JP2013087040A - Lithium compound oxide and production method of the same, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a lithium compound oxide that is suitable as a positive electrode active substance of a lithium ion secondary battery and can effectively improve a capacity retention rate and a capacity recovery rate.SOLUTION: A method for producing a lithium compound oxide comprising a matrix oxide expressed by general formula of LiMOwith addition of at least one alkaline earth metal is provided; and the method comprises carrying out steps of: (A) preparing an acidic metal salt solution containing metal salts of the whole structural metal elements of the lithium compound oxide; (B) gelling the metal salt solution by maintaining the solution at a temperature lower than a firing temperature in the following step (C); and (C) firing the obtained gel product after the step (B). In the above general formula, M represents at least one transition metal having an average valence of 4+ or more and comprises at least one selected from the group consisting of Ti, V, Cr, Mn, Fe, Co Ni and Cu; and x and y satisfy 0<x<2 and 0<y≤1.

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.

  A high-performance lithium ion secondary battery is required to be capable of charging / discharging stably even when charging / discharging at a high potential, high capacity, and repeated charging / discharging.

  In a lithium ion battery using the layered rock salt type lithium composite oxide as a positive electrode, when charging and discharging at a high potential of 4.5 V or higher are repeated, the positive electrode active material repeatedly expands and contracts, Due to instability, the structure of the positive electrode active material may collapse, the positive electrode may deteriorate, and the capacity retention rate may decrease.

Among the materials of the layered rock salt type structure, in the lithium-excess material represented by the formula Li _{1 + x} M _1-x O ₂ (where M is 0 <x as in the above case), a high value of 4.5 V or higher The layered structure can be maintained relatively even when the potential is charged and discharged repeatedly. However, the expansion and contraction of the crystal lattice is large, and the capacity retention rate may be reduced.

  For the purpose of providing a lithium ion secondary battery that can be stably charged and discharged with a high capacity even when high potential charging and discharging are repeated, the following patent document describes a positive electrode active material for a lithium ion secondary battery. Is disclosed.

Patent Document 1 contains a lithium manganese composite oxide containing at least one metal element (excluding Li) selected from the group consisting of alkali metals and alkaline earth metals of less than 10 mol% with respect to Mn. A positive electrode active material is disclosed (claim 1).
Patent Document 1 describes that the metal element is preferably unevenly distributed near the surface of the lithium manganese composite oxide particles (claim 3, paragraph 0009, etc.).
Patent Document 1 describes that cycle charge / discharge characteristics are improved (paragraph 0005 and the like).

Patent Document 1 describes that it can be applied to a layered rock salt structure, a spinel structure, and the like (paragraph 0010).
However, in the [Example] section of Patent Document 1, only a lithium composite oxide having a spinel structure containing cesium, which is an alkali metal, as the metal element M is manufactured. There is no description about concrete manufacture.
A specific method for producing a spinel-type lithium composite oxide containing cesium that is an alkali metal as the metal element M is described in paragraph 0023.

Patent Document 2 discloses a positive electrode active material represented by the following formula (claim 2).
Li [Li _x Ni _y Mn _z M _b ] O _2-a
(Where 0 <x <0.4, 0.12 <y <0.5, 0.3 <z <0.62, 0 ≦ a <0.5, M is at least 1 having a valence of 2-6) Two metal elements, x> (1-2y) / 3, 1/4 ≦ y / z ≦ 1.0, 0 <b / (y + z) ≦ 0.1, 1.0 ≦ x + y + z + b ≦ 1.1)

Patent Document 2 includes at least one selected from Mg, Al, Zr, Ti, Nb, W, and Mo as the metal element M (Claim 3).
Patent Document 2 describes that the initial charge / discharge efficiency and the discharge capacity are improved (paragraph 0079 and the like).

  The specific method for producing the positive electrode active material in the [Example] section of Patent Document 2 is described in paragraph 0020.

JP 2001-068109 A JP 2007-242581

Marcel Poubaix, 1974, National Association of Corrosion Engineers, "Atlas of electrochemical equilibria in aqueous solutions"

  Patent Documents 1 and 2 describe that in a ternary lithium composite oxide containing Mn, Co, and Ni, the cycle charge / discharge characteristics are improved by adding an alkaline earth metal. Patent Document 1 describes that the alkaline earth metal is preferably unevenly distributed on the surface of the lithium composite oxide.

