer containing a specific metal element. Has been.

JP 2007-317585 A JP 2007-242284 A JP 2007-242318 A

By the way, when Li ₂ MnO ₃ —LiMO ₂ solid solution is used as the positive electrode active material, an improvement in energy density is expected. However, when the positive electrode is exposed to a high potential during charging, the oxidation of the electrolyte on the active material surface Reduction of metal ions (especially Mn ions) is likely to occur. For this reason, the valence of Mn ions in the Li ₂ MnO ₃ domain is lowered, and disorder occurs due to the movement of Mn ions to the Li ion site, which causes deterioration of battery performance.

  In addition, as a countermeasure for such battery performance deterioration, formation of a surface protective layer with various oxides, fluorides, phosphides, etc. has been proposed, but these protective layers do not have a Li ion storage function, This contributes to a decrease in energy density and an increase in surface resistance.

The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention is a Li ₂ MnO ₃ —LiMO ₂ solid solution (M is Ni, Co, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr). , La, Ce, Sm, Mo) and a Ni-containing oxide, and the ratio of Ni to Mn (Ni / Mn) on the particle surface is higher than Ni / Mn inside the particle It is characterized by that.

  A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode including the positive electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

  The positive electrode active material according to the present invention can provide a non-aqueous electrolyte secondary battery having a high capacity and excellent charge / discharge cycle characteristics.

It is a figure which shows the positive electrode active material particle which is an example of embodiment of this invention. It is a figure which shows the positive electrode active material particle which is an example of embodiment of this invention. It is a figure which shows the positive electrode active material particle which is an example of embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail.
The drawings referred to in the description of the embodiments are schematically described, and the dimensional ratios of the components drawn in the drawings may be different from the actual products. Specific dimensional ratios and the like should be determined in consideration of the following description.

  A nonaqueous electrolyte secondary battery which is an example of an embodiment of the present invention includes a positive electrode including a particulate positive electrode active material (hereinafter referred to as “positive electrode active material particles”), a negative electrode, and a nonaqueous electrolyte including a nonaqueous solvent. With. A separator is preferably provided between the positive electrode and the negative electrode. The nonaqueous electrolyte secondary battery has, for example, a structure in which an electrode body in which a positive electrode and a negative electrode are wound via a separator and a nonaqueous electrolyte are accommodated in an exterior body.

[Positive electrode]
The positive electrode includes, for example, a positive electrode current collector such as a metal foil and a positive electrode active material layer formed on the positive electrode current collector. As the positive electrode current collector, it is preferable to use a conductive thin film, particularly a metal foil or alloy foil that is stable in the potential range of the positive electrode such as aluminum, a film having a metal surface layer such as aluminum, and the like. The positive electrode active material layer preferably contains a conductive material and a binder in addition to the positive electrode active material particles.

The positive electrode active material particles are Li ₂ MnO ₃ —LiMO ₂ solid solution (M is Ni, Co, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr, La, Ce, Sm, Mo. At least one selected) and a Ni-containing oxide. M preferably contains at least one of Ni and Co, and particularly preferably Ni and Co, or only Ni. The Ni-containing oxide preferably contains Li in addition to Ni, and more preferably contains at least one of Co and Mn in addition to Ni and Li.

Furthermore, the Li ₂ MnO ₃ —LiMO ₂ solid solution has a composition formula of Li _α Mn _x Ni _y M ^* _(1-xy) O _β (1.1 <α <1.5, 0.4 ≦ x ≦ 1.0 , 0 <y ≦ 0.6, 1.9 ≦ β ≦ 2.0, M ^* is Co, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr, La, Ce, Sm, Those represented by at least one selected from Mo) are preferred.

