JP2013020715A - Positive electrode active material for nonaqueous electrolyte secondary battery, nonaqueous electrolyte secondary battery and manufacturing method of nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material in which oxygen gas is not generated even when a battery is charged up to a high voltage, and a large discharge capacity is ensured even when the discharge capacity is large, especially when the maximum potential of the positive electrode is lower than 4.4 (vs.Li/Li) during charge, and to provide a nonaqueous electrolyte secondary battery, and a manufacturing method therefor.SOLUTION: In the nonaqueous electrolyte secondary battery, the positive electrode active material is composed of a transition metal element including Li, and Co, Ni and Mn, the mole ratio Li/Me of Li to all transition metal elements Me is 1.25-1.40, and oxygen gas is not generated when the battery is charged up to a maximum arrival potential of the positive electrode in the range of 4.5-4.6 V (vs.Li/Li). In the manufacturing method of a nonaqueous electrolyte secondary battery, charging is performed in the initial charge/discharge up to a maximum arrival potential of the positive electrode in the range of 4.5 V (vs.Li/Li) or more and less than 4.6 V (vs.Li/Li), without generating oxygen gas from a lithium transition metal composite oxide.

Description

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

Conventionally, LiCoO ₂ is mainly used as a positive electrode active material in a lithium secondary battery. However, the lithium secondary battery using LiCoO ₂ as the positive electrode active material has a discharge capacity of only about 120 to 130 mAh / g, and the thermal stability in the battery in a charged state is also inferior.

Therefore, a material obtained by forming a solid solution of LiCoO ₂ with another compound is known as an active material for a lithium secondary battery. That is, as an active material for a lithium secondary battery, LiCoO ₂ , LiNiO ₂ and LiMnO ₂ are each a solid solution having an α-NaFeO ₂ type crystal structure shown on a ternary phase diagram in which three components are arranged. Co _1-2x Ni _x Mn _x ] O ₂ (0 <x ≦ 1/2) was published in 2001. A lithium secondary battery using LiNi _1/2 Mn _1/2 O ₂ or LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ as an example of the solid solution has a discharge capacity of 150 to 180 mAh / better than g and LiCoO _2, it is superior to LiCoO ₂ in terms of thermal stability in the battery in a charged state.

  However, an active material for a lithium secondary battery having a larger discharge capacity has been demanded. Therefore, an active material having a large discharge capacity has been developed by using a lithium transition metal composite oxide composed of Li and a transition metal element (Co, Ni, Mn, etc.) as a lithium excess transition metal composite oxide. However, such an active material has a problem of generating oxygen gas during charging (see, for example, Patent Documents 1 and 2 and Non-Patent Documents 1 and 2).

Patent Document 1 includes a step of charging an electrode active material having a plateau potential in which a gas is generated in a charging section to a level equal to or higher than the plateau potential; and a step of removing the gas. The manufacturing method of a chemical element. (Claim 1), "The electrode active material has a plateau potential of 4.4 to 4.8 V, according to any one of claims 1 to 3." (Claim 4), "The gas is oxygen (O ₂ ) gas," (Claim 5), (Charging section) An electrochemical element, wherein an electrode active material having a plateau potential in which gas is generated is charged to a level equal to or higher than the plateau potential, and then the gas is removed. (Claim 6), "The electrode active material" Has a plateau potential of 4.4 to 4.8V. The electrochemical device according to Item 6, ”(Claim 7),“ After charging to the plateau potential or higher and removing the gas, the discharge capacity of the electrode active material is within a voltage range of 3.0 to 4.4V. The invention according to claim 8, characterized in that it is in the range of 100 to 280 mAh / g.
Further, as the electrode active material, chemical formula 1 _{"XLi (Li 1/3 M 2/3) O} 2 + YLiM'O ₂ in solid solution type, M = 4 + 1 or more selected from a metal having an oxidation number of M ′ = one or more elements selected from transition metals, 0 <X <1, 0 <Y <1, X + Y = 1 ”(claims 2, 8, paragraph [0024] ), “When charged above the redox potential of M ′, Li is released and oxygen is also released in order to achieve a redox balance. Therefore, the electrode active material must have a plateau potential. (Paragraph [0025]), “The compound of Formula 1 above is charged even after the gas removal step after charging to a charging voltage (4.4 to 4.8 V) higher than the plateau potential. It is preferable because it is stable as an electrode active material in the discharge cycle. " [Preferably, M is one or more elements selected from Mn, Sn, and Ti metals, and M ′ is one or more elements selected from Ni, Mn, Co, and Cr metals. (Paragraph [0027]).

Furthermore, Patent Document 1 states that “if a battery is configured by a method of performing a gas removal step after charging at least once a plateau potential according to the present invention, a high capacity can be obtained even if the battery is continuously charged above the plateau potential. In addition, the battery problem caused by gas generation can be solved, that is, after charging to a plateau potential or higher, no gas is generated from the subsequent cycle charging, and the plateau section is eliminated (see FIG. 4). " (Paragraph [0022]), and as Example 4, Li (Li _0.2 Ni _0.2 Mn _0.6 ) O ₂ (3/5 [Li (Li _1/3 Mn _2/3 ) O ₂ ] +2/5 [LiNi _1/2 Mn _1/2 ] O ₂ ) as a positive electrode active material (paragraph [0048]), “charged to 4.8 V in the first cycle and second cycle Then, “charged to 4.4 V” (paragraph [0060]) indicates that a high-capacity battery can be obtained (see FIG. 5). However, as in Comparative Example 5 and Comparative Example 6, it is suggested that when charging to 4.25 V or 4.4 V, which is lower than the plateau potential, oxygen gas is not generated, but the battery has a low discharge capacity. (Paragraph [0056], FIG. 1, Paragraph [0057], FIG. 2) shows a positive electrode active material that does not generate oxygen gas even when charged to a voltage higher than the plateau potential. I cannot say that.

In Non-Patent Documents 1 and 2, when Li [Ni _x Li _{(1 / 3-2x / 3)} Mn _{(2 / 3-x / 3)} ] O ₂ is used as a positive electrode active material, charging is more than a plateau potential. It is shown that the release of oxygen gas occurs at a voltage (4.5 to 4.7 V) (non-patent document 1, page A818, left column, line 4 to right column, second line, non-patent document 2, A785 left). Column 9 line to A788 right column 4 line) does not indicate that oxygen gas is not generated when charged to a voltage higher than the plateau potential.