In order to suppress variation in characteristics and effectively improve cycle charge / discharge characteristics, it is preferable that the alkaline earth metal is present in a substantially uniform distribution over the entire surface of the lithium composite oxide.
However, as a result of studies by the present inventors, in the method for producing a lithium composite oxide described in Patent Documents 1 and 2, the alkaline earth metal is present in a substantially uniform distribution over the entire surface of the lithium composite oxide. It was difficult to effectively improve the cycle charge / discharge characteristics (see Comparative Example 4 below).

For secondary batteries mounted on plug-in hybrid vehicles (PHV) or electric vehicles (EV), etc., in addition to the capacity maintenance rate when charging and discharging are repeated at high potential and high current density, The capacity recovery rate when charging / discharging is repeated and then charging / discharging at a high potential and low current density is also important. In the configurations of Patent Documents 1 and 2, there is a limit to improvement in both the capacity maintenance rate and the capacity recovery rate.
In this specification, high potential is defined as “4.5 V or more”.

  The present invention has been made in view of the above circumstances, and is suitable for use as a positive electrode active material of a lithium ion secondary battery, and has a capacity retention rate when charging and discharging are repeated at a high potential and high current density, and A lithium composite oxide capable of effectively improving the capacity recovery rate when charging / discharging is repeated at a high potential / high current density and then charging / discharging is performed at a high potential / low current density. It is intended to provide.

The method for producing the lithium composite oxide of the present invention comprises:
A method for producing a lithium composite oxide in which at least one alkaline earth metal is added to a base oxide represented by the following formula, including metal salts of all constituent metal elements of the lithium composite oxide, A step (A) of preparing an acidic metal salt solution;
Holding the metal salt solution at a temperature lower than the firing temperature of the step (C) to cause gelation (B);
And a step (C) of firing the gelled product obtained after the step (B).
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. .
0 <x <2, 0 <y ≦ 1)

  The first lithium composite oxide of the present invention is produced by the above-described method for producing a lithium composite oxide.

The second lithium composite oxide of the present invention is
A lithium composite oxide in which at least one alkaline earth metal is added to a base oxide represented by the following formula,
The surface of the lithium composite oxide has a crystal phase containing the alkaline earth metal, which is different from the crystal phase mainly present in the lithium composite oxide, and the crystal phase of the lithium composite oxide It exists in a substantially uniform distribution over the entire surface.
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. .
0 <x <2, 0 <y ≦ 1)

  Identification and distribution of a crystal phase different from the crystal phase mainly present in the lithium composite oxide are XPS analysis (X-ray photoelectron spectroscopy), HAADF-STEM analysis (high angle scattering dark field scanning transmission electron microscope analysis), And an analysis such as EDX analysis (energy dispersive X-ray fluorescence spectrometry).

In the HAADF-STEM image, when alkaline earth metal is mapped by EDX analysis and the alkaline earth metal is distributed over the entire surface of the lithium composite oxide, the crystal phase containing the alkaline earth metal is lithium composite. It can be said that the oxide has a substantially uniform distribution over the entire surface of the oxide (see FIG. 3A in the section “Examples” below).
For specific analysis, see the section “Examples” below.

  The first and second lithium composite oxides of the present invention are 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, it is suitable for a positive electrode active material of a lithium ion secondary battery, and has a capacity retention ratio when charging / discharging is repeated at a high potential / high current density, and a high potential / high current density. It is possible to provide a lithium composite oxide capable of effectively improving the capacity recovery rate when charging / discharging is repeated and thereafter charging / discharging at a high potential and a low current density, and a method for producing the same.

3 is an XRD pattern of a positive electrode active material obtained in Example 1. 3 is an XRD pattern of a positive electrode active material obtained in Example 2. 3 is an XRD pattern of a positive electrode active material obtained in Example 3. 4 is an XRD pattern of a positive electrode active material obtained in Example 4. 6 is an XRD pattern of a positive electrode active material obtained in Example 5. FIG. 7 is an XRD pattern of a positive electrode active material obtained in Example 6. 3 is an XRD pattern of a positive electrode active material obtained in Comparative Example 1. 3 is an XRD pattern of a positive electrode active material obtained in Comparative Example 2. 4 is an XRD pattern of a positive electrode active material obtained in Comparative Example 3. It is an XPS spectrum of the positive electrode active material obtained in Examples 1-3. It is the photograph of Ca mapping by the EDX analysis in the HAADF-STEM image obtained about the positive electrode active material obtained in Example 2, and its surface. 4 is an EDX spectrum of a precipitate in the positive electrode active material obtained in Example 2. It is a graph which shows the relationship between Ca addition amount and a capacity | capacitance maintenance factor.