The crystal structure (main structure) of the Li ₂ MnO ₃ —LiMO ₂ solid solution is a hexagonal system (space group R-3m). The Li ₂ MnO ₃ —LiMO ₂ solid solution is regularly arranged so that not only the Mn ions in the Mn—Li layer coexist with the Li ions but also form a hexagonal network structure. Due to the hexagonal network regular arrangement of Mn ions and Li ions, a peak derived from the superlattice structure is observed around 2θ = 20-25 ° in the powder X-ray diffraction pattern of the Li ₂ MnO ₃ —LiMO ₂ solid solution. Is done. In other words, a compound having a crystal structure of hexagonal system (space group R-3m) and a peak derived from the superlattice structure in the vicinity of 2θ = 20 to 25 ° of the powder X-ray diffraction pattern is Li ₂ MnO. _3- LiMO ₂ solid solution.

The positive electrode active material may contain a metal oxide other than the Li ₂ MnO ₃ —LiMO ₂ solid solution in the form of a mixture or a solid solution as long as the object of the present invention is not impaired. However, the Li ₂ MnO ₃ —LiMO ₂ solid solution in the positive electrode active material preferably exceeds 50% by volume, more preferably 70% by volume or more, and 90% by volume with respect to the total volume of the compounds constituting the positive electrode active material. The above is particularly preferable.

  The volume average particle diameter of the positive electrode active material particles is, for example, 1 to 30 μm, preferably 2 to 15 μm, more preferably 3 to 10 μm. Here, the volume average particle diameter (hereinafter referred to as “Dv50”) means a particle diameter at which the volume integrated value is 50% in the particle diameter distribution. The Dv50 of the positive electrode active material particles can be measured using a laser diffraction scattering measurement apparatus (for example, “LA-750” manufactured by HORIBA) using water as a dispersion medium.

  The positive electrode active material particles are characterized in that the ratio of Ni to Mn (Ni / Mn) on the particle surface is higher than Ni / Mn inside the particles. Thereby, even when the positive electrode is exposed to a high potential during charging, it is possible to suppress oxidation of the electrolytic solution and reduction of metal ions, particularly Mn ions, on the active material surface. Ni / Mn on the particle surface is preferably 2 times or more of Ni / Mn inside the particle, more preferably 3 times or more, particularly preferably 5 times or more, and most preferably 9 times or more. The positive electrode active material particles may have no Mn on the particle surface or Ni inside the particles, and the upper limit of the magnification is not particularly limited in terms of improving cycle characteristics.

  In FIG. 1, the cross section of the positive electrode active material particle which is an example of embodiment is shown. In FIG. 1, for convenience of explanation, a specific form of the positive electrode active material particles is not illustrated, and is extremely simplified.

The positive electrode active material particles include a base material particle 1 composed of the Li ₂ MnO ₃ —LiMO ₂ solid solution and a coating layer 2 composed of the Ni-containing oxide formed on the surface of the base material particle 1. It is suitable to have. That is, the base material particle 1 constitutes the inside of the positive electrode active material particle, and the coating layer 2 constitutes the particle surface of the positive electrode active material particle. And Ni / Mn of the coating layer 2 is higher than Ni / Mn of the base material particle 1. Specifically, Ni / Mn of the coating layer 2 is preferably 2 times or more of Ni / Mn of the base material particle 1, more preferably 3 times or more, particularly preferably 5 times or more, and more than 9 times. Most preferred.

  The maximum thickness of the coating layer 2 is preferably 10 to 500 nm, more preferably 50 to 400 nm, and particularly preferably 80 to 300 nm. The covering layer 2 is preferably formed so as to cover substantially the entire surface of the base material particle 1, and the thickness thereof is preferably substantially equal over the entire region of the layer. That is, the average thickness of the coating layer 2 is preferably 10 to 500 nm, more preferably 50 to 400 nm, and particularly preferably 80 to 300 nm. The thickness of the coating layer 2 can be measured by cross-sectional observation using a transmission electron microscope (TEM) or a scanning electron microscope (SEM). The average thickness is an average value of thicknesses measured at arbitrary 10 points.

  In addition, Dv50 of the base material particle | grains 1 is 1-30 micrometers, for example, Preferably it is 2-15 micrometers, More preferably, it is 3-10 micrometers. When obtaining the Dv50 of the base material particle 1 from the positive electrode active material particles, it can be obtained by measuring the Dv50 of the positive electrode active material particles and subtracting the average thickness of the coating layer 2 from the Dv50.

