JP2011096655A - Cathode active material and lithium secondary battery using the active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery with excellent battery characteristics through restraint of reaction of Li of a cathode active material with aqueous solution, as well as a cathode active material suitable for manufacture of the above battery. <P>SOLUTION: For forming a cathode active material layer of the lithium secondary battery, the cathode active material is used for preparation of aqueous paste containing aqueous solvent and a cathode active material. The cathode active material is structured of complex oxide at least with Li and Ni as constituent metal elements, at least a part of the surface of which is covered with a water-soluble Li compound. A covering volume of the water-soluble Li compound is so stipulated that a ratio (m<SB>Li</SB>/M<SB>all</SB>) of the number of mols m<SB>Li</SB>of Li in the compound to the total number of mols M<SB>all</SB>of all the metal elements other than the Li constituting the complex oxide is to be 0.018 to 0.032 based on the neutralization titration method. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery and a method for producing the same. Furthermore, the present invention relates to a lithium secondary battery constructed using the positive electrode active material.

In recent years, secondary batteries such as lithium secondary batteries and nickel metal hydride batteries have become increasingly important as power sources mounted on vehicles using electricity as a drive source, or power sources mounted on personal computers, portable terminals, and other electrical products. . In particular, a lithium secondary battery (typically a lithium ion battery) that is lightweight and obtains a high energy density is expected to be preferably used as a high-output power source mounted on a vehicle. Patent documents 1-4 are mentioned as technical literature relevant to a lithium secondary battery.
In one typical configuration of this type of lithium secondary battery, an electrode active material layer (specifically, a positive electrode active material layer and a negative electrode capable of reversibly inserting and extracting lithium ions on the surface of the electrode current collector). Active material layer). For example, in the case of a positive electrode, a positive electrode active material composed of a composite oxide containing lithium and a transition metal such as nickel as constituent metal elements is dispersed in an appropriate solvent (for example, an aqueous solvent such as water or various organic solvents). A paste-like composition in a state (a paste-like composition includes a slurry-like composition. Hereinafter, this type of composition is simply referred to as a paste) is applied onto a positive electrode current collector. A positive electrode active material layer.

By the way, among the composite oxides constituting the positive electrode active material of the lithium secondary battery, a so-called lithium nickel-based composite oxide composed mainly of nickel: LiNi _1-y M _y O ₂ (M is an appropriate material other than Ni). Other one or more metal elements) have a theoretical lithium ion storage capacity larger than that of conventional lithium cobalt oxide and the like, and can reduce the amount of expensive metal materials such as cobalt used. In view of the above, it has attracted attention as a preferable positive electrode material in the production of a lithium secondary battery.
For example, Patent Document 1 discloses, as a conventional technique related to this type of positive electrode active material, lithium used to improve charge / discharge cycle characteristics by suppressing an increase in internal resistance and a decrease in discharge capacity of a lithium secondary battery. A lithium secondary battery is disclosed in which the specific surface area of a nickel-based positive electrode active material is limited and the surface of the active material is coated with a water-soluble polymer.

JP 2002-42817 A JP 2006-73482 A JP 2000-30893 A JP 2004-214187 A

  By the way, when preparing the paste for forming the positive electrode active material layer, it is attracting attention to use an aqueous solvent as a solvent. Pastes that use aqueous solvents (hereinafter also referred to as “aqueous pastes”) are more organic solvents and associated industrial waste than pastes that use organic solvents (hereinafter also referred to as “solvent pastes”). This is because there is an advantage that the environmental load can be reduced as a whole because there is no need for such a system, and there is no equipment and processing costs for that purpose.

However, when an aqueous paste is prepared using a lithium nickel composite oxide as a positive electrode active material, an aqueous solvent (typically water) and a lithium nickel composite oxide typically have the following reaction formula:
H ₂ O + LiNiO ₂ → NiOOH + Li ⁺ + OH ⁻ ;
There was a possibility that the reaction shown in FIG.
As is apparent from the above reaction formula, this reaction is unfavorable because, first of all, lithium ions flow out into the aqueous solvent, resulting in a decrease in battery capacity. Secondly, the conductivity of the positive electrode active material is lowered by the progress of the reaction, and the reaction resistance in the battery (in the positive electrode) is increased by repeated charge and discharge, which is not preferable. With regard to such a problem, the technique described in Patent Document 1 described above merely restricts the contact between the positive electrode active material and water to some extent, and thus does not constitute a fundamental solution. Further, the technique described in Patent Document 2 suppresses the elution of Mn, which is a constituent element of the positive electrode active material, in the electrolytic solution from the positive electrode active material layer formed using the solvent paste. It does not solve specific problems.

  Therefore, as described above, the present invention has a problem in manufacturing a lithium secondary battery using an aqueous paste for forming a positive electrode active material layer containing a lithium nickel composite oxide as a positive electrode active material and the aqueous paste. The purpose of this is to suppress the reaction between lithium of the positive electrode active material and an aqueous solvent (typically water), and to achieve excellent battery characteristics (for example, high-rate characteristics) Another object is to provide a lithium secondary battery having cycle characteristics. Another object of the present invention is to provide a positive electrode active material for an aqueous paste suitable for the production of such a lithium secondary battery and a method for producing the same.

To achieve the above object, according to the present invention, a positive electrode for preparing an aqueous paste used for preparing an aqueous paste containing an aqueous solvent and a positive electrode active material for forming a positive electrode active material layer of a lithium secondary battery. An active material is provided. The positive electrode active material disclosed here is composed of a composite oxide (lithium nickel composite oxide) having at least lithium and nickel as constituent metal elements. At least a part of the surface of the composite oxide is coated with a water-soluble lithium compound. Coverage of the water-soluble lithium compound, the total number of moles M _all of all metallic elements other than lithium constituting the moles m _Li and the composite oxide of lithium of the water-soluble lithium compound based on neutralization titration method Based on the following formula (a):
A = m _Li / M _all (a);
The molar ratio (A) of lithium represented by the formula is specified to be 0.018 to 0.032.

In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by movement of lithium ions between the positive and negative electrodes. In general, a secondary battery referred to as a lithium ion battery is a typical example included in the lithium secondary battery in this specification.
In this specification, the “positive electrode active material” means a positive electrode side capable of reversibly occluding and releasing (typically inserting and desorbing) chemical species (that is, lithium ions) serving as charge carriers in a secondary battery. Refers to the substance.

The positive electrode active material provided by the present invention is a positive electrode active material for preparing an aqueous paste for forming a positive electrode active material layer of a lithium secondary battery. When a positive electrode active material (lithium nickel composite oxide) is mixed with an aqueous solvent (typically water) to prepare a paste for forming a positive electrode active material layer, lithium as a constituent element of the positive electrode active material is added to the aqueous solvent. The ions may flow out, and the battery capacity of the lithium secondary battery constructed using the paste may decrease. In addition, when lithium ions in the positive electrode active material flow out, the conductivity (conductive path) of the positive electrode active material decreases, and the reaction resistance in the battery (in the positive electrode) may increase due to repeated charge and discharge. In order to solve such problems peculiar to aqueous pastes that may occur due to the reaction between the positive electrode active material and the aqueous solvent, the lithium nickel composite oxide disclosed herein (hereinafter sometimes simply referred to as “composite oxide”). In the positive electrode active material for water-based paste preparation constituted by (A), at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound having a coating amount defined above. Therefore, when the positive electrode active material and the aqueous solvent are mixed to prepare an aqueous paste, an event in which lithium ions flow out from the crystal of the composite oxide can be suppressed. Typically, the lithium ion of the water-soluble lithium compound coated above flows out (dissolves) in an aqueous solvent prior to the outflow of lithium ions from the crystal, so that the positive electrode active material originally has lithium ions. Can be prevented from flowing out. Thereby, since the aqueous paste containing the positive electrode active material with the high-density lithium held in the crystal is prepared, the possibility of reducing the battery capacity of the lithium secondary battery is eliminated.
When the coating amount of the water-soluble lithium compound is too smaller than 0.018 (molar ratio), the effect of suppressing the outflow of lithium ions from the crystal is reduced, and the reaction resistance in the battery is increased as described above. It can happen. On the other hand, when the coating amount is too larger than 0.032 (molar ratio), an aqueous paste exhibiting a high pH is prepared by dissolving a large amount of lithium ions in the aqueous paste. Such a high pH aqueous paste has a high viscosity, so that it is difficult to uniformly and thinly apply to the positive electrode current collector, resulting in a decrease in battery performance. Therefore, by using the positive electrode active material for water-based paste preparation coated with the above-mentioned amount of water-soluble lithium compound, the reaction between lithium in the composite oxide and the water-based solvent is suppressed, and as a result, excellent battery characteristics (cycles) Characteristics or high rate characteristics), in particular, a lithium secondary battery having good battery characteristics (low temperature output characteristics) under low temperature and high power use conditions.

  Here, the coating amount of the water-soluble lithium compound is obtained by dispersing the positive electrode active material for water-based paste preparation according to the present invention in water and adding barium chloride to the dispersion to remove carbonate ions as barium carbonate. The free alkali component can be determined by titration with hydrochloric acid. In the titration, the first neutralization point (inflection point) near pH 8 is typically determined using a potentiometer.

