JP2013091581A - Lithium composite oxide, manufacturing method for the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium-rich type lithium composite oxide having a layered rock salt type structure capable of improving capacity retention when charging and discharging are repeated at a high potential of 4.5 V or higher, and its manufacturing method.SOLUTION: The lithium composite oxide includes a layered rock salt type structure expressed by the following formula, the lattice distortion obtained by the Halder Wafner method is 0.4% or less, and the crystallite size is 30 nm or less. The formula is LiMO(In the formula, M is at least one kind of a transition metal whose average valence is 4+, and includes at least one kind selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu, and 1.2≤x/y<2.0).

Description

  The present invention relates to a lithium composite oxide, a method for producing the same, and a lithium ion secondary battery.

  A lithium ion secondary battery is generally composed of a positive electrode and a negative electrode capable of reversibly inserting and removing lithium ions, and a liquid, gel or solid electrolyte, and has advantages such as high output and high energy density. Yes.

The positive electrode active material of the lithium ion secondary battery is represented by the formula LiMO ₂ (wherein M is at least one transition metal having an average valence of 4+) such as LiCoO ₂ , LiNiO ₂ , or a substitution system thereof. Layered rock salt type lithium composite oxides are widely used.

In a lithium composite oxide having a layered rock salt structure, the formula Li _x M _y O ₂ (wherein M is at least one transition metal having an average valence of 4+, 1.2 ≦ x / y <2.0 ) (See Examples 1 to 4 of Patent Document 1).
With a lithium-excess material, a high capacity and high energy can be obtained at a high potential charge of 4.5 V or more as compared with a lithium non-excess material.

JP 2007-220475 A JP 2004-253169 A

Atsushi Ito etc., Journal of Power Sources 195 (2010) 567-573. Marcel Poubaix, 1974, National Association of Corrosion Engineers, "Atlas of electrochemical equilibria in aqueous solutions"

With a lithium-rich lithium composite oxide having a layered rock salt structure, it is difficult to obtain a sufficient capacity unless charged to 4.5 V or higher. And, in the lithium-rich lithium composite oxide having a layered rock salt structure, the expansion and contraction of the crystal lattice is large when the charge and discharge of 4.5 V or more are repeated, and the particles are cracked and deteriorated. The capacity maintenance rate is likely to decrease.
That is, in the lithium-rich lithium composite oxide having a layered rock salt type structure, the capacity retention rate when repeating high-capacity and 4.5V or higher charge and discharge is a contradictory characteristic, and both are compatible. It ’s difficult. Refer to Non-Patent Document 1 for this problem.

As a related technique of the present invention, there is Patent Document 2.
Patent Document 2 includes a lithium composite oxide represented by the following formula, and (V + B) / M) = 0.001 to 0.00 with respect to the transition metal element (M) constituting the lithium composite oxide. A positive electrode active material containing 05 (molar ratio) vanadium (V) and / or boron (B), having a primary particle diameter of 1 μm or more, a crystallite size of 450 mm or more, and a lattice strain of 0.05% or less. (Claim 1).
Li _X M _Y O _Z-δ (wherein M represents Co or Ni, (X / Y) = 0.98 to 1.02, (δ / Z) ≦ 0.03)

  Patent Document 2 describes that the charge / discharge cycle characteristics are improved by using the positive electrode active material (paragraph 0075).

Patent Document 2 relates to a Li-excess system, and does not relate to a Li-excess system targeted by the present invention.
In the Li non-excess system in Patent Document 2, the problem of particle cracking when repeatedly charging and discharging at a high potential of 4.5 V or higher, and the decrease in capacity maintenance ratio due thereto is not so great.

  Therefore, the prescription in Patent Document 2 of Li-non-excess system in which the deterioration of the positive electrode active material is not so great when charging and discharging at a high potential of 4.5 V or more, which is the subject of the present invention, is 4.5 V or more. Even if it is applied as it is to a Li-excess system in which the positive electrode active material is likely to deteriorate when it is repeatedly charged and discharged at a high potential, cycle charge / discharge characteristics cannot be improved satisfactorily.