  FIG. 2 shows a state of the particle surface and the inside of the particle by cutting away a part of the surface of the positive electrode active material particle as an example of the embodiment.

In the positive electrode active material particles shown in FIG. 2, a large number of primary particles 3 are aggregated to form base material particles 1. The primary particles 3 are preferably composed of the Li ₂ MnO ₃ —LiMO ₂ solid solution. The base material particle 1 is formed by, for example, primary particles 3 obtained by mixing and firing the raw materials of the Li ₂ MnO ₃ —LiMO ₂ solid solution fixed to each other.

  In the positive electrode active material particles shown in FIG. 2, the coating layer 2 is formed by the coating particles 4 that are smaller than the base material particles 1. The coated particle 4 is preferably smaller than the primary particle 3 constituting the base material particle 1, and before adhering to the base material particle 1, the Dv50 is 10 to 200 nm, preferably 10 to 150 nm, more preferably. 10-100 nm.

The coated particle 4 is composed of a lithium-containing metal oxide containing Ni. The composition of the oxide is not particularly limited as long as the Ni / Mn is higher than the Ni / Mn of the base material particle 1, but contains at least one of Co and Mn in addition to Li and Ni. Is preferred. The coated particle 4 is made of, for example, LiNi _x M2 _1-x O ₂ (M2 contains at least one of Co and Mn) or the above Li ₂ MnO ₃ —LiMO ₂ solid solution having a higher Ni content than the base material particle 1. Composed.

  The positive electrode active material particles shown in FIG. 2 can be produced, for example, by fixing the covering particles 4 to the surfaces of the base material particles 1. The covering particles 4 cover substantially the entire surface of the base particle 1 and a plurality of the covering particles 4 are laminated to form the covering layer 2 having a thickness of about 80 to 300 nm.

  As a method of fixing the covering particles 4 to the surface of the base material particles 1, a method of dry mixing the base material particles 1 and the covering particles 4 is suitable. Such dry mixing can be performed using a commercially available dry mixing apparatus. The temperature at the time of mixing is not specifically limited, For example, it sets to room temperature (25 degreeC). Examples of suitable dry mixing devices include “Planet Ball Mill” manufactured by Retsch, “Hybridization System” manufactured by Nara Machinery Co., Ltd., “Nanocula”, “Nobilta”, “Mechanofusion” manufactured by Hosokawa Micron. It is done. The coated particles 4 can be fixed to the surface of the base material particles 1 by wet mixing using water, ethanol or the like as a solvent.

  By the dry mixing of the base material particle 1 and the covering particle 4, the shape of the covering particle 4 is not substantially left, and the dense covering layer 2 can be formed on the surface of the base material particle 1. For example, by applying large mechanical energy using the coated particle 4 having a small particle diameter, the base material particle 1 and the coated particle 4 and the coated particles 4 are firmly bonded to each other, and the particle interface of the coated particle 4 remains. A dense coating layer 2 can be formed.

  FIG. 3 shows a part of the positive electrode active material particles as an example of the embodiment as a cross-sectional view. The cross section is marked with dot hatching, and the higher the dot density, the higher the Ni content.

The positive electrode active material particles shown in FIG. 3 are particles in which Ni / Mn increases stepwise as the particle surface approaches the particle surface. In the example shown in FIG. 3, Ni / Mn is the lowest layer L ₃ is formed in the center of the particle, the layer L ₂ is greater Ni / Mn than layer L ₃ in the outer layer L _3, and the outermost surface of the particles A layer L ₁ having the highest Ni / Mn is formed. Such positive electrode active material particles can be prepared, for example, by preparing a plurality of compound particles having different Ni contents and sequentially dry-mixing them. The surface of the positive electrode active material particles is covered with fine particles of an oxide such as aluminum oxide (Al ₂ O ₃ ), an inorganic compound such as a phosphoric acid compound, and a boric acid compound as long as the object of the present invention is not impaired. Also good.