In a preferred embodiment of the positive electrode active material for aqueous paste preparation disclosed herein, the composite oxide constituting the positive electrode active material has the following general formula:
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
0 ≦ z ≦ 0.1,
Me is one or more elements that are not present or are selected from the group consisting of Al, Mn, Mg, Ti, Fe, Cu, Zn, and Ga. ).
The composite oxide has a lithium molar ratio x of 0.99 ≦ x ≦ 1.12 (typically 1 ≦ x ≦ 1.12), and contains an excess of lithium than the stoichiometric composition. It has a crystal structure to be obtained, and also contains nickel (Ni) as a constituent metal element other than lithium (Li). Furthermore, cobalt (Co) and / or Me element may be included as a minute content metal element. As described above, according to the present invention, even when an aqueous paste is prepared by using a composite oxide having a high nickel content as a positive electrode active material, an increase in internal resistance is suppressed and high capacity or high output is realized. An aqueous paste can be provided.

  In a preferred embodiment, the water-soluble lithium compound is lithium hydroxide. Lithium hydroxide is highly soluble in water and can be suitably employed as a starting material (lithium supply source of a raw material for producing a composite oxide described later) when producing the composite oxide. When the positive electrode active material in which the water-soluble lithium compound is coated with the composite oxide is mixed with an aqueous solvent to prepare an aqueous paste, the water-soluble lithium compound is first dissolved in the aqueous solvent. Therefore, the reaction between lithium in the complex oxide crystal and the aqueous solvent is suppressed, and the aqueous paste for forming the positive electrode active material layer can be prepared in a state where high-density lithium is retained in the crystal. As a result, a lithium secondary battery having excellent battery characteristics (cycle characteristics or low-temperature output characteristics) can be provided by using the positive electrode active material.

  By the way, the positive electrode active material having lithium hydroxide on the surface of the lithium nickel composite oxide often has lithium carbonate on the surface. When lithium carbonate is used as a raw material (Li source) of the positive electrode active material, a part of the lithium carbonate can remain, or it can be generated by reacting lithium hydroxide with carbon dioxide in the air. Therefore, if a predetermined amount of lithium hydroxide is intentionally made to exist on the surface of the composite oxide, the surface of the composite oxide may vary depending on conditions during manufacture and handling (storage, transportation, etc.) of the positive electrode active material. Relatively large amounts of lithium carbonate may be present. However, since lithium carbonate has low solubility in water, unlike lithium hydroxide, it can remain on the surface of the composite oxide even after preparation of the aqueous paste and increase internal resistance.

Therefore, in a preferred embodiment of the technology disclosed herein, in the positive electrode active material for preparing an aqueous paste, the amount of Li contained in a water-soluble lithium compound (typically lithium hydroxide) and the amount of Li contained in lithium carbonate. Defines the relationship. That is, the positive electrode active material according to the above aspect includes a lithium molar ratio (A),
Based on the carbon amount W _TC (mass%) based on the high-frequency combustion-infrared absorption method for the positive electrode active material and the total number of moles M _all of all metal elements other than lithium constituting the composite oxide, the following formula ( b):
B = W _TC × (1/12) × 2 ÷ M _all (b);
The molar ratio (B) of lithium contained in lithium carbonate (Li ₂ CO ₃ ) in an amount corresponding to W _TC calculated by the following relational expression (c):
0 <(1.2A-B) <0.03 (c);
It is characterized by satisfying. According to such a positive electrode active material, the effect of defining the coating amount of the water-soluble lithium compound (typically lithium hydroxide) within the preferable range disclosed herein can be more appropriately (more surely) exhibited. .

  In another preferable aspect of the technology disclosed herein, in the positive electrode active material for preparing an aqueous paste, the positive electrode active material has a form of secondary particles in which a plurality of primary particles are aggregated, and the primary particles are X-rays. The crystallite diameter d (Å) in the (003) plane direction determined by diffraction satisfies the following formula: 1300Å ≦ d ≦ 1500Å. Such a positive electrode active material can have a lower internal resistance than a positive electrode active material in which the primary crystallite size is larger. In general, when the SOC (State of Charge) is relatively low (for example, when the SOC is about 20%), the internal resistance is higher than when the SOC is higher (for example, about 60 to 80%). When the positive electrode active material satisfies the above crystallite size, it greatly contributes to the reduction of internal resistance in the low SOC region. Therefore, according to such a positive electrode active material, a lithium secondary battery that exhibits good output in a wider SOC range can be constructed.

Moreover, this invention provides the method of manufacturing the positive electrode active material for lithium secondary batteries comprised by the composite oxide which uses lithium and nickel as a constituent metal element as another side surface which implement | achieves the said objective. In the production method disclosed herein, a raw material for producing a composite oxide including a lithium source and a nickel source is obtained by combining the number of moles of lithium M _Li and the total number of moles M _all of all other constituent metal elements. By preparing an excess of lithium such that the molar ratio (M _Li / M _all ) is 1 <M _Li / M _all ; and firing the composite oxide-prepared raw material for preparing the composite oxide Generating a positive electrode active material comprising an oxide, wherein at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound.
Here, the amount of the lithium source used in preparing the raw material for producing the composite oxide is such that the coating amount of the water-soluble lithium compound is the molar amount of lithium in the water-soluble lithium compound based on the neutralization titration method. Based on the number m _Li and the total mole number M _all of all metal elements other than lithium constituting the composite oxide, the following formula (a):
A = m _Li / M _all (a);
The molar ratio (A) of lithium represented by is defined to be 0.018 to 0.032.
According to such a production method, lithium and all other constituent metal elements constituting the raw material for producing the composite oxide (the constituent metal elements here include nickel, cobalt, and Me element in the formula (1) described later) When the molar composition ratio (M _Li / M _all ) exceeds 1), a positive electrode active material in which at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound can be obtained. Further, in the production method disclosed herein, the amount of the lithium supply source is adjusted so that the molar ratio (A) is 0.018 to 0.032. As a result, when preparing an aqueous paste using the positive electrode active material obtained by such a manufacturing method, lithium ions of the water-soluble lithium compound coated on the surface of the composite oxide are first dissolved in the aqueous solvent, The outflow of lithium ions from the complex oxide crystal is minimized. As described above, since the reaction between lithium in the composite oxide and the aqueous solvent at the time of preparing the aqueous paste is suppressed, the battery capacity of the lithium secondary battery constructed using the paste is not reduced. Therefore, according to the technology disclosed herein, a positive electrode active material useful as a constituent material of a lithium secondary battery having excellent battery characteristics (cycle characteristics or high rate characteristics), particularly good output characteristics under low temperature and high power use conditions Can be provided.

In a preferred embodiment of the positive electrode active material disclosed herein, the raw material for producing the composite oxide is represented by the general formula:
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
0 ≦ z ≦ 0.1,
Me is one or more elements that are not present or are selected from the group consisting of Al, Mn, Mg, Ti, Fe, Cu, Zn, and Ga. ) Including all the constituent metal element sources so as to produce a composite oxide as shown in FIG.
The molar ratio x of lithium in the composite oxide is 0.99 ≦ x ≦ 1.12 (typically 1 ≦ x ≦ 1.12, such as 1.00 <x ≦ 1.12), and the chemical A cathode active material in which at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound is prepared by preparing the raw material for producing the composite oxide so that an excess amount of lithium is included rather than a stoichiometric composition. Can be generated. In addition, by providing the above raw material so that the content ratio of nickel is high, a positive electrode active material having high conductivity can be obtained, and thus a lithium secondary battery capable of realizing high capacity or high output is provided. be able to.

  In a preferred embodiment, the raw material for producing the composite oxide is prepared such that the water-soluble lithium compound that covers the surface of the composite oxide is lithium hydroxide. Lithium hydroxide has high solubility in water and can serve as a lithium supply source of the lithium element constituting the composite oxide. Therefore, the raw material for producing the composite oxide is prepared using a water-soluble lithium compound that covers at least a part of the surface of the composite oxide and lithium hydroxide that can be a lithium element in the composite oxide as a lithium supply source. Thus, it is possible to provide a positive electrode active material for a lithium secondary battery capable of realizing high capacity or high output.

  According to the present invention, there is provided an aqueous paste containing any positive electrode active material disclosed herein (can be a positive electrode active material obtained by any of the production methods disclosed herein) and an aqueous solvent. Provided. In the aqueous paste provided by the present invention, a water-soluble lithium compound (typically lithium ion) covering at least a part of the surface of the composite oxide constituting the positive electrode active material is dissolved. Therefore, since the outflow of lithium ions from the crystal of the composite oxide is minimized, the decrease in the battery capacity of the lithium secondary battery constructed using the aqueous paste can be effectively suppressed. . In addition, in the present invention, an excessive amount of lithium ions does not dissolve into the aqueous paste as in the prior art, so that an appropriate viscosity (the viscosity of the paste here depends on pH, and the higher the pH paste, the more viscous the It can be applied to the surface of the positive electrode current collector in a thin, uniform and good state. Thereby, the aqueous paste for constructing a lithium secondary battery having excellent battery characteristics (cycle characteristics or low-temperature output characteristics) with reduced reaction resistance can be provided.