Patent Document 2 states that “a positive electrode plate and a negative electrode plate provided with an active material layer capable of inserting and extracting lithium ions in a current collector are wound or laminated with a separator interposed therebetween to form an electrode group and a non-aqueous electrolyte. A non-aqueous secondary battery enclosed in a case, wherein the positive electrode plate has an active material charged at 4.3 V or less on the basis of Li / Li ⁺ and a material that generates oxygen gas when overcharged. Ion secondary battery. "(Claim 1)," Li [(Ni _0.5 Mn _0.5 ) _x Co _y (Li _1/3 Mn _1/3 ) _z ] O ₂ as a substance that generates oxygen gas during the overcharge. " The lithium ion secondary according to claim 1, wherein an active material represented by (where x + y + z = 1 z> 0) or LiαNiβMnγO ₂ (α is 1.1 or more and β: γ = 1: 1) is used. A battery. "(Claim 2) and a lithium-excess transition metal complex acid. Things (Examples 1 to 3 and _{7: Li 1.2 Ni 0.4 Mn 0.4} O 2, Example _{4~6: Li [(Ni 0.5 Mn} 0.5) 1/12 Co 1/4 (Li 1/3 Mn 2/3) _1/3 ] O ₂ (when x = 5/12, y = 1/4, z = _1/3 )) is a lithium transition metal composite oxide that is not excessive in lithium (Comparative Example 1: LiCoO ₂ , Comparative Example 2) : LiNi _0.5 Mn _0.5 O ₂ , Comparative Example 3: Compared with Li (N _1/3 Mn _1/3 Co _1/3 ) O ₂ ), oxygen gas is easily generated during overcharge (paragraph [0064] In the invention described in Patent Document 2, conversely, by utilizing the above-described properties of the lithium-excess transition metal composite oxide, “positive electrode active material layer due to gas generation during overcharge” Between the current collector and the current collector, or between the positive electrode plate and the separator, or in the positive electrode layer, so that charging is interrupted, decomposition of the electrolyte, decomposition of the positive electrode active material, In addition, it is possible to prevent a short circuit due to Li deposition on the negative electrode side (paragraph [0010]).

Further, Patent Document 2 states that “in the lithium ion secondary battery of the present invention, as a positive electrode active material that generates oxygen gas during overcharge, a lithium-excess positive electrode active material Li [(Ni _0.5 Mn _0.5 ) _x Co _y (Li _1/3 Mn _1/3 ) _z ] O ₂ (where x + y + z = 1 z> 0) or LiαNiβMnγO ₂ (where α is 1.1 or more and β: γ = 1: 1) If the above active material is used, oxygen gas is generated at about 4.5 V on the basis of Li / Li ⁺ , thereby making it possible to widen the gap between the positive electrode and the negative electrode. Since it is described, a positive electrode active material that does not generate oxygen gas when charged to 4.5 V or higher is not shown.

Patent Document 3 discloses a manufacturing method for manufacturing a lithium secondary battery in which a charging method in which the maximum potential of the positive electrode during charging is 4.3 V (vs. Li ^/ Li ⁺ ) or less is employed, lithium secondary battery using a lithium excess transition metal composite oxide as a positive electrode active material, 4.3V (vs.Li/Li ⁺⁾ beyond 4.8V (vs.Li/Li ⁺⁾ below the positive electrode potential range By adopting a manufacturing method that includes a process of charging at least a region where the potential change that appears with respect to the amount of charged electricity reaches a relatively flat region, the discharge capacity is large, particularly in a potential region of 4.3 V or less. It has been shown that a large lithium secondary battery can be obtained (claim 10, paragraphs [0062] to [0065], FIG. 9). High voltage above flat area (plateau potential) It is not shown that oxygen gas is not generated from the positive electrode active material even when charged up to.
Moreover, in the Example of patent document 3, pH in the process which manufactures a precursor by coprecipitating the compound containing Co, Ni, and Mn in a solution is 11.5, and lithium transition metal complex oxide The firing temperature is 1000 ° C.

  On the other hand, the oxygen gas generated from the positive electrode active material causes problems such as oxidation of the solvent constituting the electrolyte of the non-aqueous electrolyte secondary battery (lithium secondary battery) and battery heating. A positive electrode (positive electrode active material) for a non-aqueous electrolyte secondary battery in which gas generation is suppressed has also been developed (see, for example, Patent Documents 4 and 5).

The invention described in Patent Document 4 is that in claim 1, the difficulty in generating oxygen gas of the lithium-containing composite oxide (lithium transition metal composite oxide) is “in the gas chromatograph mass spectrometry measurement of the composite oxide. “Maximum value of oxygen generation peak” is defined as “range of 330 to 370 ° C.”, and in “GC / MS measurement”, the cathode mixture is heated from room temperature to 500 ° C. at 10 ° C./min, and the oxygen generation behavior is observed. The oxygen generation spectrum (A) obtained here is shown in Fig. 3. As is apparent from Fig. 3, in the spectrum (A), the maximum value of the oxygen generation peak is located on the higher temperature side than 350 ° C. Therefore, the positive electrode active material of the present invention is extremely stable because it generates oxygen and hardly decomposes even when exposed to a high temperature in an overcharged region with a battery voltage of 4.7 V. It is seen. "Is described as (paragraph [0041]).
However, Patent Document 4 discloses that “the element M represented by the general formula: Li _z Co _1-xy Mg _x M _y O ₂ and contained in the general formula includes Al, Ti, Sr, Mn, Ni, and X, y, and z included in the general formula are at least one selected from the group consisting of Ca, (a) 0 ≦ z ≦ 1.03, (b) 0.005 ≦ x ≦ 0.1. And (c) satisfying 0.001 ≦ y ≦ 0.03 ”is specifically described (claims 2 and 3), and an excess of lithium that does not generate oxygen gas in the overcharge region Transition metal complex oxides are not shown.

  Patent Document 5 shows that oxygen is released from the positive electrode at a high temperature by mixing or adhering an oxygen storage material to the positive electrode active material (claim 1, paragraph [0005]). , [0056]), “The oxygen desorption peak temperature of the positive electrode (samples 1 to 4) having Ce oxide or Ce—Zr oxide as the oxygen storage material is 300 ° C. or higher, and does not have the oxygen storage material. The peak temperature rose significantly compared to sample 5. This means that in samples 1 to 4, the phenomenon in which oxygen is released from the positive electrode is better (up to a higher temperature range) than in sample 5. However, a positive electrode active material (lithium transition metal composite oxide) that does not generate oxygen gas at a high temperature or the like is not shown.

JP 2009-505367 A JP 2008-226693 A JP 2010-86690 A JP 2004-220952 A JP 2006-114256 A

Journal of The Electrochemical Society, 149 (7) A815-A822 (2002) Journal of The Electrochemical Society, 149 (6) A778-A791 (2002)

The present invention has been made in view of the above problems, and does not generate oxygen gas from a lithium transition metal composite oxide even when charged to a high voltage, and has a large discharge capacity. Even when a charging method is adopted in which the maximum potential of the positive electrode at the time is lower than 4.4 (vs. Li ^/ Li ⁺ ), for example, 4.3 V (vs. Li ^/ Li ⁺ ) or less. It is an object of the present invention to provide a positive electrode active material having a large discharge capacity, and to provide a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery manufacturing method using the positive electrode active material.