The present invention has been made in view of the above circumstances, and lithium-rich lithium having a layered rock-salt structure capable of improving the capacity retention rate when charging and discharging are repeated at a high potential of 4.5 V or higher. An object of the present invention is to provide a composite oxide and a method for producing the same.
In this specification, high potential is defined as “4.5 V or more”.

The lithium composite oxide of the present invention is
A lithium composite oxide having a layered rock salt structure represented by the following formula,
It is a lithium composite oxide having a lattice strain of 0.4% or less and a crystallite size of 30 nm or less determined by the Halder Wafner method.
General formula: Li _x M _y O ₂
(In the formula, M is at least one transition metal having an average valence of 4+, and includes at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. .
1.2 ≦ x / y <2.0)

In this specification, “lattice strain” is “lattice strain obtained by the Halder Wafner method” unless otherwise specified, and is measured using analysis software PDXL manufactured by Rigaku Corporation in powder XRD measurement. .
In this specification, “crystallite size” is measured using analysis software PDXL manufactured by Rigaku Corporation in powder XRD measurement.

  The lithium composite oxide of the present invention is suitable for a positive electrode active material used for a lithium ion secondary battery.

  The lithium ion secondary battery of the present invention uses the above-described lithium composite oxide of the present invention as a positive electrode active material.

  According to the present invention, there is provided a lithium-rich lithium composite oxide having a layered rock salt structure capable of improving the capacity retention rate when charging and discharging are repeated at a high potential of 4.5 V or higher and a method for producing the same. Can be provided.

It is a XAFS pattern of the positive electrode active material obtained in Examples 1-2 and Comparative Examples 1-2. It is a TEM image of the positive electrode active material of Example 1 after cycle charging / discharging. It is a TEM image of the positive electrode active material of the comparative example 2 after cycle charging / discharging.

  Hereinafter, the present invention will be described in detail.

[Lithium composite oxide]
The lithium composite oxide of the present invention is
A lithium composite oxide having a layered rock salt structure represented by the following formula,
It is a lithium composite oxide having a lattice strain of 0.4% or less and a crystallite size of 30 nm or less determined by the Halder Wafner method.
General formula: Li _x M _y O ₂
(In the formula, M is at least one transition metal having an average valence of 4+, and includes at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. .
1.2 ≦ x / y <2.0)

  The lithium composite oxide of the present invention is a lithium-excess system that satisfies 1.2 ≦ x / y <2.0. With such a lithium-excess material, a high capacity and high energy can be obtained at a high potential charge of 4.5 V or higher, as compared with a lithium non-excess material.

  It is preferable that at least one transition metal M includes at least one selected from Mn, Co, and Ni because higher capacity and higher energy can be obtained in high potential charging of 4.5 V or more. Most preferred is a ternary system containing Ni, Co, and Ni.

  The lithium composite oxide of the present invention is suitable for a positive electrode active material used for a lithium ion secondary battery.

  As explained in the section of “Problems to be Solved by the Invention”, conventionally, a lithium-rich lithium composite oxide having a layered rock salt structure is sufficient if it is not charged to 4.5 V or more. While it is difficult to obtain a capacity, the expansion and contraction of the crystal lattice is large when charging and discharging at 4.5 V or more are repeated, the particles tend to crack and deteriorate, and the capacity retention rate tends to decrease. There is.

The lithium composite oxide of the present invention has a lattice strain of 0.4% or less and a crystallite size of 30 nm or less. The lattice strain is particularly preferably 0.3% or less.
The present inventor found that the lithium composite oxide satisfying the above regulations relaxed the stress due to the expansion and contraction of the crystal lattice, and the generation of cracks was suppressed even when charging and discharging at 4.5 V or higher were repeated. It has been found that the rate can be improved (see Table 1 in the [Examples] section below).

  In the lithium composite oxide of the present invention, the conventional lithium-rich lithium composite oxide has large expansion and contraction of the crystal lattice when charging and discharging of 4.5 V or more are repeated, and cracks are generated in the particles. This is particularly suitable when the battery is used under a charging condition of 4.5 V or more, which tends to deteriorate and cause a decrease in capacity retention rate.