In addition, according to the present invention, there is provided a lithium secondary battery comprising a positive electrode active material layer formed using the aqueous paste disclosed herein on a positive electrode current collector.
Furthermore, according to this invention, a vehicle provided with the lithium secondary battery disclosed here is provided.
As described above, the lithium secondary battery provided by the present invention exhibits battery characteristics (cycle characteristics or high rate characteristics) particularly suitable as a power source mounted on a vehicle, particularly good output characteristics under conditions of low temperature and high power use. Can be a thing. Therefore, the lithium secondary battery disclosed herein can be suitably used as a power source for a motor (electric motor) mounted on a vehicle such as an automobile equipped with an electric motor such as a hybrid vehicle or an electric vehicle.

It is a perspective view which shows typically the external shape of the lithium secondary battery which concerns on one Embodiment. It is the II-II sectional view taken on the line in FIG. It is a graph which shows the relationship between the coating amount (molar ratio (A)) of a water-soluble lithium compound, and the viscosity of an aqueous paste. It is a graph which shows the relationship between the coating amount (molar ratio (A)) of a water-soluble lithium compound, and a low temperature output. It is a graph which shows the relationship between the coating amount (molar ratio (A)) of a water-soluble lithium compound, and an internal resistance increase rate. It is a side view showing typically a vehicle (automobile) provided with a lithium secondary battery concerning one embodiment. It is a schematic diagram for demonstrating the behavior of the water-soluble lithium compound before and after the preparation of the aqueous paste. It is a graph which shows the relationship between the molar ratio (C) of lithium, and an internal resistance value. It is a graph which shows the relationship between SOC and an internal resistance value.

Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, the configuration and manufacturing method of an electrode body including a positive electrode and a negative electrode, the configuration and manufacturing of a separator and an electrolyte) General techniques related to the construction of a method, a lithium secondary battery, and other batteries) can be understood as design matters of a person skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.
In addition, in the following drawings, the same code | symbol is attached | subjected to the member and site | part which show | plays the same effect | action, and the overlapping description may be abbreviate | omitted or simplified. In addition, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships.

  The positive electrode active material provided by the present invention is a positive electrode for preparing an aqueous paste used to prepare an aqueous paste containing an aqueous solvent and a positive electrode active material for forming a positive electrode active material layer of a lithium secondary battery. It is an active material and is composed of a composite oxide containing at least lithium and nickel as constituent metal elements. Hereinafter, although the said positive electrode active material and the manufacturing method of this active material are demonstrated in detail, it is not intending limiting this invention to this embodiment.

First, the positive electrode active material for preparing an aqueous paste according to this embodiment will be described.
The positive electrode active material disclosed here is composed of a composite oxide containing at least lithium and nickel as constituent metal elements. At least a part of the surface of the composite oxide is coated with a water-soluble lithium compound. The so-called lithium-nickel composite oxide containing lithium and nickel as main constituent metal elements has a larger theoretical lithium ion storage capacity and higher energy density than lithium cobalt-based composite oxides such as lithium cobaltate. Has attracted attention as a thing. On the other hand, when preparing a water-based paste containing a water-based solvent (typically water) and a positive electrode active material for forming a positive electrode active material layer of a lithium secondary battery, the lithium nickel-based composite oxide is a water-based solvent. And the composite oxide typically has the following reaction formula:
H ₂ O + LiNiO ₂ → NiOOH + Li ⁺ + OH ⁻
Therefore, the lithium ion, which is a constituent element of the composite oxide, flows out into the aqueous solvent, the battery capacity of the lithium secondary battery is reduced, and the conductivity (conductive path) of the positive electrode active material is reduced. There is a possibility that the reaction resistance in the battery (in the positive electrode) increases and the reaction resistance increases.
However, in the positive electrode active material for preparing an aqueous paste disclosed here, since at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound, lithium ions are generated from the crystal of the composite oxide. Outflow is suppressed. Typically, the lithium ion of the coated water-soluble lithium compound flows out (dissolves) in the aqueous solvent prior to the outflow, and thus contains a complex oxide that retains a high density of lithium. An aqueous paste is prepared.

  The aqueous solvent used for the preparation of the aqueous paste is typically water, but may be any as long as it exhibits aqueous properties as a whole. For example, an aqueous solution containing a lower alcohol (methanol, ethanol, etc.) can be preferably used. That is, water or a mixed solvent mainly composed of water can be preferably used. As the solvent other than water constituting the mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. For example, a solvent in which about 80% by mass or more (more preferably about 90% by mass or more, more preferably about 95% by mass or more) of the aqueous solvent is water is preferable. A particularly preferred example is a solvent consisting essentially of water.

Examples of the water-soluble lithium compound that covers the surface of the composite oxide include lithium hydroxide, lithium carbonate, lithium sulfate, lithium nitrate, lithium peroxide, lithium phosphate, lithium oxalate, lithium acetate, and lithium iodide. some of crystal water, such as lithium hydroxide monohydrate (LiOH · _H 2 O) and the like. Further, lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ) and lithium sulfate (Li ₂ SO ₄ ) having high solubility in water are preferable, and lithium hydroxide is particularly preferable. In addition, the water-soluble lithium compound in the technique disclosed here may be any one of these, or may be two or more.

The coating amount of the water-soluble lithium compound is the number of moles of lithium m _Li in the water-soluble lithium compound based on the neutralization titration method and all the metal elements other than lithium constituting the composite oxide (positive electrode active material). the molar ratio between the total number of moles M _all of _{(a) = m Li / M} all is preferably 0.018 to 0.032, particularly preferably from 0.018 to 0.03, for example from 0.018 to 0.028 It is prescribed to become. In a preferred embodiment, the neutralization titration method is performed as follows. That is, a predetermined amount of a positive electrode active material is dispersed in water, and barium chloride is added thereto to remove carbonate ions as barium carbonate, followed by neutralization titration with hydrochloric acid to obtain a first neutralization point (inflection) near pH 8. The amount of the water-soluble lithium compound that covers the surface of the predetermined amount of the composite oxide is determined as the coating amount α (mass%) in terms of Li contained in the compound by determining the titration amount up to (point). Can do. Specifically, α can be obtained using the following equation (2).
α (mass%) = (D × F × K × M ÷ S) / 10 (2)
D: hydrochloric acid titration to the first neutralization point (mL)
F: Factor of hydrochloric acid
K: atomic weight of lithium (6.941 g / mol)
M: molar concentration of hydrochloric acid (mol / L)
S: Mass of positive electrode active material (g)
The number of moles _Li of lithium in the water-soluble lithium compound is represented by α / K. Therefore, the molar ratio (A) can be calculated from (α / K) / M _all . The positive electrode active material having a molar ratio (A) of 0.018 to 0.032 thus obtained suppresses the reaction between lithium in the composite oxide and the aqueous solvent, and thus has excellent battery characteristics (cycles). Characteristic or high rate characteristic), in particular, a positive electrode active material for a lithium secondary battery having good output characteristics under low temperature and high output use conditions.

  The neutralization titration method is preferably carried out with the time for dispersing the positive electrode active material in water (the time for eluting the water-soluble lithium compound) being substantially the same. As a result, the amount of coating can be grasped more accurately. In a preferred embodiment, the water dispersion time is set to a substantially constant time (for example, 180 seconds) set between about 120 seconds to 300 seconds. As a method for easily managing the water dispersion time, a method of separating the positive electrode active material from water by filtration or the like after a predetermined time (water dispersion time) has elapsed since the positive electrode active material was dispersed in water is exemplified. . Thereafter, barium chloride is added to the remaining water (filtrate), followed by neutralization titration.

In a preferred embodiment of the positive electrode active material for preparing an aqueous paste disclosed herein, in the positive electrode active material, the molar ratio of lithium (A),
The amount of carbon W _TC (mass%) based on the high-frequency combustion-infrared absorption method for the positive electrode active material and the total number of moles M _all of all metal elements other than lithium constituting the composite oxide (positive electrode active material) Based on the following formula (b):
B = W _TC × (1/12) × 2 ÷ M _all (b);
The molar ratio (B) of lithium contained in Li ₂ CO ₃ in an amount corresponding to W _TC calculated by the following relational expression (c):
0 <(1.2A-B) <0.03 (c);
Meet. Hereinafter, the above 1.2A-B (molar ratio) may be referred to as “Li amount (C)”. In addition, 12 in the said Formula (b) represents the atomic weight of carbon.

In a typical embodiment of the technology disclosed herein, most (possibly all) of the alkali components titrated with hydrochloric acid in the neutralization titration method are derived from LiOH on the surface of the composite compound. In this case, the molar ratio of Li (A) can be understood as a value indicating the ratio of "moles of LiOH complex compound surface" to moles M _all of the total metal elements other than Li contained in the positive electrode active material.

On the other hand, the molar ratio of Li (B) shows the ratio of "the number of moles of Li ₂ CO ₃ complex compound surface (Li conversion)" to moles M _all of the total metal elements other than Li contained in the positive electrode active material It can be grasped as a value. This molar ratio (B) is typically calculated from the amount of carbon W _TC (mass%) measured by the high-frequency combustion-infrared absorption method defined in JIS Z2615 (general rules for carbon determination of metallic materials). The Specifically, the carbon content W _TC measurement method described in Examples described later can be preferably employed. By applying the above formula (b) to this carbon amount W _TC , the Li amount (B) (molar ratio) corresponding to W _TC (mass%) can be obtained.