In the present invention, in order to solve the above problems, the following means are adopted.
(1) In the positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium transition metal composite oxide, the lithium transition metal composite oxide is composed of Li and a transition metal element containing Co, Ni, and Mn, The molar ratio Li / Me of Li to all transition metal elements Me is 1.25 to 1.40, and the maximum ultimate potential of the positive electrode is in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ). A positive electrode active material for a non-aqueous electrolyte secondary battery, characterized in that oxygen gas is not generated from the lithium transition metal composite oxide when charged to any one potential.
(2) In the positive electrode active material for a non-aqueous electrolyte secondary battery containing a lithium transition metal composite oxide, the lithium transition metal composite oxide is composed of Li and transition metal elements containing Co, Ni, and Mn, The molar ratio Li / Me of Li to all transition metal elements Me is 1.25 to 1.40, and the maximum ultimate potential of the positive electrode is in the range of 4.55 to 4.6 V (vs. Li / Li ⁺ ). A positive electrode active material for a non-aqueous electrolyte secondary battery, characterized in that oxygen gas is not generated from the lithium transition metal composite oxide when charged to any one potential.
(3) The positive electrode active material for a nonaqueous electrolyte secondary battery according to (1) or (2), wherein the charge is charge in initial charge / discharge.
(4) The positive electrode active material has a plateau potential in a charging section, and any potential in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ) is equal to or higher than the plateau potential. The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of (1) to (3), wherein:
(5) The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of (1) to (4), wherein the positive electrode active material is fired at less than 1000 ° C.
(6) A nonaqueous electrolyte secondary battery comprising the positive electrode active material according to any one of (1) to (5).
(7) In use, the charging method in which the maximum potential of the positive electrode during charging is less than 4.4 V (vs. Li ^/ Li ⁺ ) is adopted. Next battery.
(8) In a method for manufacturing a lithium secondary battery using a positive electrode active material including a lithium transition metal composite oxide and performing a process including initial charge and discharge, Li, Co, and Ni are used as the lithium transition metal composite oxide. And a transition metal element containing Mn, a lithium transition metal composite oxide having a Li / Me molar ratio Li / Me of 1.25 to 1.40 with respect to the total transition metal element Me, Charging is performed without generating oxygen gas from the lithium transition metal composite oxide, and the maximum potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or more and less than 4.6 V (vs. Li ^/ Li ⁺ ) A method for producing a non-aqueous electrolyte secondary battery, wherein the method is carried out up to any potential in the range.
(9) The positive electrode active material has a plateau potential in a charging section, and the maximum potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or more and 4.6 V (vs. Li ^/ Li ⁺ ) Any one of the potentials in the range of less than the plateau potential is equal to or higher than the plateau potential. (8) The method for producing a nonaqueous electrolyte secondary battery according to (8).

  According to the present invention, it is possible to provide a non-aqueous electrolyte secondary battery having a large discharge capacity provided with a positive electrode active material containing a lithium transition metal composite oxide that does not generate oxygen gas even when charged at a high voltage. it can.

It is a figure which shows one Embodiment of this invention, Comprising: It is a longitudinal cross-sectional view of a square lithium secondary battery. It is a figure which shows the electric potential behavior at the time of the initial stage charging / discharging process of the battery 1 in an Example. It is a figure which shows the electric potential behavior at the time of the initial stage charging / discharging process of the battery 2 in an Example. It is a figure which shows the electric potential behavior at the time of the initial stage charging / discharging process of the battery 3 in an Example. It is a figure which shows the electric potential behavior at the time of the initial stage charging / discharging process of the battery 4 in an Example.

The positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention contains a lithium transition metal composite oxide, and the lithium transition metal composite oxide is a transition metal element containing Li and Co, Ni, and Mn. The molar ratio Li / Me of Li to all transition metal elements Me is 1.25 to 1.40.
In addition, since a lithium secondary battery is typical as a nonaqueous electrolyte secondary battery, a lithium secondary battery is demonstrated below.

  When the molar ratio Li / Me of Li to all transition metal elements Me is smaller than 1.25 or larger than 1.40, the discharge capacity becomes small. Therefore, in order to obtain a lithium secondary battery having a large discharge capacity, Li / Me is set to 1.25 to 1.40. Li / Me is preferably 1.250 to 1.350.

The lithium transition metal composite oxide used as the positive electrode active material of the present invention is represented by the general formula Li _a Co _x Ni _y Mn _z O ₂ (a + x + y + z = 2), and the molar ratio of Li to the total transition metal element Me Li / Me, that is, a / (x + y + z) is 1.25 to 1.40.

  When the molar ratio Co / Me of Co to all transition metal elements Me is smaller than 0.020 or larger than 0.230, the molar ratio of Mn to all transition metal elements Me is Mn / Me from 0.625. Is smaller or larger than 0.719, the discharge capacity tends to be small. Therefore, in order to obtain a lithium secondary battery with a large discharge capacity, Co / Me is set to 0.020 to 0.230, That is, x / (x + y + z) is preferably 0.020 to 0.230, and Mn / Me, that is, z / (x + y + z) is preferably 0.625 to 0.719.

When the positive electrode active material of the present invention is charged to any potential in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ), the maximum potential of the positive electrode is the lithium transition described above. Oxygen gas is not generated from the metal composite oxide. As shown in Patent Documents 1 and 2 and Non-Patent Documents 1 and 2, the conventional positive electrode active material has a maximum positive electrode potential of 4.4 when the lithium transition metal composite oxide is a lithium-excess transition metal composite oxide. Since oxygen gas is generated from this composite oxide at about 5 V (vs. Li ^/ Li ⁺ ), it can be said that the positive electrode active material of the present invention is completely different from the conventional positive electrode active material. In the present invention, as in the examples described later, when charging was performed up to the maximum potential of the positive electrode of 4.5 V, 4.55 V, 4.6 V (vs. Li ^/ Li ⁺ ), It was confirmed that no oxygen gas was generated from the lithium transition metal composite oxide.
In the present invention, “oxygen gas is not generated” means that oxygen gas is not substantially generated. Specifically, a sealed battery including a positive electrode active material having the composition of the present invention is manufactured, was charged to one of the potential in the range of 4.5~4.6V (vs.Li/Li ^+), after discharging until 2.0V (vs.Li/Li ^+), a battery When the gas released from the battery is analyzed using gas chromatography, the volume ratio of oxygen and nitrogen should not be different from the atmospheric component (O ₂ / (N ₂ + O ₂ ) = 0.21) (Measurement error: ± 5%), that is, it means that the amount of oxygen gas generated is below the detection limit. In addition, charging / discharging of a battery is performed at normal temperature (25 degreeC), and is not performed in the state heated extremely.