  Hereinafter, the present invention will be described in detail.

[Lithium composite oxide and its production method]
The method for producing the lithium composite oxide of the present invention comprises:
A method for producing a lithium composite oxide in which at least one alkaline earth metal is added to a base oxide represented by the following formula, including metal salts of all constituent metal elements of the lithium composite oxide, A step (A) of preparing an acidic metal salt solution;
Holding the metal salt solution at a temperature lower than the firing temperature of the step (C) to cause gelation (B);
And a step (C) of firing the gelled product obtained after the step (B).
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. .
0 <x <2, 0 <y ≦ 1)

<Base oxide>
As the base oxide, at least one lithium composite oxide represented by the following formula is used.
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. .
0 <x <2, 0 <y ≦ 1)

More preferably, M includes at least one selected from the group consisting of Mn, Co, and Ni.
The base oxide is particularly preferably a ternary system containing M as Mn, Co, and Ni.

  The matrix oxide has a layered rock salt structure, and an XRD pattern using a Cu light bulb has a peak at 2θ = 20-24 °.

<Alkaline earth metal>
As the additive element, at least one alkaline earth metal is used.
The alkaline earth metal is preferably at least one selected from the group consisting of Mg, Ca, Sr, and Ba.
By adding at least one alkaline earth metal to the lithium composite oxide having a layered rock salt structure, cycle charge / discharge characteristics can be improved.

<Process (A)>
First, metal salts of all the constituent metal elements (the constituent metal of the base oxide and the alkaline earth metal) of the lithium composite oxide to be finally produced are prepared.
As the metal salt of the constituent metal of the base oxide and the metal salt of the alkaline earth metal, 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. The solution uses acid to make the pH acidic.
By making the pH of the metal salt solution acidic, the present inventor suppresses the generation of metal oxides and hydroxides in the metal salt solution, 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.

  For example, if the base oxide is a ternary system containing Mn, Co, and Ni, each ion of Mn, Co, and Ni is made to stably exist in the metal salt solution.

  P. 290 of Non-Patent Document 1 listed in the section “Background Art” describes the pH and potential at which manganese oxide and manganese hydroxide are produced in a solution containing Mn. 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.

  For any of the other metals listed as options for the transition metal M in the base oxide, the conditions under which the metal ions are stably present in the metal salt solution are the same as those for Mn, Co, and Ni.

  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.

  In the present invention, the amount of the alkaline earth metal added to all the constituent metal elements of the lithium composite oxide finally produced is preferably 2.5 mol% or less, particularly preferably 1 mol% or less.

  The cycle charge / discharge characteristics can be improved by adding an alkaline earth metal to the lithium composite oxide having a layered rock salt structure. However, if the amount added is too large, the battery performance may be deteriorated due to a relative decrease in the amount of the base oxide. Therefore, it is preferable that the addition amount of the alkaline earth metal is within the above-mentioned regulation.

<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 subsequent 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)).
In the present invention, the fired product in which the metal is uniformly dispersed can be produced by making the pH of the metal salt solution acidic, gelling it and then firing.

  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 preliminary calcination temperature is preferably 450 to 750 ° C., particularly 550 to 650 ° C., and the main calcination temperature is 750 to 900 ° C., particularly 800 to 900 ° 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.

As described above, a lithium composite oxide to which at least one alkaline earth metal is added using a lithium composite oxide represented by the following formula as a base oxide can be produced.
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. .
0 <x <2, 0 <y ≦ 1)

In the production method of the present invention, an alkaline earth metal having an ionic radius larger than that of the constituent metal of the base crystal is added. Therefore, the alkaline earth metal does not easily dissolve in the base crystal and can be precipitated as a different phase.
This heterogeneous phase is considered not to participate in the electrochemical reaction of the secondary battery. When charging and discharging at a high potential of 4.5 V or higher are repeated, it is considered that the crystal phase not involved in the electrochemical reaction exhibits a function of relieving stress generated by expansion and contraction of the base oxide.