In a preferable embodiment of the positive electrode active material for preparing an aqueous paste disclosed herein, the Li amount (C) (molar ratio) defined by “1.2A-B” satisfies 0 <C <0.03. It is prescribed. A positive electrode active material in which C satisfies 0.003 ≦ C ≦ 0.02 (for example, 0.01 <C <0.02) is more preferable. In addition to the above-mentioned molar ratio (A) satisfying a predetermined numerical range, by defining the relationship between A and B within the above range, an appropriate amount of LiOH (typically 0.018 ≦ A ≦ 0.032), and in consideration of the amount of LiOH, the amount of Li ₂ CO ₃ that can reduce battery performance (for example, output performance) was suppressed to an appropriate range. A positive electrode active material for preparing an aqueous paste can be realized. Further, in the positive electrode active material in which C is in the above numerical range, for example, as shown in FIG. 8, a strong correlation is observed between C and the internal resistance value. Therefore, by managing the value of C in the positive electrode active material, it is possible to accurately manage the internal resistance of a lithium secondary battery manufactured using an aqueous paste prepared from the positive electrode active material.

The significance of defining the Li amount (C) (molar ratio) in the predetermined range as described above will be described more specifically with reference to the schematic diagram shown in FIG. That is, the positive electrode active material obtained by firing the composite oxide production raw material adjusted so as to satisfy 1 <M _Li / M _all is shown in FIG. 7A when only LiOH is used as the Li source, for example. As described above, even if most of the water-soluble lithium compound 84 covering the surface of the composite oxide 82 is LiOH, and a part of LiOH contains Li ₂ CO ₃ generated by reaction with CO ₂ in the environment, It is desirable to keep the amount small. The fact that the amount of Li (C) is in the preferred range described above is related to the state in which the positive electrode active material 80 is imaged from FIG. By adjusting the positive electrode active material 80 in such a state to an aqueous paste, as shown in FIG. 7B, LiOH dissolves in water, the surface of the composite oxide 82 is exposed, and lithium ions enter and exit. A suitable surface condition is provided. Therefore, a lithium secondary battery constructed using such a positive electrode active material 80 can have an internal resistance effectively reduced.

On the other hand, the positive electrode active material whose Li amount (C) (molar ratio) is too smaller than the above-described preferable range is associated with the positive electrode active material 180 imaged from FIG. For example, even when Li ₂ CO ₃ is used as part or all of the Li source, or when only LiOH is used as the Li source, part or all of LiOH is CO in the environment depending on the storage atmosphere and storage time. _By reacting with ₂ to produce Li ₂ CO ₃ , a positive electrode active material 180 as shown in FIG. 7C can be obtained. When the positive electrode active material 180 shown in FIG. 7C is adjusted to an aqueous paste as shown in FIG. 7D, LiOH is dissolved in water, but Li ₂ CO ₃ having low solubility in water is Most of them remain on the surface of the composite oxide 82, which may be a factor that hinders the reaction between the electrolyte and the composite oxide (lithium ion insertion / desorption reaction). Therefore, a lithium secondary battery constructed using such a positive electrode active material 180 tends to have a high internal resistance. In addition, when the amount of Li (C) is too larger than the preferable range mentioned above, the viscosity may become too high when preparing an aqueous paste.

In a preferred embodiment of the technology disclosed herein, the carbon amount W _TC satisfies W _TC ≦ 0.13 mass% (typically W _TC <0.13 mass%). More preferably, W _TC ≦ 0.12% by mass. According to such a positive electrode active material for preparing an aqueous paste, a lithium secondary battery having lower internal resistance (and thus excellent output performance) can be constructed. In a preferred embodiment, the Li source is included in a molar number of 1.04 to 1.12 times the stoichiometric amount (in other words, 1.04 ≦ M _Li / M _all ≦ 1.12). And so on) adjusting the raw material for producing the composite oxide. Usually, a preferable result can be realized by setting 1.04 ≦ M _Li / M _all ≦ 1.08.

The molar ratios (A), (B), and (C) are, for example, the type of Li source to be used, the value of M _Li / M _{all in} the raw material for producing the composite oxide, firing conditions, cooling conditions, and storage conditions. Etc. can be adjusted. Specific contents of the firing, cooling, and storage conditions include temperature, time, atmosphere (for example, CO ₂ concentration and humidity), and the like.

Furthermore, as a preferable embodiment of the composite oxide constituting the positive electrode active material, the following general formula:
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
0 ≦ z ≦ 0.1,
Me is absent or from the group consisting of aluminum (Al), manganese (Mn), magnesium (Mg), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn) and gallium (Ga). One or more elements selected. ).
That is, the molar ratio x of lithium (Li) in the composite oxide is 0.99 ≦ x ≦ 1.12 (typically 1 ≦ x ≦ 1.12), which is more than the stoichiometric composition. The positive electrode active material is composed of a composite oxide that has a crystal structure that can contain lithium and has a high lithium content. Moreover, nickel (Ni) may be included as a constituent metal element other than lithium, and cobalt (Co) and / or Me element may be included as a minute metal element. Further, the molar ratio of nickel is larger than the molar ratio of cobalt or Me element, or the total molar ratio of cobalt and Me element, and the content molar ratio is higher next to lithium.

  The element represented by Me in the above formula (1) does not exist or is aluminum (Al), manganese (Mn), magnesium (Mg), titanium (Ti), iron (Fe), copper (Cu). Zinc (Zn) and gallium (Ga) are preferable, and Al is a preferable example. Further, the Me element may be one or more elements selected from the above-described element group. However, when two or more kinds of Me elements are selected, z indicating the total number of moles of each element needs to satisfy 0 ≦ z ≦ 0.1. By containing these elements in an appropriate molar ratio, the conductivity of the positive electrode active material can be improved, and as a result, an increase in internal resistance of the lithium secondary battery can be suppressed.

  The positive electrode active material for preparing an aqueous paste disclosed herein typically has a form of secondary particles in which a plurality of primary particles are aggregated. In a preferred embodiment, the primary particles have a crystallite diameter d (Å) in the (003) plane direction determined by X-ray diffraction (for example, X-ray diffraction using CuKα rays) of 1300Å ≦ d ≦ 1500Å (typical Satisfies the following condition: 1300Å <d ≦ 1500Å The crystallite diameter d may be a value obtained from the half-value width of the diffraction peak corresponding to the (003) plane according to the Sherler formula. May have a lower internal resistance than a positive electrode active material having a larger primary particle crystallite diameter, and the fact that the positive electrode active material satisfies the above crystallite diameter particularly reduces the internal resistance in a low SOC region. Therefore, according to such a positive electrode active material, excellent output performance is exhibited in a wider SOC range (in other words, internal resistance or S of the output performance). Lithium secondary battery can be constructed with less C dependence.From the viewpoint of the effect of reducing internal resistance in a low SOC range (for example, SOC 20%), d is preferably 1350１３ ≦ d ≦ 1500Å, more preferably 1400 Å ≦ d ≦ 1500 例 え ば (for example, 1400 Å <d ≦ 1500 Å).

The crystallite diameter d can be adjusted by various methods. For example, as a method of increasing d: M _Li / M _all in the composite oxide production raw material disclosed herein is made larger; the firing temperature of the composite oxide production raw material is made higher; Examples of such a method include making the firing time of the raw material for manufacturing a product longer. These methods can be employed alone or in appropriate combination.

  According to the study by the present inventors, the reduction of internal resistance by defining one or both of the molar ratios (A) and (C) within the preferred range disclosed herein is from the low SOC region to the middle. It was confirmed that the same effect was exhibited up to the high SOC range. Therefore, by applying one or both of the rules A and C and the crystallite diameter d in combination, as a synergistic effect by such a combination, a wider SOC range (in particular, a wider SOC range on the low SOC side). ), And a positive electrode active material for adjusting an aqueous paste suitable for manufacturing the battery can be provided.

Next, a method for producing a positive electrode active material for a lithium secondary battery composed of a composite oxide containing at least lithium and nickel as constituent metal elements will be described in detail.
The manufacturing method disclosed herein includes the following steps:
(1) A step of preparing a raw material for producing a composite oxide including a lithium supply source and a nickel supply source, and (2) a step of firing the raw material for producing the composite oxide.

First, a raw material for producing a composite oxide including a lithium supply source and a nickel supply source for supplying main constituent elements of the composite oxide is prepared. One preferred embodiment of the composite oxide is a composite oxide represented by the following formula (1).
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
0 ≦ z ≦ 0.1,
Here, Me is one or more elements that are not present or are selected from the group consisting of Al, Mn, Mg, Ti, Fe, Cu, Zn, and Ga. )
And the composite oxide shown by said Formula (1) is mixed by the molar ratio set so that it might be manufactured by baking, and the raw material for composite oxide manufacture which consists of the supply source of all the structural metal elements is prepared. .

As raw materials for producing the composite oxide, a lithium supply source and a nickel supply source that are essential constituent elements, and a cobalt supply source and a Me element supply source that are selective constituent elements are prepared.
As the compound including the lithium supply source, a material in which the surface of the positive electrode active material to be produced is coated with a water-soluble lithium compound is used. For example, lithium hydroxide, lithium carbonate, lithium sulfate, lithium nitrate, lithium peroxide, lithium phosphate, lithium oxalate, lithium acetate and lithium iodide, or lithium hydroxide monohydrate (LiOH · H ₂ O ) With crystal water such as Among these, lithium hydroxide (LiOH), lithium carbonate (Li ₂ CO ₃ ) or lithium sulfate (Li ₂ SO ₄ ) is preferable, and lithium hydroxide is particularly preferable. In addition, you may use these individually by 1 type or in mixture of 2 or more types.