As described above, when the maximum potential of the positive electrode is in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ), oxygen gas is not generated even if charging is performed up to any potential. In the manufacturing process of the lithium secondary battery, when this charging is performed as charging in initialization (initial charging / discharging), the maximum potential of the positive electrode exceeds 4.5 V (vs. Li ^/ Li ⁺ ). It is preferable to carry out to any potential in the range of less than 6 V (vs. Li ^/ Li ⁺ ), more preferably 4.55 V or a potential close thereto.
When the maximum potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or less, the amount of electricity that can be discharged in a discharge region of 4.3 V (vs. Li ^/ Li ⁺ ) or less is reduced. When the ultimate potential is 4.6 V (vs. Li ^/ Li ⁺ ) or more, the discharge capacity increases, but the amount of gas generated due to the decomposition of the electrolytic solution increases, and the battery performance deteriorates.
Oxygen gas is generated when the maximum potential of the positive electrode is charged to 4.55 V (vs. Li ^/ Li ⁺ ) as the charge in the initial charge / discharge (first charge / discharge) as in the examples described later. The lithium secondary battery manufactured in this way has a maximum potential of 4.3 V (vs. Li / Li) at the time of charging when used. ^When a charging method that is ⁺ ) is adopted, a large discharge capacity can be obtained.

In addition, the positive electrode active material in the present invention has a plateau potential in the charging section, and any potential in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ) is a plateau potential. That's it. Here, the plateau potential means a “region in which the potential change appearing with respect to the amount of charge in the positive electrode potential range is relatively flat” as shown in FIG. It has the same meaning as the plateau potential described in.
The present invention includes a lithium transition metal composite oxide in which a molar ratio Li / Me of Li to all transition metal elements Me (Co, Ni, and Mn) is 1.25 to 1.40, and a plateau potential in a charging section. Even when the charge in the initial charge / discharge (first charge / discharge) of the lithium secondary battery using the positive electrode active material having the above is performed at a plateau potential or higher, oxygen gas is not substantially generated from the lithium transition metal composite oxide. It is characterized by.

  The lithium transition metal composite oxide of the present invention is represented by the above general formula, and is essentially a composite oxide composed of Li, Co, Ni and Mn. It does not exclude the inclusion of a small amount of other metals such as alkali metals such as Na and Ca, alkaline earth metals, and transition metals represented by 3d transition metals such as Fe and Zn, as long as they are not impaired.

In addition, the lithium transition metal composite oxide of the present invention essentially has a crystal structure belonging to hexagonal crystals. The space group can be assigned to P3 ₁ 12 or R3-m. Here, P3 ₁ 12 is a crystal structure model in which the atomic positions of the 3a, 3b, and 6c sites in R3-m are subdivided, and when ordering is recognized in the atomic arrangement in R3-m, the P3 ₁ 12 model Is adopted. Note that “R3-m” should be represented by adding a bar “-” on “3” of “R3m”.

Next, a method for producing the active material for a lithium secondary battery of the present invention will be described.
The active material for a lithium secondary battery of the present invention basically includes a raw material containing a metal element (Li, Mn, Co, Ni) constituting the active material according to the composition of the active material (oxide). It can be obtained by adjusting and baking this. However, with respect to the amount of the Li raw material, it is preferable to add an excess of about 1 to 5% in anticipation of a part of the Li raw material disappearing during firing.
In producing an oxide having a desired composition, a so-called “solid phase method” in which salts of Li, Co, Ni, and Mn are mixed and fired, or Co, Ni, and Mn were previously present in one particle. A “coprecipitation method” is known in which a coprecipitation precursor is prepared, and a Li salt is mixed and fired therein. In the synthesis process by the “solid phase method”, especially Mn is difficult to uniformly dissolve in Co and Ni, so it is difficult to obtain a sample in which each element is uniformly distributed in one particle. In literatures and the like, many attempts have been made to dissolve Mn in a part of Ni or Co (LiNi _1-x Mn _x O ₂ etc.) by solid phase method, but the “coprecipitation method” is selected. It is easier to obtain a homogeneous phase at the atomic level. Therefore, the “coprecipitation method” is employed in the examples described later.

When preparing a coprecipitation precursor, Mn is easily oxidized among Co, Ni and Mn, and it is not easy to prepare a coprecipitation precursor in which Co, Ni and Mn are uniformly distributed in a divalent state. Uniform mixing at the atomic level of Co, Ni and Mn tends to be insufficient. Particularly in the composition range of the examples, since the Mn ratio is higher than the Co and Ni ratios, it is particularly important to remove dissolved oxygen in the aqueous solution. Examples of the method for removing dissolved oxygen include a method of bubbling a gas not containing oxygen. The gas not containing oxygen is not limited, but nitrogen gas, argon gas, carbon dioxide (CO ₂ ), or the like can be used. Among these, when preparing a coprecipitated carbonate precursor as in the examples described later, it is preferable to employ carbon dioxide as a gas not containing oxygen because an environment in which carbonate is more easily generated is provided. .

  Moreover, it is preferable to control the pH in the step of producing a precursor by coprecipitation of a compound containing Co, Ni and Mn in a solution. When the coprecipitation precursor is prepared as a coprecipitation carbonate precursor, the tap density can be increased to 1.25 g / cc or more by adjusting the pH to 9.4 or less, and high-rate discharge is achieved. Characteristics can be improved. The pH is preferably 8.5 to 9.4.

  The coprecipitation precursor is preferably made of a compound in which Mn, Ni and Co are uniformly mixed. However, the precursor is not limited to carbonates, and other precursors such as hydroxides and citrates that are uniformly soluble at the atomic level may be used in the same manner as carbonates. it can. In addition, a precursor having a larger bulk density can be produced by using a crystallization reaction using a complexing agent. At that time, a higher density active material can be obtained by mixing and firing with a Li source, so that the energy density per electrode area can be improved.

  The raw materials for the coprecipitation precursor include manganese oxide, manganese carbonate, manganese sulfate, manganese nitrate, manganese acetate, etc. as the Mn compound, and nickel hydroxide, nickel carbonate, nickel sulfate, nickel nitrate, nickel acetate as the Ni compound. As examples of the Co compound, cobalt sulfate, cobalt nitrate, cobalt acetate, and the like can be given as examples.

  As a raw material used for the preparation of the coprecipitation precursor, any form can be used as long as it forms a precipitation reaction with an alkaline aqueous solution, but a metal salt having high solubility is preferably used. .

  The active material for a lithium secondary battery in the present invention can be suitably prepared by mixing the coprecipitation precursor and the Li compound, followed by heat treatment. As a Li compound, it can manufacture suitably by using lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate, etc.

In obtaining an active material having a large reversible capacity, the selection of the firing temperature is extremely important.
When the firing temperature is too high, the obtained active material collapses with an oxygen releasing reaction, and in addition to the hexagonal crystal of the main phase, the monoclinic Li [Li _1/3 Mn _2/3 ] O ₂ type is obtained. The prescribed phase tends to be observed as a phase separation rather than as a solid solution phase, and such materials are not preferred because the reversible capacity of the active material is greatly reduced. In such materials, impurity peaks are observed around 35 ° and 45 ° on the X-ray diffraction pattern. Therefore, it is important that the firing temperature is lower than the temperature at which the oxygen release reaction of the active material affects. The oxygen release temperature of the active material is approximately 1000 ° C. or higher in the composition range according to the present invention. However, there is a slight difference in the oxygen release temperature depending on the composition of the active material. It is preferable to keep it. In particular, it is confirmed that the oxygen release temperature of the precursor shifts to the lower temperature side as the amount of Co contained in the sample increases. As a method for confirming the oxygen release temperature of the active material, a mixture of a coprecipitation precursor and Li ₂ CO ₃ may be subjected to thermogravimetric analysis (DTA-TG measurement) in order to simulate the firing reaction process. Although this method is good, there is a possibility that platinum used in the sample chamber of the measuring instrument is corroded by the Li component volatilized, and the instrument may be damaged. The obtained composition is preferably subjected to thermogravimetric analysis.