  In the production method of the present invention, the metal salt solution containing the precursor is made acidic, the precipitation of metal oxides and hydroxides is suppressed, and then the gelation step is performed, followed by firing. The reaction can proceed, and the heterogeneous phase containing the alkaline earth metal can be distributed substantially uniformly over the entire surface of the lithium composite oxide.

  According to the production method of the present invention, the surface of the lithium composite oxide has an alkaline earth metal and has a crystal phase different from the crystal phase mainly present in the lithium composite oxide, and the crystal phase is It is possible to stably produce a lithium composite oxide that exists in a substantially uniform distribution over the entire surface of the lithium composite oxide.

  In the production method of the present invention, since the heterogeneous phase containing the alkaline earth metal can be dispersed substantially uniformly over the entire surface of the lithium composite oxide, than any of the patent documents listed in the section of “Background Art”, Variation in characteristics can be suppressed, and cycle charge / discharge characteristics can be effectively improved.

  According to the manufacturing method of the present invention, even if charging and discharging at a high potential of 4.5 V or more are repeated, cracks generated in the positive electrode active material are suppressed, deterioration of the positive electrode is suppressed, capacity retention rate and capacity Both recovery rates can be improved.

  If the amount of the alkaline earth metal added to the base oxide becomes too large, the battery capacity decreases due to the relatively small amount of the base crystal phase involved in the electrochemical reaction of the secondary battery. Conceivable. Therefore, the addition amount of the alkaline earth metal is preferably 2.5 mol% or less, particularly preferably 1.0 mol% or less, with respect to all the constituent metal elements of the lithium composite oxide.

  The lithium composite oxide produced by the production method of the present invention can be preferably used as a positive electrode active material for a lithium ion secondary battery.

  As described above, according to the present invention, it is suitable for a positive electrode active material of a lithium ion secondary battery, and has a capacity retention rate when charging / discharging is repeated at a high potential / high current density, and a high Provided is a lithium composite oxide capable of effectively improving the capacity recovery rate when repeated charging / discharging at a high potential / low current density and then charging / discharging at a high potential / low current density, and a method for producing the same be able to.

  The charging potential in repeated charging / discharging is not particularly limited, and is preferably 4.5 to 5.0 V, for example. In the present invention, good battery performance can be obtained even when charging and discharging at a high potential of 4.5 V or higher are repeated.

[Lithium ion secondary battery]
The lithium ion secondary battery of the present invention uses a lithium composite oxide produced by the production method 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 produced by the production method of the present invention is used as the positive electrode active material.

As the positive electrode active material, a known positive electrode active material other than the lithium composite oxide produced by the production method of the present invention may be used in combination. However, the higher the amount of the lithium composite oxide produced by the production method of the present invention, the higher the effect.
Known no particular limitation on the positive electrode active material, for _{example, LiCoO 2, LiMnO 2, LiMn} 2 O 4, LiNiO 2, LiNi x Co (1-x) O 2, and _{LiNi x Co y Mn (1-} x-y _And lithium-containing composite oxides such as O ₂ (where 0 <x <1, 0 <y <1).

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 a lithium composite oxide produced by the production method of the present invention as a positive electrode active material.
According to the present invention, the capacity retention rate when charging / discharging is repeated at a high potential / high current density, and charging / discharging is repeated at a high potential / high current density, and then charging / discharging is performed at a high potential / low current density. It is possible to provide a lithium ion secondary battery capable of effectively improving the capacity recovery rate at the time of charging.

  According to the present invention, the discharge capacity after one cycle of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 300 mA / g, and the following cycle charge / discharge Thus, a lithium ion secondary battery having a capacity retention rate of 79.0% or more obtained from the ratio to the discharge capacity after performing (see Table 1 in the [Example] section) can be provided.

  According to the present invention, after performing the following cycle charge / discharge, one cycle charge / discharge was further performed under the conditions of 25 ° C., a charge voltage of 4.6 V, a discharge voltage of 2.5 V, and a current density of 50 mA / g. A lithium ion secondary battery having a capacity recovery rate of 58.0% or more obtained by dividing the subsequent discharge capacity by the charge capacity of the final cycle of the following cycle charge / discharge can be provided ([Example ] In Table 1).