  The conductive material is used to increase the electrical conductivity of the positive electrode active material layer. Examples of the conductive material include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. These may be used alone or in combination of two or more. The content of the conductive material is preferably 0% by mass to 30% by mass with respect to the total mass of the positive electrode active material layer, more preferably 0% by mass to 20% by mass, and particularly preferably 0% by mass to 10% by mass. preferable.

  The binder is used to maintain a good contact state between the positive electrode active material particles and the conductive material and to increase the binding properties of the positive electrode active material particles and the like to the surface of the positive electrode current collector. As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride, polyvinyl acetate, polymethacrylate, polyacrylate, polyacrylonitrile, polyvinyl alcohol, or a mixture of two or more thereof are used. The binder may be used in combination with a thickener such as carboxymethyl cellulose (CMC) or polyethylene oxide. The content of the binder is preferably 0% by mass to 30% by mass, more preferably 0% by mass to 20% by mass, and more preferably 0% by mass to 10% by mass with respect to the total mass of the positive electrode active material layer. Particularly preferred.

The positive electrode potential in a fully charged state of the positive electrode having the above structure can be set to a high potential of 4.0 V (vs. Li ^/ Li ⁺ ) or higher. The charge termination potential of the positive electrode is preferably 4.5 V (vs. Li ^/ Li ⁺ ) or more, more preferably 4.6 V (vs. Li ^/ Li ⁺ ) or more, from the viewpoint of increasing the capacity. The upper limit of the charge termination potential of the positive electrode is not particularly limited, but is preferably 5.0 V (vs. Li ^/ Li ⁺ ) or less from the viewpoint of suppressing decomposition of the nonaqueous electrolyte.

[Negative electrode]
The negative electrode includes, for example, a negative electrode current collector such as a metal foil, and a negative electrode active material layer formed on the negative electrode current collector. As the negative electrode current collector, it is preferable to use a conductive thin film, particularly a metal foil or alloy foil that is stable in the potential range of the negative electrode such as copper, a film having a metal surface layer such as copper, or the like. The negative electrode active material layer preferably contains a binder in addition to the negative electrode active material capable of inserting and extracting lithium ions. As the binder, PTFE or the like can be used as in the case of the positive electrode, but it is preferable to use a styrene-butadiene copolymer or a modified body thereof. The binder may be used in combination with a thickener such as CMC.

  Examples of the negative electrode active material include natural graphite, artificial graphite, lithium, silicon, carbon, tin, germanium, aluminum, lead, indium, gallium, lithium alloy, carbon and silicon in which lithium is previously occluded, and alloys and mixtures thereof. Etc. can be used.

[Non-aqueous electrolyte]
The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The nonaqueous electrolyte is not limited to a liquid electrolyte (nonaqueous electrolyte solution), and may be a solid electrolyte using a gel polymer or the like. As the non-aqueous solvent, for example, esters, ethers, nitriles, amides, a mixed solvent of two or more thereof, and the like can be used. As the non-aqueous solvent, halogen-substituted products obtained by substituting hydrogen in various solvents with halogen atoms such as fluorine may be used. For example, a fluorinated cyclic carbonate, a fluorinated chain carbonate, or a mixed solvent thereof can be used.

  Examples of the esters include cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, Examples thereof include carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone.

  Examples of the ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4 -Cyclic ethers such as dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether , Dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxy toluene, benzyl ethyl ether, diphenyl ether, dibenzyl Ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, tri Examples thereof include chain ethers such as ethylene glycol dimethyl ether and tetraethylene glycol dimethyl.

  Examples of the nitriles include acetonitrile, and examples of the amides include dimethylformamide.

The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , LiN (C ₁ F _{2l + 1} SO ₂ ) (C _m F _{2m + 1} SO ₂₎ (l, m is an integer of 1 or _{more), LiC (C P F 2p} + 1 SO 2) (C q F 2q + 1 SO 2) (C r F 2r + 1 SO 2) (p, q, r Is an integer of 1 or more), Li [B (C ₂ O ₄ ) ₂ ] (bis (oxalate) lithium borate (LiBOB)), Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ], and a mixture of two or more thereof.