  In addition, as a compound including a nickel supply source, a compound such as a hydroxide, oxide, sulfide, various salts (for example, carbonate), halide (for example, fluoride) having nickel as a constituent element is selected. obtain. Examples thereof include nickel carbonate, nickel oxide, nickel sulfate, nickel nitrate, nickel hydroxide, nickel oxalate, and nickel oxyhydroxide.

  Furthermore, although it is not an essential constituent element, it may contain cobalt as a constituent element. As a compound including such a cobalt supply source, a compound such as hydroxide, oxide, sulfide, various salts (for example, carbonate), halide (for example, fluoride) having cobalt as a constituent element can be selected. . Although it does not specifically limit, For example, cobalt carbonate, cobalt oxide, cobalt sulfate, cobalt nitrate, cobalt hydroxide, cobalt oxyhydroxide etc. are mentioned.

  Preferably, a hydroxide containing the constituent elements to be supplied (nickel and cobalt) can be used instead of preparing the nickel source and the cobalt source as separate compounds. Such a hydroxide is obtained by mixing two or more kinds of constituent elements by mixing each compound in a predetermined molar ratio and reducing the raw material compound under an alkaline condition, preferably in a non-oxidizing atmosphere such as an inert gas atmosphere. A hydroxide containing can be produced. By using the hydroxide thus produced, it is well mixed with the lithium source and promotes crystal growth.

  Further, the lithium nickel-based composite oxide disclosed herein may contain cobalt as a constituent metal element other than lithium and nickel, but a part of these metal elements is substituted, and Me in the above formula (1) 1 represented by the group consisting of aluminum (Al), manganese (Mn), magnesium (Mg), titanium (Ti), iron (Fe), copper (Cu), zinc (Zn) and gallium (Ga) It may contain seeds or two or more metal elements. The molar composition ratio z of the Me element is a number that satisfies 0 ≦ z ≦ 0.1. That is, the Me element does not exist or is contained in a molar ratio smaller than any main constituent metal element (lithium, nickel). Among these, addition of an aluminum supply source containing Al as a constituent element is particularly preferable. By adding these elements at an appropriate molar ratio, the conductivity of the positive electrode active material can be improved.

Next, the above-prepared supply sources of the constituent elements are weighed and mixed to prepare a composite oxide manufacturing raw material. At this time, each of the above-mentioned supply sources is adjusted (weighed) so as to have a molar ratio in the above formula (1) to prepare a composite oxide production raw material. Preferably, the lithium is such that the molar ratio (M _Li / M _all ) between the number of moles of lithium M _Li and the total number of moles of all other constituent metal elements (M _all ) is 1 <M _Li / M _all Prepare in excess. Further, preferably, the lithium source is added so that the molar ratio (A) of Li is 0.018 to 0.032. As a result, a positive electrode active material containing a lithium in an amount exceeding the stoichiometric composition in the crystal structure of the composite oxide and having at least a part of the surface of the composite oxide coated with a water-soluble lithium compound is obtained. be able to.

  In addition, when mixing each supply source and preparing the raw material for complex oxide manufacture, stirring (a kneading | mixing and a grinding | pulverization) can be performed as needed. The apparatus used for mixing is not particularly limited. For example, by using a planetary mixer, a planetary stirrer, a desper, a ball mill, a kneader, a mixer, etc., each supply source is uniformly diffused or permeated and stabilized. A mixed state can be formed.

After various raw material compounds are mixed to prepare a raw material for producing a complex oxide, the raw material for producing a complex oxide is calcined at an appropriate temperature. The firing is desirably performed in an oxidizing atmosphere, for example, in the air or in an atmosphere richer in oxygen than air. Moreover, although a calcination temperature is not specifically limited, It is preferable to determine the maximum calcination temperature within the range of 650 degreeC or more and 850 degrees C or less in an oxidizing atmosphere. More preferably, the prepared composite oxide production raw material is first calcined at an intermediate firing temperature set in a temperature range of 400 ° C. or higher and lower than 500 ° C., and then set in a temperature range of 650 ° C. or higher and 850 ° C. or lower. The temperature is raised to the highest firing temperature and fired. By pre-baking in such a temperature range, the rapid growth of the crystal is suppressed, and further by raising the temperature and baking, a composite oxide containing lithium in excess of the stoichiometric composition in the crystal structure, A positive electrode active material in which at least a part of the surface is coated with a water-soluble lithium compound is obtained.
In addition, although it does not specifically limit about baking time, After heating up to the set maximum baking temperature, it is preferable to bake for about 5 to 24 hours (for example, 7 hours) in this temperature range. Further, after firing, it is preferable to block the contact with water as much as possible and take residual heat in a low humidity environment.

In the positive electrode active material for a lithium secondary battery obtained by the manufacturing method according to this embodiment, at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound. Such a coating amount can be easily confirmed by a conventionally known neutralization titration method. For example, as an example of the neutralization titration method, the positive electrode active material produced above is dispersed in water, and barium chloride is added thereto to remove carbonate ions as sparingly soluble barium carbonate. By performing Japanese titration and determining the neutralization point using a potentiometer, the coating amount α (% by mass) in terms of Li of the water-soluble lithium compound can be determined. Specifically, it can be obtained from the following equation.
α (mass%) = (D × F × K × M ÷ S) / 10 (2)
D: Hydrochloric acid titration (mL)
F: Factor of hydrochloric acid K: Atomic weight of lithium (6.941 g / mol)
M: molar concentration of hydrochloric acid (mol / L)
S: Mass of positive electrode active material (g)
Using the above α, the coating amount in terms of Li (molar ratio (A)) of the water-soluble lithium compound represented by A = (α / K) / M _all (ie, A = m _Li / M _all ) The positive electrode active material, preferably 0.018 to 0.032, particularly preferably 0.018 to 0.03, for example 0.018 to 0.028, is a reaction between lithium in the composite oxide and an aqueous solvent. Therefore, a positive electrode active material for a lithium secondary battery having excellent battery characteristics (cycle characteristics or high-rate characteristics), in particular, good output characteristics under low temperature and high power use conditions is provided.

  Hereinafter, an aqueous paste using a positive electrode active material for a lithium secondary battery (typically a positive electrode active material for preparing an aqueous paste) obtained by the production method disclosed herein, a positive electrode using the paste, and the positive electrode An embodiment of a lithium secondary battery including the above will be described with reference to the schematic diagrams shown in FIGS. 1 and 2, but the present invention is not intended to be limited to such an embodiment.

  FIG. 1 is a perspective view schematically showing a rectangular lithium secondary battery according to an embodiment, and FIG. 2 is a cross-sectional view taken along line II-II in FIG. As shown in FIGS. 1 and 2, the lithium secondary battery 100 according to the present embodiment includes a rectangular parallelepiped battery case 10 and a lid body 14 that closes the opening 12 of the case 10. A flat electrode body (wound electrode body 20) and an electrolyte can be accommodated in the battery case 10 through the opening 12. The lid 14 is provided with a positive terminal 38 and a negative terminal 48 for external connection, and a part of the terminals 38 and 48 protrudes to the surface side of the lid 14. Also, some of the external terminals 38 and 48 are connected to the internal positive terminal 37 or the internal negative terminal 47, respectively, inside the case.

  As shown in FIG. 2, in this embodiment, a wound electrode body 20 is accommodated in the case 10. The electrode body 20 includes a positive electrode sheet 30 in which a positive electrode active material layer 34 is formed on the surface of a long sheet-like positive electrode current collector 32, and a negative electrode active material layer 44 on the surface of a long sheet-like negative electrode current collector 42. Are formed of a negative electrode sheet 40 on which is formed, and a long sheet-like separator 50. Then, the positive electrode sheet 30 and the negative electrode sheet 40 are overlapped and wound together with the two separators 50, and the obtained wound electrode body 20 is formed into a flat shape by crushing and ablating from the side surface direction.

  Further, in the wound positive electrode sheet 30, one end portion 35 along the longitudinal direction is a portion where the positive electrode current collector 32 is exposed without forming the positive electrode active material layer 34 (positive electrode active material layer non-formation portion 36). In the negative electrode sheet 40 that is wound, one end 45 along the longitudinal direction is a portion where the negative electrode current collector 42 is exposed without the negative electrode active material layer 44 being formed (negative electrode active material layer non-forming). Part 46). An internal positive electrode terminal 37 is joined to the positive electrode active material layer non-forming portion 36 of the positive electrode current collector 32, and an internal negative electrode terminal 47 is joined to the exposed end portion of the negative electrode current collector 42 to form the flat shape. The wound electrode body 20 is electrically connected to the positive electrode sheet 30 or the negative electrode sheet 40. The positive and negative electrode terminals 37 and 47 and the positive and negative electrode current collectors 32 and 42 can be joined by, for example, ultrasonic welding, resistance welding, or the like.