Cycle charge / discharge conditions: 3 cycles of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 15 mA / g, followed by the same temperature, same charge voltage, same discharge voltage And 50 cycles of charge and discharge under the condition of a current density of 300 mA / g.

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

Example 1
<Synthesis of positive electrode active material>
Lithium acetate dihydrate as Li source, Manganese acetate tetrahydrate as Mn source, Cobalt acetate tetrahydrate as Co source, Nickel acetate tetrahydrate as Ni source, Calcium acetate as Ca source Monohydrate (both manufactured by Nacalai Tesque Co., Ltd.) was used.
These raw materials were weighed so as to be Li / Mn / Co / Ni / Ca = 60 / 26.75 / 6.5 / 6.5 / 0.025 (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 for 1 hour, and then fired again at 900 ° C.
As described above, a positive electrode active material composed of ternary lithium composite oxide particles 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 particles, and acetylene black (Electrochemical Industry Co., Ltd.) as a conductive agent. ) HS-100) and PVDF (Kureha KF Polymer # 1120) 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 150 ° 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.

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

(Examples 2-6, Comparative Examples 1-4)
A lithium ion secondary battery was manufactured in the same manner as in Example 1 except that the composition ratio was changed to the following in the synthesis of the positive electrode active material.

(Example 2)
Li / Mn / Co / Ni / Ca = 60 / 26.5 / 6.5 / 6.5 / 0.5 (molar ratio)
(Example 3)
Li / Mn / Co / Ni / Ca = 60/26 / 6.5 / 6.5 / 1 (molar ratio)
Example 4
Li / Mn / Co / Ni / Mg = 60 / 26.5 / 6.5 / 6.5 / 0.5 (molar ratio)
(Example 5)
Li / Mn / Co / Ni / Mg = 60/26 / 6.5 / 6.5 / 1 (molar ratio)
(Example 6)
Li / Mn / Co / Ni / Sr = 60/26 / 6.5 / 6.5 / 1 (molar ratio)
(Example 7)
Li / Mn / Co / Ni / Ca = 60 / 24.5 / 6.5 / 6.5 / 2.5 (molar ratio)

(Comparative Example 1)
Li / Mn / Co / Ni = 60/27 / 6.5 / 6.5 (molar ratio)
(Comparative Example 2)
Li / Mn / Co / Ni / Ti = 60/26 / 6.5 / 6.5 / 1.0 (molar ratio)
(Comparative Example 3)
Li / Mn / Co / Ni / Fe = 60/26 / 6.5 / 6.5 / 1.0 (molar ratio)

(Comparative Example 4)
First, a precursor of a positive electrode active material was produced by the same coprecipitation method as in Example 1 of Patent Document 2 listed in the “Background Art” section. Specifically, Mn / Co / Ni composite hydroxide was obtained by adding lithium hydroxide to a mixed solution of Mn sulfate, Co sulfate and Ni sulfate and coprecipitation.

Next, lithium carbonate and magnesium carbonate and the composite hydroxide obtained above were mixed so as to have the following composition ratio.
Li / Mn / Co / Ni / Ca = 60/26 / 6.5 / 6.5 / 1 (molar ratio)

The obtained mixture was baked in two steps under 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 for 1 hour, and then fired again at 900 ° C.
As described above, a positive electrode active material composed of lithium composite oxide particles was obtained.
Using this positive electrode active material, a lithium ion secondary battery was produced in the same manner as in Example 1.

(Evaluation)
<XRD analysis>
About the positive electrode active material obtained in Examples 1-6 and Comparative Examples 1-3, the powder XRD pattern was obtained using the X ray diffraction (XRD) apparatus, and the crystal phase was identified.
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.

<XPS analysis>
The surface of the positive electrode active material obtained in Examples 1 to 3 was subjected to XPS analysis (X-ray photoelectron spectroscopic analysis) using PHI5800 manufactured by ULVAC-PHI. The measurement was performed under the conditions of the radiation source Mg and 400 W.

<HAADF-STEM analysis and EDX analysis>
The positive electrode active material obtained in Example 2 was subjected to HAADF-STEM analysis (high angle scattering dark field scanning transmission electron microscope analysis) and EDX analysis (energy dispersive X-ray fluorescence X-ray spectroscopy analysis) using TITAN 80-300 manufactured by FEI. ). The acceleration voltage was 300 kV.