  By using the lithium composite oxide of the present invention as a positive electrode active material, the present inventor can charge 100 cycles under the conditions of 25 ° C., a charging voltage of 4.6 V, a discharging voltage of 2.5 V, and a current density of 300 mA / g. Provided is a lithium ion secondary battery having a capacity retention ratio of 70.0% or more obtained from a ratio between a discharge capacity after discharge and a discharge capacity after 10 cycles of charge and discharge under the same conditions (See Table 1 in the [Examples] section below).

  The lithium composite oxide of the present invention having a lattice strain of 0.4% or less, preferably 0.3% or less and a crystallite size of 30 nm or less is subjected to surface oxidation treatment after synthesizing the lithium composite oxide, for example. It can be manufactured.

  The surface oxidation treatment may be performed after the lithium ion secondary battery is assembled or before the lithium ion secondary battery is assembled.

The surface oxidation treatment method is not particularly limited.
As the surface oxidation treatment method, for example, a method of charging to a potential of 4.5 V or more after assembling the lithium ion secondary battery and holding this potential for 5 minutes or more is preferable. When the oxidation treatment is performed by this method, the battery may be charged to a potential of 4.5 V or higher and held at this potential for 5 minutes or more, and then discharged or not.

  The present inventor has confirmed by XAFS analysis that the surface oxidation treatment can be satisfactorily performed by this method in the section [Example] described later (see FIG. 1 and Table 1 in the section [Example] below). reference). Further, it was confirmed by XRD analysis that the lithium composite oxide subjected to the surface oxidation treatment by this method has a lattice strain of 0.4% or less, preferably 0.3% or less, and a crystallite size of 30 nm or less. (See Table 1 in the [Examples] section below).

As a method of subjecting the lithium composite oxide to surface oxidation treatment before assembling the lithium ion secondary battery, a method of chemically subjecting the lithium composite oxide to surface oxidation treatment using a gas such as NO ₂ BF ₄ , I ₂ , O ₃ , HClO, etc. Is mentioned.

  In the lithium composite oxide, a plurality of types of surface oxidation treatment methods may be combined.

[Method for producing lithium composite oxide]
Hereinafter, an example of the method for producing the lithium composite oxide will be described.

<Process (A)>
First, metal salts of all constituent metal elements of the lithium composite oxide to be finally produced are prepared.
For example, in the case of a ternary lithium composite oxide, a Li source, a Mn source, a Co source, and a Ni source are prepared. As the Li source, Mn source, Co source, and Ni source, for example, acetate is preferable.

Metal salts of all the constituent metal elements of the lithium composite oxide to be finally produced are blended at a desired metal composition ratio, and dissolved using water or the like. It is preferable that the solution is acidified with an acid.
The present inventor suppresses the generation of metal oxides and hydroxides in the metal salt solution by making the pH of the metal salt solution acidic, and allows the metal ions to exist stably in the liquid. It has been found that a metal salt solution in which each metal is well dispersed can be obtained.
The pH of the metal salt solution may be acidic, specifically less than 6 and particularly preferably 3 or less.

  In p. 290 of Non-Patent Document 2 mentioned in [Background Art], the pH and potential at which manganese oxide and manganese hydroxide are produced in a solution containing Mn are described. In the aqueous solution, a substance within the range of line a to line b in the figure is stably present. The valence of Mn in the raw material is 2+. In order for the Mn ions to stably exist as ions in the metal salt solution, the conditions on the left side of the lines 12, 16, 18, and 20 need to be satisfied. The pH can be 8 or less, and the conditions on the left side of the line 12 can be set. Lines 16, 18, and 20 indicate regions where oxygen and Mn in the air can react. The line 20 may be exceeded under conditions of a high concentration oxygen gas atmosphere, but the line 20 is not exceeded in a normal air atmosphere. In order not to exceed the lines 16, 18, the pH of the solution is preferably less than 6, and 3 or less is particularly preferable.

  The same figure is shown also about Co and Ni in p.325 and p.333 of the literature. As with Mn, it has been shown that the pH is preferably less than 6 and particularly preferably 3 or less in order for Co ions and Ni ions to be stably present as ions in the metal salt solution.

  The acid used for pH adjustment is not particularly limited, and examples thereof include nitric acid, hydrochloric acid, sulfuric acid, perchloric acid, and combinations thereof.