  The positive electrode (typically, the positive electrode sheet 30) has a configuration in which a positive electrode active material layer 34 containing a positive electrode active material is formed on a long positive electrode current collector 32. As the positive electrode current collector 32, a conductive member made of a highly conductive metal is preferably used. For example, aluminum or an alloy containing aluminum as a main component can be used. The shape of the positive electrode current collector 32 may vary depending on the shape of the lithium secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape.

Next, a method for preparing an aqueous paste for forming the positive electrode active material layer 34 will be described.
The aqueous paste disclosed herein includes an aqueous solvent and a positive electrode active material comprising a lithium nickel composite oxide in which at least a part of the surface obtained by using the above-described production method is coated with a water-soluble lithium compound. In addition to such materials, one or two or more binders, conductive materials, and the like that can be blended in a general lithium secondary battery can be included as necessary.

  First, as the conductive material, a conductive powder material such as carbon powder or carbon fiber is preferably used. For example, acetylene black, furnace black, ketjen black, graphite powder and the like are preferable, and only one of these may be used or two or more may be used in combination.

  The binder is preferably a water-soluble polymer material that dissolves in water. Examples thereof include carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), hydroxypropylmethylcellulose phthalate (HPMCP), and polyvinyl alcohol (PVA). In addition, a polymer material that is dispersed in water (water-dispersible) can be suitably used. For example, polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE) ), Etc., rubbers such as vinyl acetate copolymer, styrene butadiene block copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), and gum arabic.

  The aqueous paste disclosed herein is obtained by adding the positive electrode active material according to the present invention, the above-exemplified conductive material, and an additive such as a binder to an appropriate aqueous solvent, and dispersing or dissolving the mixture. Can be prepared. In the aqueous paste thus prepared, a water-soluble lithium compound (typically lithium ion) covering at least a part of the surface of the composite oxide constituting the positive electrode active material is dissolved. Therefore, elution of lithium ions from the composite oxide is minimized, and a decrease in battery capacity of a lithium secondary battery constructed using the aqueous paste is suppressed. Furthermore, since an excessive amount of lithium ions does not dissolve into the aqueous paste as in the past, it has an appropriate viscosity (viscosity-dependent viscosity, and usually the higher the pH paste, the higher the viscosity). It can be applied to the surface of the positive electrode current collector in a good state.

Then, the prepared aqueous paste is applied to the positive electrode current collector 32, the aqueous solvent is volatilized and dried, and then compressed (pressed). As a result, a positive electrode for a lithium secondary battery having the positive electrode active material layer 34 formed using the aqueous paste on the positive electrode current collector 32 is obtained.
Here, as a method of applying the aqueous paste to the positive electrode current collector 32, a technique similar to a conventionally known method can be appropriately employed. For example, the aqueous paste can be suitably applied to the positive electrode current collector 32 by using an appropriate application device such as a slit coater, a die coater, a gravure coater, or a comma coater. Moreover, when drying a solvent, it can dry favorably by using natural drying, a hot air, low-humidity air, a vacuum, infrared rays, far-infrared rays, and an electron beam individually or in combination. Furthermore, as a compression method, a conventionally known compression method such as a roll press method or a flat plate press method can be employed. In adjusting the thickness, the thickness may be measured with a film thickness measuring instrument, and the press pressure may be adjusted to compress a plurality of times until a desired thickness is obtained.

  On the other hand, the negative electrode (typically, the negative electrode sheet 40) may have a configuration in which a negative electrode active material layer 44 is formed on a long negative electrode current collector 42 (for example, a copper foil). As the negative electrode active material, one or more of materials conventionally used in lithium secondary batteries can be used without any particular limitation. A preferred example is carbon particles. A particulate carbon material (carbon particles) containing a graphite structure (layered structure) at least partially is preferably used. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. obtain.

  Moreover, the negative electrode active material layer 44 may contain, in addition to the negative electrode active material, materials such as one or two or more binders that can be blended in a general lithium secondary battery as required. it can. Then, a negative electrode current collector 42 is prepared by adding a negative electrode active material, a binder, or the like to an appropriate solvent (water, an organic solvent, and a mixed solvent thereof), and dispersing or dissolving the paste or the slurry-like composition. It can preferably be prepared by applying to the substrate, drying the solvent and compressing.

  Moreover, as a suitable separator sheet 50 used between the positive / negative electrode sheets 30 and 40, what was comprised with porous polyolefin resin is mentioned. For example, a porous separator sheet made of synthetic resin (for example, made of polyolefin such as polyethylene) can be suitably used. When a solid electrolyte or a gel electrolyte is used as the electrolyte, there may be a case where a separator is unnecessary (that is, in this case, the electrolyte itself can function as a separator).

Moreover, the electrolyte can use the thing similar to the nonaqueous electrolyte conventionally used for a lithium secondary battery without limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than seeds can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ). _3. Lithium compounds (lithium salts) such as LiI can be used. In addition, the density | concentration of the support salt in a nonaqueous electrolyte solution may be the same as that of the nonaqueous electrolyte solution used with the conventional lithium secondary battery, and there is no restriction | limiting in particular. An electrolyte containing an appropriate lithium compound (supporting salt) at a concentration of about 0.1 to 5 mol / L can be used.

  In addition, a rough procedure for constructing the lithium secondary battery 100 according to an embodiment will be described. The prepared positive electrode sheet 30 and negative electrode sheet 40 are stacked and wound together with two separators 50, and are crushed and crushed from the stacking direction to form the electrode body 20 into a flat shape and accommodate in the battery case 10. After injecting the electrolyte, the lid 14 is attached to the case opening 12 and sealed, whereby the lithium secondary battery 100 of this embodiment can be constructed. There is no particular limitation on the structure, size, material (for example, metal or laminate film) of the battery case 10 and the like.

  As described above, the lithium secondary battery constructed in this manner exhibits excellent battery characteristics (battery capacity, cycle characteristics or high rate characteristics), particularly good cycle characteristics under low temperature and high power use conditions. obtain.

Note that the matters disclosed by this specification include the following.
(1) The process of grasping | ascertaining the coating amount (alpha) mentioned above about the positive electrode active material which is evaluation object;
A step of comparing the covering amount α grasped above with a target numerical value (predetermined acceptance criteria);
A step of determining that the positive electrode active material is acceptable when the coating amount α satisfies the target value;
A step of preparing an aqueous paste containing a positive electrode active material determined to be acceptable and an aqueous solvent;
Applying the aqueous paste to a positive electrode current collector to produce a positive electrode having a positive electrode active material layer on the current collector; and
Building a lithium secondary battery using the positive electrode;
A method for producing a lithium secondary battery.

(2) A method for evaluating the quality of a positive electrode active material used for preparing an aqueous paste for producing a positive electrode of a lithium secondary battery,
A step of grasping the above-described coating amount α for the positive electrode active material to be evaluated;
A step of comparing the covering amount α grasped above with a target numerical value (predetermined acceptance criteria); and
A step of determining that the positive electrode active material is acceptable when the coating amount α satisfies the target value;
A method for evaluating the quality of a positive electrode active material.

  The positive electrode active material determined to be unacceptable in the above (1) and (2) may be used for the preparation of an aqueous paste after being treated so that α satisfies the above range. And an organic solvent may be used to prepare a solvent-based paste (composition for forming a positive electrode active material layer). In addition, the coating amount α can be grasped by actually measuring each time using the above-described neutralization titration method, but is not limited to such an embodiment, for example, based on past results. Then, the covering amount α may be grasped (estimated). As a criterion for the above pass / fail judgment, in addition to or in addition to whether or not the coating amount α satisfies the above range, other parameters (preferred examples include Li amount (C) and crystallite diameter). .)) May be adopted as to whether or not one or more of the above satisfy the preferred range disclosed herein. Further, the molar ratio (A) is grasped from the coating amount α (mass%), and the molar ratio (A) is compared with a predetermined acceptance standard (for example, 0.018 to 0.032 (molar ratio)). Pass / fail may be determined.

  In the following test examples, a lithium secondary battery (sample battery) was constructed using a positive electrode active material in which at least a part of the surface of the composite oxide disclosed herein was coated with a water-soluble lithium compound, and its performance Evaluation was performed.

<Test Example 1: Production of positive electrode active material>
First, a positive electrode active material was manufactured by the following procedure. That is, nickel sulfate (NiSO ₄ ) as a nickel supply source and cobalt sulfate (CoSO ₄ ) as a cobalt supply source were mixed at a predetermined molar ratio to prepare a sulfate aqueous solution. Next, while heating to about 50 ° C., the solution was neutralized while supplying an aqueous ammonia (NH ₃ ) solution and an aqueous sodium hydroxide solution (NaOH) little by little, and adjusted to about pH 11 to prepare a slurry. . The slurry is washed with water, filtered, and then dried at about 70 ° C. to thereby obtain a nickel cobalt composite hydroxide (Ni _1-y Co _y (OH) ₂ ) (where y is 0 ≦ y ≦ 0.25). Is obtained).
The nickel cobalt composite hydroxide is dispersed in an aqueous solution in which sodium hydroxide (NaOH) and sodium aluminate (NaAlO ₂ ) are dissolved, neutralized with an aqueous sulfuric acid solution (H ₂ SO ₄ ) while stirring, and nickel cobalt A slurry in which aluminum hydroxide was deposited on the surface of the composite hydroxide was prepared. The slurry was washed with water, filtered, and then dried at about 100 ° C., and then heated to 700 ° C. and fired to obtain a nickel cobalt aluminum-containing composite oxide ((Ni _1-y Co _y ) _1-z Al _z O. (Wherein y and z are numbers satisfying all 0 ≦ y ≦ 0.25 and 0 ≦ z ≦ 0.1).