On the other hand, if the firing temperature is too low, crystallization does not proceed sufficiently and the electrode characteristics are also greatly deteriorated. The firing temperature needs to be at least 800 ° C. or higher. Sufficient crystallization is important for reducing the resistance of the grain boundaries and promoting smooth lithium ion transport. Visual observation using a scanning electron microscope is mentioned as a method of determining the degree of crystallization. When the positive electrode active material of the present invention was observed with a scanning electron microscope, the sample synthesis temperature was 800 ° C. or less, and it was formed from nano-order primary particles. By further increasing the sample synthesis temperature, It was crystallized to a submicron level, and large primary particles that lead to improved electrode characteristics were obtained.
In addition, the inventors have analyzed the half width of the diffraction peak of the active material of the present invention in detail, and in the sample synthesized at a temperature up to 800 ° C., strain remains in the lattice, and at a temperature higher than that, It was confirmed that almost all strains could be removed by synthesis. The crystallite size was increased in proportion to the increase in the synthesis temperature. Therefore, even in the composition of the active material of the present invention, a favorable discharge capacity can be obtained by aiming at a particle having almost no lattice distortion in the system and having a sufficiently grown crystallite size. Specifically, it has been found that it is preferable to employ a synthesis temperature (firing temperature) such that the strain amount affecting the lattice constant is 2% or less and the crystallite size is grown to 100 nm or more. Although changes due to expansion and contraction can be seen by charging and discharging by molding these as electrodes, it is preferable as an effect that the crystallite size is maintained at 50 nm or more in the charging and discharging process. That is, an active material having a remarkably large reversible capacity can be obtained only by selecting the firing temperature as close as possible to the oxygen release temperature of the active material.

  As described above, since the preferred firing temperature varies depending on the oxygen release temperature of the active material, it is generally difficult to set a preferred range for the firing temperature. However, in the present invention, the firing temperature may be less than 1000 ° C. Preferably, it is 800-940 degreeC. By baking at less than 1000 ° C., the BET specific surface area is increased, and the initial charge / discharge efficiency is improved.

  The nonaqueous electrolyte used for the lithium secondary battery according to the present invention is not limited, and those generally proposed for use in lithium batteries and the like can be used. Nonaqueous solvents used for the nonaqueous electrolyte include cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate and vinylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; dimethyl carbonate, Chain carbonates such as diethyl carbonate and ethyl methyl carbonate; chain esters such as methyl formate, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxy Ethers such as ethane, 1,4-dibutoxyethane and methyldiglyme; Nitriles such as acetonitrile and benzonitrile; Dioxolane or derivatives thereof; Ethylene sulfide, sulfolane, sultone or derivatives thereof Examples thereof include a conductor alone or a mixture of two or more thereof, but are not limited thereto.

Examples of the electrolyte salt used for the nonaqueous electrolyte include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiSCN, LiBr, LiI, Li ₂ SO ₄ , Li ₂ B ₁₀ Cl ₁₀ , NaClO ₄ , NaI, NaSCN, NaBr. , KClO ₄ , KSCN, and other inorganic ion salts containing one of lithium (Li), sodium (Na), or potassium (K), LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , (CH ₃ ) ₄ NBF ₄ , ( CH ₃ ) ₄ NBr, (C ₂ H ₅ ) ₄ NClO ₄ , (C ₂ H ₅ ) ₄ NI, (C ₃ H ₇ ) ₄ NBr, (n-C ₄ H ₉ ) ₄ NClO ₄ , (n-C _{4 H 9) 4 NI, (} C 2 H 5) 4 N-mal _{ate, (C 2 H 5)} 4 N-benzoate, (C 2 H 5) 4 N-phtalate, lithium stearyl sulfonate, lithium octyl sulfonate, organic ion salts of lithium dodecyl benzene sulfonate, and the like. These These ionic compounds can be used alone or in admixture of two or more.

Furthermore, by using a mixture of LiBF ₄ and a lithium salt having a perfluoroalkyl group such as LiN (C ₂ F ₅ SO ₂ ) ₂ , the viscosity of the electrolyte can be further reduced, so that the low temperature characteristics are further improved. It can be increased and self-discharge can be suppressed, which is more desirable.

  Moreover, you may use normal temperature molten salt and an ionic liquid as a nonaqueous electrolyte.

  The concentration of the electrolyte salt in the non-aqueous electrolyte is preferably 0.1 mol / l to 5 mol / l, more preferably 0.5 mol / l to 2 in order to reliably obtain a non-aqueous electrolyte battery having high battery characteristics. .5 mol / l.

The negative electrode material is not limited, and any negative electrode material that can deposit or occlude lithium ions may be selected. For example, titanium-based materials such as lithium titanate having a spinel crystal structure represented by Li [Li _1/3 Ti _5/3 ] O ₄ , alloy-based materials such as Si, Sb, and Sn-based lithium metal, lithium alloys (Lithium metal-containing alloys such as lithium-silicon, lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloys), lithium composite oxide (lithium-titanium), silicon oxide In addition, an alloy capable of inserting and extracting lithium, a carbon material (for example, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.) can be used.

  It is desirable that the positive electrode active material powder and the negative electrode material powder have an average particle size of 100 μm or less. In particular, the positive electrode active material powder is desirably 10 μm or less for the purpose of improving the high output characteristics of the non-aqueous electrolyte battery. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier is used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill or a sieve is used. At the time of pulverization, wet pulverization in the presence of water or an organic solvent such as hexane may be used. There is no particular limitation on the classification method, and a sieve, an air classifier, or the like is used as needed for both dry and wet methods.

  The positive electrode active material and the negative electrode material, which are the main components of the positive electrode and the negative electrode, have been described in detail above. In addition to the main components, the positive electrode and the negative electrode include a conductive agent, a binder, a thickener, and a filler. Etc. may be contained as other constituents.

  The conductive agent is not limited as long as it is an electron conductive material that does not adversely affect the battery performance. Usually, natural graphite (such as scaly graphite, scaly graphite, earthy graphite), artificial graphite, carbon black, acetylene black, Conductive materials such as ketjen black, carbon whisker, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) powder, metal fiber, and conductive ceramic material can be included as one kind or a mixture thereof. .