<Process (B)>
Next, the acidic metal salt solution obtained in the above step is gelled by being held at a temperature lower than the firing temperature in the next step (C).
There is no restriction | limiting in particular in gelling temperature, For example, 45-100 degreeC is preferable.
For example, the acidic metal salt solution obtained in the above step can be kept at 45 to 100 ° C. for about one night for gelation.
By passing through this gelation step, the next firing step can be carried out while maintaining a uniform dispersion state of metal ions.

<Process (C), process (D)>
Next, the gelled product obtained after the above step is baked (step (C)).
By making the pH of the metal salt solution prepared in the step (A) acidic and gelling it, a fired product in which the metal is uniformly dispersed can be generated.

  The firing atmosphere is not particularly limited, and may be an air atmosphere or an inert atmosphere such as an Ar atmosphere.

The firing step (C) may be performed in one stage or in a plurality of stages.
The pulverization step (D) can be performed after the one-step baking step, after the plurality of final baking steps, or between the plurality of steps of baking step.

For example, the firing step (C) is preferably performed in two stages, a temporary firing step (C1) and a main firing step (C2), and the pulverization step (D) is preferably performed between them.
The temporary firing temperature and the main firing temperature are not particularly limited.
In order to promote good crystallization and suppress the formation of undesired crystals, for example, the pre-baking temperature is preferably 450 to 750 ° C., particularly 550 to 650 ° C., and the main baking temperature is 750 to 1000 ° C., particularly 800 to 950 ° C is preferred.

The method for pulverizing the fired product (which may be a pre-fired product) is not particularly limited, and pulverization using a ball mill or the like is preferable.
The grinding time by the ball mill is not particularly limited, and is preferably 1 to 10 hours, particularly preferably 1 to 3 hours.

<Process (E)>
As described above, a lithium composite oxide is synthesized.
The synthesized lithium composite oxide is preferably subjected to surface oxidation treatment. By the surface oxidation treatment, the lattice strain can be reduced and the crystallite size can be reduced.

The surface oxidation treatment may be performed after the lithium ion secondary battery is actually assembled or before the lithium ion secondary battery is assembled.
Since the surface oxidation treatment method has been described above, the description thereof is omitted here.

  As described above, according to the present invention, a lithium-rich lithium composite oxide having a layered rock-salt structure capable of improving the capacity retention rate when charging and discharging are repeated at a high potential of 4.5 V or higher. And a manufacturing method thereof.

  For example, 4.5 to 5.0 V is preferable as the charging potential in repeated charging and discharging. In the present invention, even when such high potential charging and discharging are repeated, good battery performance can be obtained.

[Lithium ion secondary battery]
The lithium ion secondary battery of the present invention uses the above-described lithium composite oxide of the present invention as a positive electrode active material.
A lithium ion secondary battery can be produced by a known method using a positive electrode, a negative electrode, a separator, a nonaqueous electrolyte, and an outer package.

<Positive electrode>
The positive electrode can be manufactured by applying a positive electrode active material to a positive electrode current collector such as an aluminum foil by a known method.
In the present invention, the lithium composite oxide of the present invention is used as a positive electrode active material.

  As the positive electrode active material, a known positive electrode active material other than the lithium composite oxide of the present invention may be used in combination. However, the higher the amount of the lithium composite oxide of the present invention, the higher the effect.

For example, using a dispersant such as N-methyl-2-pyrrolidone, mixing a positive electrode active material, a conductive agent such as carbon powder, and a binder such as polyvinylidene fluoride (PVDF) to obtain a slurry, This slurry can be applied onto a current collector such as an aluminum foil, dried, and pressed to obtain a positive electrode.
The basis weight of the positive electrode is not particularly limited and is preferably 1.5 to 15 mg / cm ² . If the basis weight of the positive electrode is too small, uniform application is difficult, and if it is too large, there is a risk of peeling from the current collector.

<Negative electrode>
The negative electrode active material is not particularly limited, and a material having a lithium storage capacity of 2.0 V or less on the basis of Li / Li + is preferably used. As the negative electrode active material, carbon such as graphite, metallic lithium, lithium alloy, transition metal oxide / transition metal nitride / transition metal sulfide capable of doping / dedoping lithium ions, and these A combination etc. are mentioned.