Next, lithium hydroxide (LiOH) as a lithium supply source is added to the composite oxide, the number of moles of Li, and the total number of moles of all other constituent metal elements (Ni, Co, Al) (M _all ). Were mixed in such amounts that various molar ratios (M _Li / M _all ) were obtained to prepare a plurality of samples of composite oxide production raw materials. The prepared raw materials are heated in oxygen from room temperature to a temperature range of about 700 to 800 ° C. and fired in the temperature range for about 7 hours, so that at least part of the surface of the composite oxide has water-soluble lithium. Positive electrode active material (Li _x (Ni _1-y Co _y ) _1-z Al _z O ₂ ) coated with a compound (where x, y and z are 0.99 ≦ x ≦ 1.12, 0 ≦ y ≦ 0.25 and 0 ≦ z ≦ 0.1 are all satisfied).

About each positive electrode active material manufactured in this way, the coating amount of the water-soluble lithium compound coated on the surface of the composite oxide was measured. In the measurement method, 2.01 g to 2.04 g of the positive electrode active material was weighed and dispersed (dissolved) in ion-exchanged water with stirring to make a total amount of 100 mL. To this, 2 mL of 10% barium chloride solution was added, neutralized with 1 mol / L hydrochloric acid while stirring, and the first neutralization point of pH 8 was judged using a potentiometer, whereby the coating amount α of the water-soluble lithium compound was determined. (Mass%) was calculated | required using the following (2) Formula.
α (mass%) = (D × F × K × M ÷ S) / 10 (2)
D: hydrochloric acid titration to the first neutralization point (mL)
F: Factor of hydrochloric acid
K: atomic weight of lithium (6.941 g / mol)
M: molar concentration of hydrochloric acid (1 mol / L)
S: Mass of positive electrode active material (g)
Furthermore, the Li amount (molar ratio (A)) corresponding to the coating amount α was calculated from the α by the following formula: (α / K) / M _all ;

<Test Example 2: Preparation of aqueous paste>
An aqueous paste was prepared using each of the manufactured positive electrode active materials. That is, in forming the positive electrode active material layer in the positive electrode, the positive electrode active material, acetylene black as a conductive material, carboxymethyl cellulose (CMC) as a binder, and polytetrafluoroethylene (PTFE), The material was weighed so that the mass% ratio of the material was 88: 10: 1: 1, and added to an aqueous solvent (ion-exchanged water) so that the solid content ratio of the material was 54 mass%. At this time, the molar composition ratio between the positive electrode active material and the aqueous solvent (H ₂ O) is such that water is about 0.75 to 1.05 (for example, about 0.97) with respect to the positive electrode active material 1. It was prepared as follows. Subsequently, it mixed for 50 minutes with the planetary mixer, and prepared the aqueous paste for positive electrode active material layer formation.

<Test Example 3: Viscosity of aqueous paste>
The viscosity of the prepared aqueous paste was determined using a viscometer. And the relationship between the lithium coating amount which coat | covers the surface of the composite oxide of each positive electrode active material, and the viscosity of aqueous paste was investigated. FIG. 3 shows the result.

In FIG. 3, the horizontal axis represents the coating amount (molar ratio) of the water-soluble lithium compound that coats the surface of the composite oxide, and the vertical axis represents the viscosity of the aqueous paste.
As is clear from FIG. 3, it was confirmed that the viscosity of the aqueous paste tends to increase as the lithium coating amount increases. Furthermore, when the coating amount (molar ratio) is about 0.018 to 0.03, the viscosity of the aqueous paste is about 3000 to 5000 mPa · s, but when the coating amount (molar ratio) is 0.036 or more, the viscosity is extremely high. (About 13000 mPa · s).

<Test Example 4: Construction of a test lithium secondary battery using an aqueous paste>
The prepared aqueous paste was applied to both surfaces of an aluminum foil having a thickness of about 15 μm as a positive electrode current collector so that the total coating amount (in terms of solid content) was approximately 9.5 mg / cm ² . Next, the moisture in the paste is dried and stretched into a sheet with a roller press to prepare a layer thickness of 60 μm (including the thickness of the positive electrode current collector) to form a positive electrode active material layer A positive electrode (positive electrode sheet) for a lithium secondary battery was produced.

Next, a negative electrode for a lithium secondary battery was produced. That is, graphite (coated with amorphous carbon) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) in a mass% ratio of these materials of 98: 1: 1 was mixed with ion-exchanged water to prepare a paste for forming a negative electrode active material layer. And it apply | coated so that the total application amount (in solid content conversion) of this paste might be in the range of about 9.0 mg / cm < ² > on both surfaces of the copper foil about 10 micrometers thick as a negative electrode collector. Next, the moisture in the paste is dried and stretched into a sheet shape with a roller press to adjust the layer thickness to 60 μm (including the thickness of the negative electrode current collector) to form a negative electrode active material layer A negative electrode (negative electrode sheet) for a lithium secondary battery was produced.

The prepared positive electrode sheet and negative electrode sheet were overlapped and wound together with two porous separators, and crushed from the stacking direction to be crushed to form an electrode body into a flat shape. The electrode body is housed in a case, and an electrolyte in which the supporting salt LiPF ₆ is dissolved at a concentration of 1 mol / L in a mixed solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1: 1 is prepared. Injected. Next, a lithium secondary battery for testing was constructed by charging up to 4.1 V with a constant current of 2 A as a conditioning treatment.

<Test Example 5: Low-temperature output characteristics>
Next, the output characteristics of each lithium secondary battery under low temperature conditions were examined. That is, after a constant current discharge up to 3.0 V under a temperature condition of 25 ° C., the battery was charged with a constant current and a constant voltage and adjusted to 40% SOC (State of Charge). Thereafter, the current was appropriately changed at −30 ° C., the voltage 2 seconds after the start of discharge was measured, and an IV characteristic graph of the sample battery was created. The discharge cut voltage was 2.0V. The output value (W) was obtained from the IV characteristic graph. Then, the relationship between the amount of lithium coating covering the surface of the composite oxide of each positive electrode active material and the low temperature output was examined. FIG. 4 shows the result.

In FIG. 4, the horizontal axis represents the coating amount (molar ratio) of the water-soluble lithium compound that coats the surface of the composite oxide, and the vertical axis represents the low-temperature output value.
As is clear from FIG. 4, it was confirmed that the output value increased as the lithium coating amount (molar ratio) increased. Also, a lithium secondary battery having a coating amount (molar ratio) of 0.018 to 0.02 has an output of approximately 110 W, indicating that the low-temperature output characteristics are good.

<Test Example 6: Internal resistance increase rate>
Next, the internal resistance increase rate of each lithium secondary battery was examined. That is, each battery was adjusted to a SOC 60% charge state by constant current constant voltage (CC-CV) charging under a temperature condition of 25 ° C. Thereafter, the battery was discharged at 60 ° C. at 25 ° C., and a voltage value 10 seconds after the start of discharge was plotted to prepare an IV characteristic graph. From the slope of the IV characteristic graph, the initial internal resistance value (mΩ) at 25 ° C. was calculated.
Then, after adjusting each battery to SOC 60% under the same conditions, at 60 ° C., these batteries were charged at a constant current (10 A) to SOC 80% and discharged at a constant current (10 A) to SOC 30%. The charge / discharge cycle was repeated 2000 cycles. For these batteries after 2000 cycles, the internal resistance value was measured in the same manner as the measurement of the initial internal resistance value. Then, a graph with the horizontal axis as the square root of the number of cycles and the vertical axis as the internal resistance value was prepared, and the slope of the graph was determined as the increase rate of the internal resistance value before and after the charge / discharge cycle. The relationship between the amount of lithium coating covering the surface of the composite oxide of each positive electrode active material and the rate of increase in internal resistance was investigated. The result is shown in FIG.

In FIG. 5, the horizontal axis represents the coating amount (molar ratio) of the water-soluble lithium compound covering the surface of the composite oxide, and the vertical axis represents the internal resistance increase rate.
As is clear from FIG. 5, it was confirmed that when the charge / discharge cycle was repeated 2000 times, the internal resistance increase rate was increased as the lithium coating amount (molar ratio) was decreased. However, even in a lithium secondary battery having a small coating amount (molar ratio) of about 0.02 to 0.022, no significant increase in internal resistance was observed.

<Test Example 7: Effect of lithium carbonate amount>
In the same manner as in the production of the positive electrode active material according to Test Example 1, a plurality of types of positive electrode active materials having different M _Li / M _all in the composite oxide production raw material were produced. The coating amount α and the carbon amount W _TC were measured for the positive electrode active material samples (total 22 types) after being stored under various conditions in which one or both of the storage atmosphere and the storage time were different. The obtained α and W _TC were substituted into formulas (a) and (b), respectively, to determine the molar ratios (A) and (B) of Li, and the Li amount (C) was further calculated. Α was measured in the same manner as in Test Example 1. _WTC was measured by the following method.