  Among these, as the conductive agent, acetylene black is desirable from the viewpoints of electron conductivity and coatability. The addition amount of the conductive agent is preferably 0.1% by weight to 50% by weight, and particularly preferably 0.5% by weight to 30% by weight with respect to the total weight of the positive electrode or the negative electrode. In particular, it is desirable to use acetylene black by pulverizing into ultrafine particles of 0.1 to 0.5 μm because the required carbon amount can be reduced. These mixing methods are physical mixing, and the ideal is uniform mixing. Therefore, powder mixers such as V-type mixers, S-type mixers, crackers, ball mills, and planetary ball mills can be mixed dry or wet.

  The binder is usually a thermoplastic resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene. Polymers having rubber elasticity such as rubber (SBR) and fluororubber can be used as one kind or a mixture of two or more kinds. The addition amount of the binder is preferably 1 to 50% by weight, particularly preferably 2 to 30% by weight, based on the total weight of the positive electrode or the negative electrode.

  As the filler, any material that does not adversely affect the battery performance may be used. Usually, olefin polymers such as polypropylene and polyethylene, amorphous silica, alumina, zeolite, glass, carbon and the like are used. The addition amount of the filler is preferably 30% by weight or less with respect to the total weight of the positive electrode or the negative electrode.

  The positive electrode and the negative electrode are obtained by kneading the main components (positive electrode active material in the positive electrode, negative electrode material in the negative electrode) and other materials into a mixture and mixing them in an organic solvent such as N-methylpyrrolidone and toluene. The obtained mixed liquid is applied on a current collector described in detail below, or pressed and heat-treated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours. About the application method, for example, it is desirable to apply to any thickness and any shape using means such as roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, etc. It is not limited.

  As the separator, it is preferable to use a porous film or a non-woven fabric exhibiting excellent high rate discharge performance alone or in combination. Examples of the material constituting the separator for a nonaqueous electrolyte battery include polyolefin resins typified by polyethylene and polypropylene, polyester resins typified by polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, and vinylidene fluoride-hexa. Fluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, fluorine Vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride - tetrafluoroethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer.

  The porosity of the separator is preferably 98% by volume or less from the viewpoint of strength. Further, the porosity is preferably 20% by volume or more from the viewpoint of charge / discharge characteristics.

  The separator may be a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, polyvinylidene fluoride, and an electrolyte. Use of the non-aqueous electrolyte in the gel state as described above is preferable in that it has an effect of preventing leakage.

  Furthermore, it is desirable that the separator be used in combination with the above-described porous film, non-woven fabric, or the like and a polymer gel because the liquid retention of the electrolyte is improved. That is, by forming a film in which the surface of the polyethylene microporous membrane and the microporous wall are coated with a solvophilic polymer having a thickness of several μm or less, and holding the electrolyte in the micropores of the film, Gels.

  Examples of the solvophilic polymer include polyvinylidene fluoride, an acrylate monomer having an ethylene oxide group or an ester group, an epoxy monomer, a polymer having a monomer having an isocyanate group, and the like crosslinked. The monomer can be subjected to a crosslinking reaction using a radical initiator in combination with heating or ultraviolet rays (UV), or using an actinic ray such as an electron beam (EB).

  The configuration of the lithium secondary battery is not particularly limited, and examples thereof include a cylindrical battery having a positive electrode, a negative electrode, and a roll separator, a square battery, a flat battery, and the like.

In a conventional lithium secondary battery, it is generally used by charging so that the maximum potential of the positive electrode reaches 4.5 V (vs. Li ^/ Li ⁺ ) or more. The substance can also be charged / discharged when the positive electrode potential reaches around 4.5 V (vs. Li ^/ Li ⁺ ). However, depending on the type of nonaqueous electrolyte used, if the positive electrode potential during charging is too high, the nonaqueous electrolyte may be oxidized and decomposed, resulting in a decrease in battery performance. Therefore, in use, a lithium secondary battery capable of obtaining a sufficient discharge capacity even when a charging method is adopted in which the maximum potential of the positive electrode during charging is 4.3 V (vs. Li ^/ Li ⁺ ) or less. May be required. When the active material of the present invention is used, for example, 4.3 V (vs. Li ^/ Li) such that the maximum potential of the positive electrode during charging is lower than 4.4 V (vs. Li ^/ Li ⁺ ) during use. ⁺ ) Even if a charging method such as the following is adopted, it is possible to take out the amount of discharge electricity exceeding the capacity of the conventional positive electrode active material of 197 mAh / g or more.

In order for the positive electrode active material according to the present invention to have a high discharge capacity, the transition metal element constituting the lithium transition metal composite oxide is present in a portion other than the transition metal site of the layered rock salt type crystal structure. It is preferable that the ratio is small. This is because the transition metal elements such as Co, Ni, and Mn are sufficiently uniformly distributed in the precursor to be subjected to the firing process, and the conditions of an appropriate firing process for promoting the crystallization of the active material sample. You can achieve it by choosing. When the distribution of the transition metal in the precursor to be subjected to the firing step is not uniform, a sufficient discharge capacity cannot be obtained. Although it is not necessarily clear about this reason, when the distribution of the transition metal in the precursor subjected to the firing step is not uniform, the obtained lithium transition metal composite oxide is a portion other than the transition metal site of the layered rock salt type crystal structure, That is, the present inventors speculate that it is derived from the so-called cation mixing in which a part of the transition metal element is present at the lithium site. The same inference can be applied to the crystallization process in the firing step. If the crystallization of the active material sample is insufficient, cation mixing in the layered rock salt type crystal structure is likely to occur. When the distribution of the transition metal element is high, the intensity ratio of the diffraction peaks of the (003) plane and the (104) plane as measured by X-ray diffraction tends to be large. In the present invention, the intensity ratio of diffraction peaks of the (003) plane and the (104) plane measured by X-ray diffraction measurement is preferably I ₍₀₀₃₎ / I ₍₁₀₄₎ ≧ 1.20, and I ₍₀₀₃₎ / It is more preferable that I ₍₁₀₄₎ ≧ 1.40, and it is particularly preferable that I ₍₀₀₃₎ / I ₍₁₀₄₎ ≧ 1.50. Further, it is preferable that I ₍₁₀₃₎ / I ₍₁₀₄₎ > 1 in the state after discharge after charging and discharging. If the precursor synthesis conditions and procedure are inadequate, the peak intensity ratio will be smaller and often less than 1.
By adopting the synthesis conditions and synthesis procedures described in the present specification, a high-performance positive electrode active material as described above can be obtained. In particular, when the charge upper limit potential is set lower than 4.4 V, a positive electrode active material for a nonaqueous electrolyte secondary battery capable of obtaining a high discharge capacity even when a charge upper limit potential such as 4.3 V is set is obtained. it can.