[Separator]
As the separator, a porous sheet having ion permeability and insulating properties is used. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. As the material of the separator, polyolefin such as polyethylene and polypropylene is suitable.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited to these Examples.

<Example 1>
[Preparation of base material particle A1]
Nickel sulfate (NiSO ₄ ) and manganese sulfate (MnSO ₄ ) are mixed in an aqueous solution so as to have a molar weight ratio of 1: 3, and coprecipitated to obtain (Ni, Mn) (OH) ₂ as a precursor substance. Obtained. Thereafter, the precursor material and lithium hydroxide monohydrate (LiOH.H ₂ O) were mixed at a molar weight ratio of 1: 1.5, and the mixture was calcined at 850 ° C. for 12 hours. Base material particles A1 of the positive electrode active material were obtained.

The crystal structure of the lithium-containing transition metal oxide constituting the base particles A1, the powder X-ray diffraction method (manufactured by Rigaku Corporation, using a powder XRD measuring apparatus RINT2200 (source Cu-K _alpha). Forth.) Analysis did. As a result, the crystal structure (main structure) of the lithium-containing transition metal oxide composing the base material particle A1 was a hexagonal crystal (space group R-3m). In addition, a peak derived from the superlattice structure was observed in the vicinity of 2θ = 20 to 25 °.

The composition of the lithium-containing transition metal oxide constituting the base material particle A1 was analyzed by ICP emission analysis (Thermo Fisher Scientific, ICP emission spectroscopic analyzer iCAP6300 was used. The same applies hereinafter). As a result, the composition of the lithium-containing transition metal oxide constituting the base material particle A1 was Li _1.2 Mn _0.6 Ni _0.2 O ₂ .

  The base material particle A1 was a spherical secondary particle formed by adhering a large number of primary particles to each other, and Dv50 was about 7 μm. Dv50 was measured using a laser diffraction / scattering type measurement apparatus (manufactured by HORIBA, LA-750).

[Preparation of coated particle B1]
NiSO ₄ and MnSO ₄ were mixed in an aqueous solution so as to have a molar weight ratio of 1: 1 and coprecipitated, and (Ni, Mn) (OH) ₂ and LiOH · H ₂ O had a molar weight ratio of 1 : 1 to mix. The mixture was fired at 850 ° C. for 12 hours, and then crushed using a planetary ball mill (manufactured by Retsch, PM400) until Dv50 was 100 nm or less, whereby the coated particles B1 constituting the coating layer of the positive electrode active material were obtained. Obtained.

The composition of the lithium-containing transition metal oxide constituting the coated particle B1 was analyzed by ICP emission analysis. As a result, the composition of the lithium-containing transition metal oxide constituting the coated particle B1 was LiMn _0.5 Ni _0.5 O ₂ .

[Preparation of Positive Electrode Active Material Particle C1]
The base material particle A1 and the covering particle B1 were dry mixed to produce the positive electrode active material particle C1. Details of the dry mixing step are as follows.
Apparatus: Rolling ball mill (manufactured by Retsch)
Temperature condition: 25 ° C
Mixing time: 12 hours

As a result of surface observation by SEM, the surface of the positive electrode active material particle C1 was covered with a coating layer composed of a large number of coating particles B1, and the exposure of the surface of the base material particle A1 could not be confirmed. As a result of cross-sectional observation of the positive electrode active material particles C1 by TEM, the average thickness of the coating layer was about 150 nm. The average thickness was an average value of thicknesses measured at arbitrary 10 points.
Ni / Mn of the particle surface (base material particle A1) of the positive electrode active material particle C1 and the inside of the particle (coating particle B1) is as follows.
Particle surface Ni / Mn = 1.00
Ni / Mn inside the particle = 0.33
(Ni / Mn on particle surface) / (Ni / Mn inside particle) = about 3

[Production of Test Cell D1]
The test cell D1 shown in FIG. 3 was produced by the following procedure.
First, using the positive electrode active material particles C1 as the positive electrode active material, acetylene black as the conductive material, and polyvinylidene fluoride as the binder, the mass ratio of the positive electrode active material, the conductive material, and the binder is 80:10:10. And slurried with N-methyl-2-pyrrolidone. Next, this slurry was applied on an aluminum foil current collector as a positive electrode current collector, and vacuum dried at 110 ° C. to produce a working electrode (positive electrode).