The negative electrode can be produced, for example, by applying a negative electrode active material to a negative electrode current collector such as a copper foil by a known method.
For example, using a dispersant such as water, a negative electrode active material, a binder such as a modified styrene-butadiene copolymer latex, and a thickener such as carboxymethyl cellulose Na salt (CMC) are mixed as necessary. Thus, a slurry can be obtained, and this slurry can be applied onto a current collector such as a copper foil, dried, and pressed to obtain a negative electrode.
The basis weight of the negative electrode is not particularly limited and is preferably 1.5 to 15 mg / cm ² . If the basis weight of the negative electrode is too small, uniform application is difficult, and if it is too large, there is a risk of peeling from the current collector.

  When metallic lithium or the like is used as the negative electrode active material, metallic lithium or the like can be used as it is as the negative electrode.

<Nonaqueous electrolyte>
As the non-aqueous electrolyte, known ones can be used, and liquid, gel-like or solid non-aqueous electrolytes can be used.
For example, a lithium-containing electrolyte is dissolved in a mixed solvent of a high dielectric constant carbonate solvent such as propylene carbonate or ethylene carbonate and a low viscosity carbonate solvent such as diethyl carbonate, methyl ethyl carbonate, or dimethyl carbonate. A water electrolysis solution is preferably used.
As the mixed solvent, for example, a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) is preferably used.
Examples of the lithium-containing electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂ SiF ₆ , LiOSO ₂ C _k F _{(2k + 1)} (k = 1 to 8), LiPF _n {C _k F _{(2k + 1) )} (6-n) (} n = 1~5 integer, k = 1 to 8 integer) lithium salts such as, and combinations thereof.

<Separator>
The separator may be a film that electrically insulates the positive electrode and the negative electrode and is permeable to lithium ions, and a porous polymer film is preferably used.
As the separator, for example, a porous film made of polyolefin such as a porous film made of PP (polypropylene), a porous film made of PE (polyethylene), or a laminated porous film of PP (polypropylene) -PE (polyethylene) is preferably used. It is done.

<Exterior body>
A well-known thing can be used as an exterior body.
As a type of the secondary battery, there are a cylindrical type, a coin type, a square type, a film type, and the like, and an exterior body can be selected according to a desired type.

The lithium ion secondary battery of the present invention uses the above-described lithium composite oxide of the present invention as a positive electrode active material.
ADVANTAGE OF THE INVENTION According to this invention, the lithium ion secondary battery which can improve the capacity | capacitance maintenance factor at the time of repeating charging / discharging with the high electric potential of 4.5 V or more can be provided.

  According to the present invention, the discharge capacity after 100 cycles of charge / discharge under the conditions of 25 ° C., charge voltage 4.6 V, discharge voltage 2.5 V, and current density 300 mA / g, and 10 cycles under the same conditions. A lithium ion secondary battery having a capacity retention rate of 70.0% or more obtained from the ratio to the discharge capacity after charging / discharging can be provided (see Table 1 in the [Examples] section). reference).

  Examples and comparative examples according to the present invention will be described.

Example 1
<Synthesis of positive electrode active material>
Lithium acetate dihydrate as the Li source, manganese acetate tetrahydrate as the Mn source, cobalt acetate tetrahydrate as the Co source, nickel acetate tetrahydrate as the Ni source (both Nacalai Tesque ).
These raw materials were weighed so as to be Li / Mn / Co / Ni = 60/27 / 6.5 / 6.5 (molar ratio) and dissolved in pure water.
The solution was adjusted to pH 3 with nitric acid.
The resulting solution was kept overnight at 80 ° C. with stirring to obtain a gel.

<Baking>
The gel obtained above was baked in two steps in an air atmosphere.
First, it was calcined at 600 ° C. for 5 hours. Then, after cooling to room temperature, it was pulverized into particles using a ball mill and then fired again at 900 ° C.
As described above, a particulate lithium composite oxide was obtained.

<Manufacture of positive electrode>
Using N-methyl-2-pyrrolidone (manufactured by Wako Pure Chemical Industries, Ltd.) as a dispersant, a positive electrode active material composed of the above lithium composite oxide, and acetylene black (electrochemical industry) as a conductive agent HS-100) and PVDF (Kureha KF Polymer # 1120) as a binder were mixed at 85/10/5 (mass ratio) to obtain a slurry.
The slurry was applied onto an aluminum foil as a current collector by a doctor blade method, dried at 80 ° C. for 30 minutes, and pressed using a press machine to obtain a positive electrode. The positive electrode had a basis weight of 5.5 mg / cm ² and a thickness of 18 μm.