(Synthesis of active material)
Cobalt sulfate heptahydrate, nickel sulfate hexahydrate and manganese sulfate pentahydrate were weighed so that the molar ratio of Co, Ni and Mn was 12.5: 19.94: 67.56, and ion-exchanged water. A 2M aqueous solution of sulfate was prepared by dissolving in the solution. On the other hand, a 15 L reactor was prepared. The reaction layer is provided with a discharge port so that the solution is discharged from the discharge port when the liquid level inside the reaction tank exceeds a certain height. In addition, a stirring blade is provided in the reaction tank, and a cylindrical convection plate for fixing vertical convection during stirring is fixed. 7 L of ion exchange water was put into the reaction vessel, and CO ₂ gas was bubbled for 30 minutes to sufficiently dissolve the CO ₂ gas in the ion exchange water. The CO ₂ gas bubbling was continued until the sulfate aqueous solution was dropped. Next, the reaction layer was set to 50 ° C., and the stirring blade was operated at a rotation speed of 1000 rpm. A 2 L aqueous sulfate solution was gradually added dropwise into the reaction vessel. The stirring was continued during the dropwise addition. Further, the pH in the reaction vessel was constantly monitored, and an aqueous solution in which 2 M sodium carbonate and 0.2 M ammonia were dissolved was added so that the pH was in the range of 8.6 ± 0.2. While dropping the sulfate aqueous solution, a part of the solution containing the reaction product is discharged from the discharge port to the outside of the reaction tank, but the discharged solution until the entire amount of 2 L sulfate aqueous solution has been dropped is It was discarded without returning to the reaction vessel. After completion of the dropwise addition, the coprecipitation product was separated from the solution containing the reaction product by suction filtration, and washed with ion-exchanged water in order to remove the attached sodium ions. Next, it was dried in an atmosphere at 100 ° C. in an oven under normal pressure. After drying, the mixture was pulverized for several minutes in a mortar so that the particle diameters were uniform. Thus, a coprecipitated carbonate precursor powder was obtained.

Lithium carbonate was added to the coprecipitated carbonate precursor to prepare a mixed powder having a Li: Me (Co, Ni, Mn) molar ratio of 1.3: 1.0. Here, the lithium carbonate was made to have an excess of 3% with respect to the stoichiometric ratio. The mixed powder was transferred to a mortar and placed in a firing furnace. The temperature of the firing furnace was raised from room temperature to 900 ° C. over 4 hours, and was fired at 900 ° C. for 10 hours under normal pressure. After returning the temperature of the firing furnace to room temperature, the fired product was taken out and pulverized in a mortar to the same particle size. Li [Li _1.13 Co _0.109 Ni _0.173 Mn _0.588 ] O ₂ (Li / Me ratio: 1.30) was produced as described above.

(Preparation of prismatic lithium secondary battery)
FIG. 1 is a schematic cross-sectional view of a prismatic lithium secondary battery used in this example. This square lithium secondary battery 1 has a positive electrode plate 3 having a positive electrode mixture layer containing a positive electrode active material in an aluminum foil current collector, and a negative electrode mixture layer containing a negative electrode active material in a copper foil current collector. A power generating element including a flat wound electrode group 2 in which a negative electrode plate 4 is wound via a separator 5 and a nonaqueous electrolyte containing an electrolyte salt is formed in a battery case 6 having a width of 34 mm, a height of 50 mm, and a thickness of 5.2 mm. It is something that is stored.
A battery lid 7 provided with a safety valve 8 is attached to the battery case 6 by laser welding, the negative electrode plate 4 is connected to a negative electrode terminal 9 via a negative electrode lead 11, and the positive electrode plate 3 is connected to a battery via a positive electrode lead 10. Connected to the lid.

(Positive electrode plate)
Using Li [Li _1.13 Co _0.109 Ni _0.173 Mn _0.588 ] O ₂ produced as described above as a positive electrode active material, a square lithium secondary battery was produced according to the following procedure.
Using N-methylpyrrolidone as a dispersion medium, a positive electrode paste in which the positive electrode active material, acetylene black (AB) and polyvinylidene fluoride (PVdF) were kneaded and dispersed at a mass ratio of 90: 5: 5 was prepared. The positive electrode paste was applied to both sides of a 15 μm thick aluminum foil current collector and dried. Next, the positive electrode plate was produced by roll-pressing so that a mixture filling density might be 2.6 g / cc.

(Negative electrode plate)
On the other hand, negative electrode paste in which graphite, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) as negative electrode active materials were kneaded and dispersed at a mass ratio of 97: 2: 1 using ion-exchanged water as a dispersion medium was prepared. . The negative electrode paste was applied to both sides of a 10 μm thick copper foil current collector and dried. Next, the negative electrode plate was produced by roll-pressing so that a mixture filling density might be 1.4 g / cc.

(Electrolyte)
As an electrolytic solution, a solution in which LiPF ₆ was dissolved in a mixed solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 3: 7 so as to have a concentration of 1 mol / l was used.

(Separator)
As the separator, a polyethylene microporous membrane (H6022 manufactured by Asahi Kasei) having a thickness of 20 μm was used.

  The square lithium secondary battery (battery 1) produced by the above procedure was subjected to the following test.

(Battery thickness measurement)
The thickness of the battery was measured so that the center of the long side surface of the prismatic lithium secondary battery after fabrication was sandwiched with calipers from the vertical direction (from the short side surface to the horizontal direction) with respect to the long side surface. The measured value at this time was recorded as “battery thickness (mm) before the test”.
Regarding the battery thickness measurement, the battery thickness was measured in the same manner as described above even after the following discharge capacity test. The measured value at this time was recorded as “battery thickness after test (mm)”.

(Discharge capacity test)
First, one cycle of initial charge / discharge (initial charge / discharge) was performed at 25 ° C. Here, charging was constant current constant voltage charging with a current of 0.2 CA and a voltage of 4.5 V, charging time was 8 hours, and discharging was a constant current discharge with a current of 0.2 CA and a final voltage of 2.0 V. Subsequently, a discharge capacity test was performed. The conditions for the discharge capacity test consisted of one cycle of charge / discharge under the same conditions as the initial charge / discharge except that the charge voltage was changed to 4.2V. The amount of electricity discharged at this time was recorded as “discharge capacity (mAh)”.

(Gas analysis)
The discharged battery was disassembled in liquid paraffin, and all the gas released from the battery was collected in the manner of water replacement. This gas was analyzed for gas components using a gas chromatography (HP5890 series II gas chromatograph manufactured by HEWLETT PACKARD) equipped with MolecularSieve13X and Porapak Q (both manufactured by SPELCO) in the column.

(Battery 2)
Using the lithium secondary battery (battery 2) produced in the same procedure as battery 1, the discharge capacity test and gas analysis were performed in the same way as battery 1 except that the charge voltage in the initial charge / discharge was 4.45V. did.

(Battery 3)
Using a lithium secondary battery (battery 3) produced in the same procedure as battery 1, the discharge capacity test and gas analysis were performed in the same manner as battery 1 except that the charge voltage in the initial charge / discharge was set to 4.40V. did.

(Battery 4)
Using a lithium secondary battery (battery 4) produced in the same procedure as battery 1, a discharge capacity test was performed in the same manner as battery 1 except that the charge voltage in the initial charge / discharge was set to 4.20V.

  Table 1 shows the results of the battery thickness measurement and the discharge capacity test for battery 1, battery 2, battery 3, and battery 4.

  Table 2 shows the analysis results of the gas components in the battery 1, the battery 2, and the battery 3.