A test cell D1 which is a non-aqueous electrolyte secondary battery is produced using the working electrode, the counter electrode (negative electrode), the separator, the non-aqueous electrolyte, and an exterior body that accommodates them under dry air with a dew point of −50 ° C. or less. did. Details of each component are as follows.
Counter electrode; Lithium metal separator; Polyethylene separator Nonaqueous electrolyte; Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 1: 1 to obtain a nonaqueous solvent. A nonaqueous electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte salt in the nonaqueous solvent so as to have a concentration of 1.0 mol / l.

<Example 2>
In the same manner as in Example 1, except that the coated particle B2 having a composition of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (Ni / Mn = 1.67) was used instead of the coated particle B1, positive electrode active material particles C2 and Test cell D2 was produced.

<Example 3>
In the same manner as in Example 1, except that the coated particle B3 belonging to the space group Fd-3m and having the composition LiNi _1.5 Mn _0.5 O ₂ (Ni / Mn = 3) was used instead of the coated particle B1, the positive electrode active Material particles C3 and test cell D3 were prepared.

<Example 4>
Cathode active material particles C4 and test cells D4 were prepared in the same manner as in Example 1 except that coated particles B4 having a composition of LiNi _0.8 Co _0.2 O ₂ (not containing Mn) were used instead of the coated particles B1. Produced.

<Comparative Example 1>
A positive electrode active material particle Y1 (= base material particle A1) and a test cell Z1 were produced in the same manner as in Example 1 except that the coating layer was not formed.

<Comparative Example 2>
In the same manner as in Example 1, except that the coated particle X2 having a composition of LiNi _0.1 Co _0.3 Mn _0.6 O ₂ (Ni / Mn = 0.167) was used instead of the coated particle B1, positive electrode active material particles Y2 and Test cell Z2 was produced.

[Evaluation of cycle characteristics]
The prepared nonaqueous electrolyte secondary battery was charged (0.2 C) until the battery voltage reached 4.7 V, and then discharged (0.2 C) until the battery voltage reached 2.0 V, thereby charging the battery. The discharge capacity (mAh) was measured. The battery voltage 4.7V corresponds to a positive electrode potential 4.6V (vs. Li ^/ Li ⁺ ). Thereafter, the above charge / discharge was repeated, and as the cycle characteristics for repeating charge / discharge, the capacity retention rate was evaluated by multiplying 100 by the value obtained by dividing the discharge capacity after 15 cycles by the discharge capacity at the first cycle.

  Table 1 shows capacity retention rates after 15 cycles for test cells D1 to D4 of Examples 1 to 4 and test cells Z1 and Z2 of Comparative Examples 1 and 2.

※母材粒子の組成；（１）Ｌｉ_1.2Ｍｎ_0.6Ｎｉ_0.2Ｏ₂
※被覆粒子の組成；
（２）ＬｉＭｎ_0.5Ｎｉ_0.5Ｏ₂
（３）ＬｉＮｉ_0.5Ｃｏ_0.2Ｍｎ_0.3Ｏ₂
（４）ＬｉＮｉ_1.5Ｍｎ_0.5Ｏ₂
（５）ＬｉＮｉ_0.8Ｃｏ_0.2Ｏ₂
（６）ＬｉＮｉ_0.1Ｃｏ_0.3Ｍｎ_0.6Ｏ₂

* Base material particle composition; (1) Li _1.2 Mn _0.6 Ni _0.2 O ₂
* Composition of coated particles;
(2) LiMn _0.5 Ni _0.5 O ₂
(3) LiNi _0.5 Co _0.2 Mn _0.3 O ₂
(4) LiNi _1.5 Mn _0.5 O ₂
(5) LiNi _0.8 Co _0.2 O ₂
(6) LiNi _0.1 Co _0.3 Mn _0.6 O ₂