<Measurement of capacity maintenance rate and capacity recovery rate>
The following cycle charging / discharging was implemented with respect to the lithium ion secondary battery obtained in each case.
3 cycles of charge / discharge under conditions of a charging voltage of 4.6 V (vs. Li / Li +), a discharge voltage of 2.5 V (vs. Li / Li +), and a current density of 15 mA / g in a constant temperature bath at 25 ° C. Subsequently, 50 cycles of charge and discharge were performed in a constant temperature bath at the same temperature under the conditions of the same charge voltage, the same discharge voltage, and a current density of 50 mA / g.

  In the thermostat of the same temperature as above, the discharge capacity after performing one cycle of charge / discharge under the conditions of the same charge voltage, the same discharge voltage, and a current density of 300 mA / g, and the above cycle charge / discharge were performed. The ratio with the subsequent discharge capacity was determined as the capacity retention rate.

  After performing the above-mentioned cycle charge / discharge, from the discharge capacity after performing one cycle of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 50 mA / g, A value obtained by dividing the charge capacity in the final cycle of the above cycle charge / discharge was determined as a capacity recovery rate.

(result)
1A to 1I show the XRD patterns of Examples 1 to 6 and Comparative Examples 1 to 3. FIG. In either case, the desired crystal structure (layered rock salt structure) was obtained.

In FIG. 2, the XPS spectrum of the positive electrode active material obtained in Examples 1-3 is shown.
It was confirmed that Ca was present on the surface of the positive electrode active material obtained in Examples 1 to 3.

FIG. 3A shows a HAADF-STEM image obtained for the positive electrode active material obtained in Example 2 and a Ca mapping photograph by EDX analysis on the surface thereof. In the figure, the portion that appears whitish is where a crystal phase containing Ca exists. FIG. 3B shows an EDX spectrum of the crystal phase containing Ca. The Cu peak in FIG. 3B is derived from the substrate.
In the positive electrode active material obtained in Example 2, the crystal phase containing Ca was distributed almost uniformly over the entire surface of the base crystal, and it was confirmed that the composition was CaO. CaO has a crystal phase different from the parent crystal.

  The analysis results of XPS, HAADF-STEM, and EDX are the same as in Examples 1 to 3 and 7 using Ca and Examples 4 to 6 using an alkaline earth metal other than Ca. The crystal phase containing alkaline earth metal was distributed almost uniformly over the entire surface, and it was confirmed that the composition was an oxide of alkaline earth metal.

  Table 1 shows the measurement results of the types and addition amounts of alkaline earth metals and the capacity retention ratio and capacity recovery ratio in each example. The relationship between the Ca addition amount and the capacity retention rate is shown in FIG. FIG. 4 is a plot of the results of Comparative Example 1 without Ca addition and Examples 1 to 3 and 7 with Ca addition.

As shown in Table 1 and FIG. 4, in Examples 1 to 7 to which the alkaline earth metal was added, a higher capacity retention rate and a higher capacity recovery rate were obtained than Comparative Example 1 in which the metal element was not added. .
In particular, in Examples 1 to 6 in which 0.25 to 1.0 mol% of alkaline earth metal was added, Comparative Example 1 in which no metal element was added, Comparative Example 2 in which a metal element other than alkaline earth metal was added, A capacity retention rate and a capacity recovery rate higher than 3 were obtained.

  In Comparative Examples 2 and 3, Fe or Ti was used as the additive metal of the positive electrode active material. Since these metal elements have an ionic radius close to that of Ni, Mn, and Co forming the base crystal, it is considered that these metal elements easily dissolve in the crystal structure of the base oxide. Therefore, it is difficult to deposit as a heterogeneous phase on the surface.

  On the other hand, the alkaline earth metal used in Examples 1 to 7 has a larger ionic radius than Ni, Mn, and Co, and can be precipitated as a different phase. This heterogeneous phase is considered not to participate in the electrochemical reaction of the secondary battery. When charging and discharging at a high potential of 4.5 V or higher are repeated, it is considered that the crystal phase not involved in the electrochemical reaction exhibits a function of relieving stress generated by expansion and contraction of the base oxide. As a result, even if charging and discharging at a high potential of 4.5 V or more are repeated, cracks generated in the positive electrode active material are suppressed, deterioration of the positive electrode is suppressed, and both the capacity retention rate and the capacity recovery rate are improved. It is thought that it is done.