From Tables 1 and 2, the following can be seen.
The battery 1 swells after the discharge capacity test and has a larger amount of collected gas (corresponding to the amount of gas in the battery) than the batteries 2 and 3, but the volume ratio of oxygen and nitrogen (O ₂ / ( N ₂ + O ₂ )) is not changed from normal atmospheric components, and it is not considered that oxygen was generated. In the battery 1, since the CO gas volume ratio (CO / collected amount) is increased, the cause of the battery swelling is considered to be due to the oxidative decomposition of the electrolyte in the positive electrode field.
That is, even when the battery voltage is 4.50 V [positive electrode potential is 4.60 V (vs. Li / Li ⁺ )] in the initial charge / discharge (first charge / discharge), the oxygen volume ratio is compared with that before the test. There is no change in oxygen gas.

The charge voltage at the initial charge and discharge of the battery 4.45 V, [4.55V at the positive electrode potential ^{(vs.Li/Li +), 4.50V (vs.Li/Li} +) ] 4.40V The test was carried out in the In Battery 2 and Battery 3, the battery swelled less than Battery 1 charged at 4.50 V, and the amount of gas collected (corresponding to the amount of gas in the battery) decreased. In addition, there is no change in the ratio of oxygen to nitrogen (O ₂ / (N ₂ + O ₂ )), and the ratio of CO gas in the collected gas (CO / collected amount) is decreased in conjunction with the decrease in the positive electrode potential. Yes.

Battery voltage: 4.2 V [positive electrode potential: 4.3 V (vs. Li ^/ Li ⁺ )] With regard to the discharge capacity during charging, the maximum potential of the positive electrode is 4.60 V (vs. Li / Li ⁺ ). 55V battery 1 was charged in the first charging and discharging (vs.Li/Li ^+), in the battery 2, but as large as, to charge in initial charge and discharge at 4.50V (vs.Li/Li ⁺⁾ In addition, the battery 3 is slightly smaller, and is extremely small when charged in the initial charge / discharge at 4.30 V (vs. Li / Li ⁺ ). Therefore, when the positive electrode active material of the present invention is used and a lithium secondary battery is manufactured by performing a process including initial charge / discharge (initialization), the maximum potential of the positive electrode for charging in the initial charge / discharge is 4.5 V or more. It is preferable that

From FIG. 2 to FIG. 4 showing the potential behavior during the initial charge / discharge process of the battery 1, the battery 2 and the battery 3, it can be seen that the plateau potential of the positive electrode active material of the present invention is around 4.5V. It was confirmed that the active material had a plateau potential in the charging section, and oxygen gas was not generated above the plateau potential.
However, even when an active material that does not generate oxygen gas during charging is used, if the initial charge is performed at a maximum potential of 4.6 V (vs. Li ^/ Li ⁺ ) or higher, gas is generated due to decomposition of the electrolyte. .
In addition, when the initial charge is performed with the maximum reached potential of the positive electrode not passing through the plateau potential being less than 4.5 V (vs. Li ^/ Li ⁺ ), the battery voltage is 4.2 V [the positive electrode potential is 4.3 V (vs. .Li / Li ⁺ ) There is a problem that the discharge capacity during charging is small.
Therefore, even when a positive electrode active material that does not generate oxygen gas during charging is used, the maximum potential of the positive electrode is 4.5 V (vs. Li ^/ Li ⁺ ) or more and 4.6 V (vs. Li ^/ Li ⁺ ). In particular, it is considered that the optimum initial formation (initial charge / discharge) condition is about 4.55 V (vs. Li ^/ Li ⁺ ).

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 2 Electrode group 3 Positive electrode 4 Negative electrode 5 Separator 6 Battery case 7 Lid 8 Safety valve 9 Negative electrode terminal 10 Positive electrode lead-11 Negative electrode lead

  By using the positive electrode active material containing the novel lithium transition metal composite oxide of the present invention, it is possible to provide a nonaqueous electrolyte secondary battery having a large discharge capacity that does not generate oxygen gas during charging. The electrolyte secondary battery is useful as a lithium secondary battery for hybrid vehicles and electric vehicles.

Claims (9)

In a positive electrode active material for a non-aqueous electrolyte secondary battery including a lithium transition metal composite oxide, the lithium transition metal composite oxide is composed of Li and transition metal elements including Co, Ni, and Mn, and all transition metals thereof Either the molar ratio Li / Me of the element Me is 1.25 to 1.40, and the maximum potential of the positive electrode is in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ). A positive electrode active material for a nonaqueous electrolyte secondary battery, characterized in that oxygen gas is not generated from the lithium transition metal composite oxide when charged to a potential of In a positive electrode active material for a non-aqueous electrolyte secondary battery including a lithium transition metal composite oxide, the lithium transition metal composite oxide is composed of Li and transition metal elements including Co, Ni, and Mn, and all transition metals thereof Either the molar ratio Li / Me of the element Me is 1.25 to 1.40, and the maximum potential of the positive electrode is in the range of 4.55 to 4.6 V (vs. Li / Li ⁺ ). A positive electrode active material for a nonaqueous electrolyte secondary battery, characterized in that oxygen gas is not generated from the lithium transition metal composite oxide when charged to a potential of   The positive electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein the charge is charge in initial charge / discharge. The positive electrode active material has a plateau potential in a charging section, and any potential in the range of 4.5 to 4.6 V (vs. Li / Li ⁺ ) is equal to or higher than the plateau potential. The positive electrode active material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 3.   The positive electrode active material for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material is fired at a temperature lower than 1000 ° C.   The nonaqueous electrolyte secondary battery provided with the positive electrode active material of any one of Claims 1-5. The nonaqueous electrolyte secondary battery according to claim 6, wherein a charging method in which the maximum potential of the positive electrode during charging is less than 4.4 V (vs. Li ^/ Li ⁺ ) in use is employed. . In the method for manufacturing a lithium secondary battery using a positive electrode active material containing a lithium transition metal composite oxide and performing a process including initial charge and discharge, Li, Co, Ni and Mn are used as the lithium transition metal composite oxide. Using a lithium transition metal composite oxide that is composed of transition metal elements including, and whose molar ratio Li / Me of Li to all transition metal elements Me is 1.25 to 1.40, and charging in the initial charge and discharge, Oxygen gas is not generated from the lithium transition metal composite oxide, and the maximum potential of the positive electrode is in the range of 4.5 V (vs. Li ^/ Li ⁺ ) or more and less than 4.6 V (vs. Li ^/ Li ⁺ ). A method for producing a non-aqueous electrolyte secondary battery, wherein the method is performed up to a certain potential. The positive electrode active material has a plateau potential in a charging interval, and the maximum ultimate potential of the positive electrode is in a range of 4.5 V (vs. Li ^/ Li ⁺ ) or more and less than 4.6 V (vs. Li ^/ Li ⁺ ) The method for producing a non-aqueous electrolyte secondary battery according to claim 8, wherein any one of the potentials is at least the plateau potential.