From Table 1, it can be seen that the test cells D1 to D4 of the examples have higher capacity retention ratios after 15 cycles and excellent cycle characteristics than the test cells Z1 and Z2 of the comparative examples. In the non-aqueous electrolyte secondary battery using the Li ₂ MnO ₃ —LiMO ₂ solid solution positive electrode active material, the test cells D1 to D4 of the examples can be expected to have a high capacity but are inferior in cycle characteristics. This is a significant improvement in cycle characteristics, which was a problem.

In the positive electrode active material particles C1 to C4 used in the test cells D1 to D4 of the examples, Ni / Mn on the particle surface is made higher than Ni / Mn inside the particle, specifically, Ni on the surface of the positive electrode active material particle By forming a rich coating layer, it succeeded in suppressing the reduction of Mn in the Li ₂ MnO ₃ —LiMO ₂ solid solution. Thereby, it is considered that good cycle characteristics were obtained even when the positive electrode was exposed to a high potential. The Ni-rich coating layer has a Li ion occlusion function, and thus does not cause adverse effects such as a decrease in energy density and an increase in surface resistance.

  The remarkable effect of this example can be obtained only when Ni / Mn on the particle surface is made higher than Ni / Mn inside the particle, and even when Ni is present on the particle surface, it is obtained when the condition is not satisfied. (Comparative Example 2).

  In addition, cycling characteristics improve, so that the magnification of Ni / Mn in the particle | grain surface with respect to Ni / Mn inside a particle | grain becomes high. From the viewpoint of improving cycle characteristics, the magnification is preferably 5 times (Example 2) rather than 3 times (Example 1) and 9 times or more (Examples 3 and 4) than 5 times.

  1 base material particle, 2 coating layer, 3 primary particle, 4 coating particle.

Claims (7)

A particulate positive electrode active material used for a non-aqueous electrolyte secondary battery,
Li ₂ MnO ₃ —LiMO ₂ solid solution (M is at least one selected from Ni, Co, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr, La, Ce, Sm, Mo) When,
A Ni-containing oxide;
Including
A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the ratio of Ni to Mn (Ni / Mn) on the particle surface is higher than that of Ni / Mn inside the particle. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1,
The positive electrode active material for a non-aqueous electrolyte secondary battery, wherein Ni / Mn on the particle surface is twice or more Ni / Mn inside the particle. In the positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 or 2,
Base material particles composed of the Li ₂ MnO ₃ —LiMO ₂ solid solution,
A coating layer formed of the Ni-containing oxide formed on the surface of the base material particles;
Have
A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein Ni / Mn of the coating layer is higher than Ni / Mn of the base material particles. In the positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 1 or 3,
The Li ₂ MnO ₃ —LiM ₁ O ₂ solid solution is Li _α Mn _x Ni _y M ^* _(1-xy) O _β (1.1 <α <1.5, 0.4 ≦ x ≦ 1.0, 0 <Y ≦ 0.6, 1.9 ≦ β ≦ 2.0, M ^* is from Co, Fe, Al, Mg, Ti, Cr, Zr, W, B, Nb, Sr, La, Ce, Sm, Mo A positive electrode active material for a non-aqueous electrolyte secondary battery represented by at least one selected. In the positive electrode active material for nonaqueous electrolyte secondary batteries according to claim 3 or 4,
The coating layer is a positive electrode active material for a non-aqueous electrolyte secondary battery formed of particles smaller than the base material particles.   A nonaqueous electrolyte secondary battery comprising: a positive electrode including the positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1; a negative electrode; and a nonaqueous electrolyte. The nonaqueous electrolyte secondary battery according to claim 6,
The non-aqueous electrolyte secondary battery in which a charge end potential of the positive electrode is 4.5 V or more and 5.0 V or less (vs. Li ^/ Li ⁺ ).

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