(Measurement of carbon content _WTC )
measuring device:
LECO carbon sulfur analyzer, model “CS-600”
Measurement condition:
Carrier gas Oxygen (> 99.5%)
Crucible LECO ceramic crucible standard sample Japan Iron and Steel Federation JSS 061-1 (C: 0.63%)
JSS 173-7 (C: 0.038%)
Analysis operations:
1) Place the crucible on the precision balance, set the zero point, and confirm that the mass does not change for 5 seconds.
2) Weigh 0.2 to 0.3 g of sample to the order of 0.1 mg and place in a crucible.
3) Add W and Sn as a _combustor and measure _WTC (n = 2)

(Measurement of internal resistance)
A test lithium secondary battery was constructed in the same manner as in Test Examples 2 and 4 using each positive electrode active material sample. Each battery was adjusted to SOC 60% by constant current constant voltage (CC-CV) charge under the temperature condition of 25 degreeC. The battery was discharged at a discharge rate of 12 C (60 A) under the same temperature condition, and a voltage value 10 seconds after the start of discharge was plotted to prepare an IV characteristic graph. From the slope of this IV characteristic graph, the internal resistance value (SOC 60% internal resistance value at normal temperature) of each battery was calculated.

  FIG. 8 shows a graph in which the relationship between the Li amount (C) (molar ratio) of each positive electrode active material sample and the IV resistance of a lithium secondary battery constructed using the sample is plotted. As can be seen from this graph, the value (molar ratio) of the Li amount (C) is 0 <C <0.03 (more specifically 0.003 ≦ C ≦ 0.02, particularly 0.01 <C <0. It was confirmed that the battery using the positive electrode active material in the range of .02) has a low internal resistance value and a high correlation between the Li amount (C) and the internal resistance value.

<Test Example 8: Influence of crystallite size>
In the same manner as in the production of the positive electrode active material according to Test Example 1, except that the value of M _Li / M _all , the firing temperature and the firing time in the raw material for producing the composite oxide are adjusted, so that crystals in the (003) plane direction are obtained. Four types of positive electrode active material samples (samples 8a to 8d) having different core diameters d were produced. The crystallite diameter d was determined by an X-ray diffractometer (manufactured by PANalytical, X'Pert PRO). For each sample, the molar ratio (A), the carbon amount W _TC , the molar ratio (B), and the molar ratio (C) were determined in the same manner as in Test Example 7. The results are shown in Table 1.

(Measurement of internal resistance and low temperature output)
A test lithium secondary battery was constructed in the same manner as in Test Examples 2 and 4 using each positive electrode active material sample. Each battery was adjusted to SOC 20%, 40%, 60% or 80% by constant current constant voltage (CC-CV) charging under a temperature condition of 25 ° C. The battery was discharged at a discharge rate of 12 C (60 A) under the same temperature condition, and a voltage value 10 seconds after the start of discharge was plotted to prepare an IV characteristic graph. From the slope of the IV characteristic graph, the internal resistance value of each battery was calculated. For each battery, the low temperature output was measured in the same manner as in Test Example 5. The obtained results are shown in Table 1 and FIG.

  As shown in these charts, the battery using any positive electrode active material sample had a higher internal resistance when the SOC was 20% than when the SOC was 60% to 80%, but the crystallite diameter was less than 1300 mm. Compared with the sample a which does not, the samples 8b to 8d showed less increase in internal resistance due to low SOC. In particular, in sample 8c, the internal resistance value at 20% SOC was significantly reduced, and as a result, a lithium secondary battery showing flatter output characteristics in a wider SOC range was realized. Further, as shown in Table 1, it was confirmed that the effect of reducing the internal resistance was well reflected in the low temperature output characteristics.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment and Example are only illustrations and what changed and changed the above-mentioned specific example is contained in the invention disclosed here. For example, the battery of the various content from which an electrode body structural material and electrolyte differ may be sufficient. Further, the size and other configurations of the battery can be appropriately changed depending on the application (typically for in-vehicle use).

  The lithium secondary battery constructed using the positive electrode active material obtained by the method of the present invention has excellent battery characteristics (cycle characteristics or low-temperature output characteristics) as described above, and is therefore mounted on vehicles such as automobiles. It can be suitably used as a power source for a motor (electric motor). Therefore, as schematically shown in FIG. 6, the present invention provides a vehicle 1 (typically an automobile, particularly a hybrid) provided with such a lithium secondary battery (typically, a battery pack formed by connecting a plurality of series batteries) 100 as a power source. Automobiles, electric vehicles, automobiles equipped with electric motors such as fuel cell vehicles).

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Battery case 12 Opening part 14 Cover body 20 Winding electrode body 30 Positive electrode sheet 32 Positive electrode collector 34 Positive electrode active material layer 38 External positive electrode terminal 40 Negative electrode sheet 42 Negative electrode collector 44 Negative electrode active material layer 48 External negative electrode terminal 50 Separator 100 Lithium secondary battery

Claims (12)

A positive electrode active material for preparing an aqueous paste used to prepare an aqueous paste containing an aqueous solvent and a positive electrode active material for forming a positive electrode active material layer of a lithium secondary battery,
The positive electrode active material is composed of a composite oxide having at least lithium and nickel as constituent metal elements,
At least a portion of the surface of the composite oxide is coated with a water-soluble lithium compound;
Here, the coating amount of the water-soluble lithium compound is the number of moles of lithium m _Li in the compound based on the neutralization titration method and the total number of moles M _all of all metal elements other than lithium constituting the composite oxide. Based on the following formula (a):
A = m _Li / M _all (a);
A positive electrode active material for preparing an aqueous paste, characterized in that the molar ratio (A) of lithium represented by the formula is specified to be 0.018 to 0.032. The composite oxide constituting the positive electrode active material has the following general formula:
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
0 ≦ z ≦ 0.1,
Me is one or more elements that are not present or are selected from the group consisting of Al, Mn, Mg, Ti, Fe, Cu, Zn, and Ga. )
The positive electrode active material according to claim 1, wherein the positive electrode active material is a composite oxide represented by the formula:   The positive electrode active material according to claim 1, wherein the water-soluble lithium compound is lithium hydroxide. A molar ratio (A) of the lithium;
Based on the carbon amount W _TC (mass%) based on the high-frequency combustion-infrared absorption method for the positive electrode active material and the total number of moles M _all of all metal elements other than lithium constituting the composite oxide, the following formula ( b):
B = W _TC × (1/12) × 2 ÷ M _all (b);
The molar ratio (B) of lithium contained in lithium carbonate in an amount corresponding to _WTC calculated by the following relational expression (c):
0 <(1.2A-B) <0.03 (c);
The positive electrode active material according to claim 1, wherein: It takes the form of secondary particles in which multiple primary particles are assembled,
5. The primary particle according to claim 1, wherein a crystallite diameter d (Å) in a (003) plane direction obtained by X-ray diffraction satisfies the following formula: 1300Å ≦ d ≦ 1500Å. Positive electrode active material. A method for producing a positive electrode active material for a lithium secondary battery comprising a composite oxide having at least lithium and nickel as constituent metal elements, comprising:
A raw material for producing a complex oxide including a lithium source and a nickel source is a molar ratio between the number of moles of lithium M _Li and the total number of moles of all other constituent metal elements M _all (M _Li / M _all ). Is prepared in excess of lithium so that 1 <M _Li / M _all ; and by firing the composite oxide-producing raw material prepared in excess of lithium, Producing a positive electrode active material in which at least a part of the surface of the composite oxide is coated with a water-soluble lithium compound;
Including
Here, the amount of the lithium source used in preparing the raw material for producing the composite oxide is such that the coating amount of the water-soluble lithium compound is the number of moles of lithium m _Li in the compound based on the neutralization titration method and Based on the total number of moles M _all of all metal elements other than lithium constituting the composite oxide, the following formula (a):
A = m _Li / M _all (a);
The molar ratio (A) of lithium represented by the formula is specified to be 0.018 to 0.032. The raw material for producing the composite oxide has a general formula:
Li _x (Ni _1-y Co _y ) _1-z Me _z O ₂ (1)
(X, y and z in the formula (1) are
0.99 ≦ x ≦ 1.12,
0 ≦ y ≦ 0.25,
A number that satisfies all 0 ≦ z ≦ 0.1,
Me is one or more elements that are not present or are selected from the group consisting of Al, Mn, Mg, Ti, Fe, Cu, Zn, and Ga. )
The production method according to claim 6, comprising a source of all constituent metal elements so as to produce a composite oxide represented by the formula (II) and being prepared in excess of lithium.   The production according to claim 6 or 7, wherein the raw material for producing the complex oxide is prepared so that the water-soluble lithium compound covering the surface of the complex oxide becomes lithium hydroxide. Method. An aqueous paste containing a positive electrode active material and an aqueous solvent for forming a positive electrode active material layer of a lithium secondary battery,
The positive electrode active material according to any one of claims 1 to 5 or the positive electrode active material obtained by the production method according to any one of claims 6 to 8 is included as the positive electrode active material. A water-based paste.   A lithium secondary battery comprising a positive electrode active material layer formed using the aqueous paste according to claim 9 on a positive electrode current collector.   The lithium secondary battery according to claim 10, which is used as a driving power source for a vehicle.   A vehicle comprising the lithium secondary battery according to claim 10.

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