  If the amount of the alkaline earth metal added to the base oxide becomes too large, the battery capacity decreases due to the relatively small amount of the base crystal phase involved in the electrochemical reaction of the secondary battery. Conceivable. Therefore, the addition amount of the alkaline earth metal is preferably 2.5 mol% or less, particularly preferably 1.0 mol% or less, with respect to all the constituent metal elements of the lithium composite oxide.

  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 (15)

A method for producing a lithium composite oxide in which at least one alkaline earth metal is added to a base oxide represented by the following formula, including metal salts of all constituent metal elements of the lithium composite oxide, A step (A) of preparing an acidic metal salt solution;
Holding the metal salt solution at a temperature lower than the firing temperature of the step (C) to cause gelation (B);
The manufacturing method of lithium composite oxide which has the process (C) which bakes the gelled material obtained after the process (B).
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. .
0 <x <2, 0 <y ≦ 1)   The method for producing a lithium composite oxide according to claim 1, wherein in the step (A), the amount of the alkaline earth metal added to all the constituent metal elements of the lithium composite oxide is 2.5 mol% or less.   The method for producing a lithium composite oxide according to claim 2, wherein in the step (A), the amount of the alkaline earth metal added to all the constituent metal elements of the lithium composite oxide is 1.0 mol% or less.   The method for producing a lithium composite oxide according to any one of claims 1 to 3, wherein the pH of the metal salt solution is 3 or less in the step (A).   The method for producing a lithium composite oxide according to any one of claims 1 to 4, wherein in the step (B), the gelation temperature is 45 to 100 ° C.   The lithium composite oxide according to any one of claims 1 to 5, wherein the step (C) includes a step (C1) of preliminary firing at 450 to 750 ° C and a step (C2) of main firing at 750 to 900 ° C. Manufacturing method.   Between the step (C1) and the step (C2), there is a step (D) of pulverizing the temporarily fired product obtained after the step (C1) into particles, and the pulverized product obtained after the step (D) The method for producing a lithium composite oxide according to claim 6, wherein the step (C2) is performed.   The method for producing a lithium composite oxide according to claim 1, wherein the lithium composite oxide is for a positive electrode active material used in a lithium ion secondary battery.   A lithium composite oxide produced by the method for producing a lithium composite oxide according to claim 1. A lithium composite oxide in which at least one alkaline earth metal is added to a base oxide represented by the following formula,
The surface of the lithium composite oxide has a crystal phase containing the alkaline earth metal, which is different from the crystal phase mainly present in the lithium composite oxide, and the crystal phase of the lithium composite oxide A lithium composite oxide that exists in a substantially uniform distribution over the entire surface.
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. .
0 <x <2, 0 <y ≦ 1)   The lithium composite oxide according to claim 10, wherein an addition amount of the alkaline earth metal with respect to all constituent metal elements of the lithium composite oxide is 2.5 mol% or less.   The lithium composite oxide according to claim 11, wherein an addition amount of the alkaline earth metal with respect to all constituent metal elements of the lithium composite oxide is 1.0 mol% or less.   A lithium ion secondary battery using the lithium composite oxide according to any one of claims 9 to 12 as a positive electrode active material. Discharge capacity after one cycle of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 300 mA / g, and discharge after performing the following cycle charge / discharge The lithium ion secondary battery according to claim 13, wherein a capacity maintenance ratio obtained from a ratio with the capacity is 79.0% or more.
Cycle charge / discharge conditions: 3 cycles of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 15 mA / g, followed by the same temperature, same charge voltage, same discharge voltage And 50 cycles of charge and discharge under the condition of a current density of 300 mA / g. After performing the following cycle charging / discharging, from the discharge capacity after further charging / discharging one cycle under the conditions of 25 ° C., charging voltage 4.6 V, discharging voltage 2.5 V, and current density 50 mA / g, The lithium ion secondary battery according to claim 13 or 14, wherein a capacity recovery rate obtained by dividing a charge capacity of a final cycle of the following cycle charge / discharge is 58.0% or more.
Cycle charge / discharge conditions: 3 cycles of charge / discharge under the conditions of 25 ° C., charge voltage 4.6V, discharge voltage 2.5V, and current density 15 mA / g, followed by the same temperature, same charge voltage, same discharge voltage And 50 cycles of charge and discharge under the condition of a current density of 300 mA / g.