<Negative electrode>
Metal lithium was used as the negative electrode active material.
This was used as a negative electrode as it was.

<Separator>
A commercially available separator made of a PP (polypropylene) porous film was prepared.

<Nonaqueous electrolyte>
Using a mixed solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate = 3/3/4 (volume ratio) as a solvent, LiPF ₆ as a lithium salt as an electrolyte was dissolved at a concentration of 1 mol / L. A non-aqueous electrolysis solution was prepared.

<Exterior body>
As an exterior body, a SUS 2032 type coin cell was prepared.

<Assembly of lithium ion secondary battery>
A lithium ion secondary battery was assembled by a known method using the positive electrode, the negative electrode, the separator, the non-aqueous electrolyte, and the outer package.

<Surface oxidation treatment>
After assembling the lithium ion secondary battery, this was charged to 4.8 V (lithium metal standard), then held for 20 hours, discharged to 2.5 V, and the positive electrode active material was oxidized.

(Example 2)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the surface oxidation treatment of the positive electrode active material was performed under the following conditions.
Oxidation treatment: After assembling the lithium ion secondary battery, after charging to 4.8 V (lithium metal standard), this was held for 20 hours to oxidize the positive electrode active material.

(Example 3)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the surface oxidation treatment of the positive electrode active material was performed under the following conditions.
Oxidation treatment: After assembling the lithium ion secondary battery, this was charged to 4.8 V (lithium metal standard), then held for 5 minutes and discharged to 2.5 V to oxidize the positive electrode active material.

(Comparative Example 1)
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the surface oxidation treatment of the positive electrode active material was performed under the following conditions.
Oxidation treatment: After the lithium ion secondary battery was assembled, it was charged to 4.4 V (lithium metal standard), then held for 5 minutes, and discharged to 2.5 V to oxidize the positive electrode active material.

(Comparative Example 2)
A lithium ion secondary battery was manufactured in the same manner as in Example 1 except that after the lithium ion secondary battery was assembled, the surface oxidation treatment of the positive electrode active material by charging (discharging) was not performed.

(Evaluation)
<XRD analysis>
In Examples 1 to 4 and Comparative Example 1, the lithium ion secondary battery after surface oxidation treatment of the positive electrode active material and before cycle charge / discharge was disassembled in a glove box, washed with dimethyl carbonate (DMC), and then dried. . Thereafter, the positive electrode active material was peeled from the current collector, and powder XRD analysis was performed on the positive electrode active material using an X-ray diffraction (XRD) apparatus.
In Comparative Example 2, the synthesized positive electrode active material was measured as it was.
The measurement was performed using Rigaku Ultima 4 as a measuring device, using CuKα rays, and using a primary detector. Measurement conditions were set to 2θ = 10 to 80 °, 10 ° / min, and 3 times integration.
Lattice strain and crystallite size were measured using analysis software PDXL manufactured by Rigaku Corporation.

<XAFS analysis>
The surface oxidation state of the positive electrode active material was evaluated by X-ray absorption fine structure (XAFS) analysis.
In Examples 1 and 2 and Comparative Example 1, the lithium ion secondary battery after the surface oxidation treatment of the positive electrode active material and before cycle charge / discharge was disassembled in a glove box, washed with DMC, and dried. Thereafter, the positive electrode active material was peeled from the current collector, and XAFS analysis of the Mn K edge was performed on this positive electrode active material. In Comparative Example 2, the synthesized positive electrode active material was measured as it was.
The absorption edge position was defined by the midpoint between the peak rising position and the peak apex.

<Cycle charge / discharge>
The following cycle charging / discharging was implemented with respect to the lithium ion secondary battery obtained in each case.
In a constant temperature bath at 25 ° C., 100 cycles of charge and discharge were performed under the conditions of a charge voltage of 4.6 V (lithium metal standard), a discharge voltage of 2.5 V (lithium metal standard), and a current density of 300 mA / g.

<Measurement of capacity retention>
In the above cycle charge / discharge, the discharge capacity at the 10th cycle and the discharge capacity at the 100th cycle were determined, and the capacity retention rate defined by the following formula was determined.
Capacity retention rate (%) = (discharge capacity at 100th cycle) / (discharge capacity at 10th cycle)

<TEM (transmission electron microscope) observation>
In Example 1 and Comparative Example 2, the lithium ion secondary battery after the cycle charge / discharge was disassembled in a glove box, washed with DMC, and dried. Thereafter, the positive electrode active material was peeled from the current collector and attached to a Cu mesh for TEM observation. This TEM image was observed.

(result)
Table 1 shows the presence / absence and method of surface oxidation treatment of the positive electrode active material, the measurement results of the lattice strain and crystallite size of the positive electrode active material, and the measurement result of the capacity retention ratio for each example.

  As shown in Table 1, an example using a lithium composite oxide having a lattice strain obtained by the Halder Wafner method of 0.4% or less, preferably 0.3% or less, and a crystallite size of 30 nm or less. In 1-3, the capacity maintenance rate of 70% or more higher than the comparative examples 1 and 2 using the lithium complex oxide outside the range prescribed | regulated by this invention was obtained.

The XAFS patterns obtained in Examples 1-2 and Comparative Examples 1-2 are shown in FIG.
The absorption edge position in each example is shown in Table 1.
The absorption edge positions of Examples 1 and 2 were 6555.0 eV or less, and the absorption edge positions of Comparative Examples 1 and 2 were more than 6555.0 eV.

  In Examples 1 and 2 in which the surface of the positive electrode active material was oxidized by charging the lithium ion secondary battery to a potential of 4.5 V or higher after maintaining the potential for 5 minutes or more. The surface of the positive electrode active material is better oxidized to a higher level than Comparative Example 1 in which only 4.4 V was charged in the oxidation treatment and Comparative Example 2 in which the surface oxidation treatment of the positive electrode active material was not performed. Was confirmed.

2A and 2B show TEM images of the positive electrode active materials of Example 1 and Comparative Example 2 after the 100-cycle charge / discharge test.
Although no crack was found in the positive electrode active material of Example 2, cracks were seen in the positive electrode active material of Comparative Example 2.

  The method for producing a lithium composite oxide of the present invention can be preferably applied to a lithium ion secondary battery or the like mounted on a plug-in hybrid vehicle (PHV) or an electric vehicle (EV).

Claims (10)

A lithium composite oxide having a layered rock salt structure represented by the following formula,
A lithium composite oxide having a lattice strain of 0.4% or less and a crystallite size of 30 nm or less determined by a Halder Wafner method.
General formula: Li _x M _y O ₂
(In the formula, M is at least one transition metal having an average valence of 4+, and includes at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. .
1.2 ≦ x / y <2.0)   The lithium composite oxide according to claim 1, wherein the lattice strain is 0.3% or less.   The lithium composite oxide according to claim 1 or 2, which has been subjected to surface oxidation treatment.   The lithium composite oxide according to claim 3, wherein the XAFS measurement at the Mn K edge has an absorption edge at 6555.0 eV or less when the absorption edge position is defined by the midpoint between the peak rising position and the peak apex.   The lithium composite oxide according to any one of claims 1 to 4, wherein the at least one transition metal M includes at least one selected from Mn, Co, and Ni.   The lithium composite oxide according to any one of claims 1 to 5, which is used for a positive electrode active material used in a lithium ion secondary battery.   The lithium composite oxide according to claim 6, which is used under a charging condition of 4.5 V or more.   A lithium ion secondary battery using the lithium composite oxide according to claim 1 as a positive electrode active material.   The discharge capacity after 100 cycles of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 300 mA / g, and 10 cycles of charge / discharge were performed under the same conditions. The lithium ion secondary battery according to claim 8, wherein a capacity maintenance ratio obtained from a ratio with a subsequent discharge capacity is 70.0% or more. A method for producing a lithium ion secondary battery according to claim 8 or 9,
A method of manufacturing a lithium ion secondary battery in which the positive electrode active material is subjected to surface oxidation treatment by charging to a potential of 4.5 V or more after assembly of the lithium ion secondary battery and maintaining the potential for 5 minutes or more.