JP2003249217A - Positive electrode active material and its manufacturing method, and lithium secondary battery using the positive electrode active material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material, a manufacturing method of the positive electrode active material capable of fabricating a lithium secondary battery in which inconveniences such as rupture/ignition or the like are difficult to occur even under an overcharged condition and which is superior in safety, and provide the lithium secondary battery in which the positive electrode active material is used and which is superior in safety. <P>SOLUTION: This is the positive active material used for the positive electrode of the lithium secondary battery. This is a lithium transition metal oxide in which weight reduction in 220 to 400°C is within a range of 0 to 2 wt.% and in which its chemical formula is expressed by the general formula LiXMYOZ (where M is a transition metal element, and X, Y and Z show a constituting ratio of respective elements in one molecule). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a positive electrode active material, a method for producing the same, and a lithium secondary battery using the positive electrode active material and having excellent safety.

[0002]

2. Description of the Related Art In recent years, portable electronic devices such as mobile phones, VTRs, and notebook personal computers have been rapidly reduced in size and weight. As a power source battery, a lithium composite oxide is used as a positive electrode active material. Secondary batteries using a carbonaceous material as a negative electrode active material and an organic electrolytic solution in which a Li ion electrolyte is dissolved in an organic solvent as an electrolytic solution have been used.

Such a battery is generally called a lithium secondary battery or a lithium-ion battery, has a large energy density, and has a high unit cell voltage of about 4 V. Electric vehicles (hereinafter referred to as “E”), which have been actively spread to the general public as low-pollution vehicles due to recent environmental problems
V ”. ) Or a hybrid electric vehicle (hereinafter,
It is written as "HEV". ) Is attracting attention as a motor drive power supply.

In such a lithium secondary battery,
Battery characteristics such as the battery capacity and charge / discharge cycle characteristics depend largely on the material characteristics of the positive electrode active material used. Here, as the lithium composite oxide used as the positive electrode active material, specifically, lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNi
O ₂ ) or lithium transition metal oxides such as lithium manganate (LiMn ₂ O ₄ ) can be mentioned.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention When a lithium secondary battery in which LiCoO ₂ or LiNiO ₂ which is an example of a lithium transition metal oxide is used as a positive electrode active material is overcharged, those lithium It is assumed that there may be a problem with the safety of the secondary battery. For example, L
When a lithium secondary battery in which iCoO ₂ is used as a positive electrode active material is overcharged, LiCoO ₂ becomes CoO ₂
Becomes Since CoO ₂ is chemically unstable and easily releases oxygen, it easily reacts with a flammable non-aqueous electrolytic solution, and the lithium secondary battery may burst or ignite.

On the other hand, when a lithium secondary battery in which LiMn ₂ O ₄ is used as a positive electrode active material is overcharged, LiMn ₂ O ₄ becomes MnO ₂ , but MnO ₂ is chemically stable. It is known that, even if it comes into contact with a flammable non-aqueous electrolyte, it is unlikely that ignition due to combustion will occur. However, hydrogen (H ₂ ), acetylene (C ₂ H ₂ ), and the like, which are decomposition gases of organic solvents (ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), etc.) that compose the non-aqueous electrolyte solution. In the case of the occurrence of these, these decomposition gases and LiM
Contact with n ₂ O ₄ under high temperature conditions may cause ignition. In addition, when the lithium secondary battery is overcharged, these decomposed gases have a terminal voltage of, for example, 4.5.
It is known to occur when V or more.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to prevent failures such as rupture and ignition even under an overcharge condition and to improve safety. Provided is a positive electrode active material capable of producing an excellent lithium secondary battery, a method for producing the positive electrode active material, and a lithium secondary battery excellent in safety using the positive electrode active material. It is in.

[0008]

Means for Solving the Problems That is, according to the present invention, a positive electrode active material used for a positive electrode of a lithium secondary battery, wherein the weight loss at 220 to 400 ° C. is within a range of 0 to 2 mass%. And its chemical formula is the general formula L
i _X M _Y O _Z (where M is a transition metal element, X, Y, and Z
Indicates the composition ratio of each element in one molecule. The present invention provides a positive electrode active material, which is a lithium transition metal oxide represented by In the present invention, 220 to 40
The weight loss at 0 ° C is preferably within the range of 0 to 1% by mass.

In the present invention, the general formula Li_XM_YO_Z
(However, M is a transition metal element, and X, Y, and Z are in one molecule.
The composition ratio of each element in is shown. ), X is 1, Y
When is 2, it is preferred that Z is between 3.6 and 3.8.
, The general formula Li _XM_YO_Z(However, M is a transition metal element,
X, Y, and Z represent the composition ratio of each element in one molecule.
You It is preferable that the transition metal element M in) is Mn.
Good Furthermore, the positive electrode active material of the present invention has a cubic spinel structure.
It is preferably manufactured.

In the present invention, the lithium transition metal oxide contains a part of Mn, Ti, and Li,
Fe, Ni, Mg, Zn, Co, Cr, Al, B, S
_A general formula Li _X MA substituted with two or more transition metal elements consisting of one or more elements selected from the group consisting of i, Sn, P, V, Sb, Nb, Ta, Mo, and W M
n _YA O _Z (where M is a transition metal element, A is a substitution amount, X,
Y and Z represent the composition ratio of each element in one molecule. ) Is preferable. In the present invention,
The Li / Mn ratio is preferably more than 0.5.

According to the invention, the general formula Li _X M _Y
O _Z (where M is a transition metal element, A is the substitution amount, X,
Y and Z represent the composition ratio of each element in one molecule. ), The salts and / or oxides of the respective elements constituting the lithium transition metal oxide represented by the formula (1) are mixed at a predetermined ratio to form a mixture, and the mixture is mixed in an oxidizing atmosphere, 650-1000.
In the temperature range of 0 ° C., primary firing is performed at least once for 5 to 50 hours to obtain a primary firing, and the primary firing is performed in a nitrogen atmosphere,
Secondary firing is performed in a temperature range of 50 to 900 ° C. to obtain a secondary fired body, and then the secondary fired body is heated to 600 ° C. up to 10 ° C.
There is provided a method for producing a positive electrode active material, which comprises cooling at a cooling rate of not less than / min.

In the present invention, the transition metal element M is M
The general formula Li _x Mn _Y O _Z (where X, Y, and Z are
Indicates the composition ratio of each element in one molecule. ) A salt of each element constituting the lithium transition metal oxide represented by
Alternatively, it is preferable to mix oxides in a predetermined ratio. Further, a part of Mn contains Ti, and Li, F
e, Ni, Mg, Zn, Co, Cr, Al, B, Si,
_A general formula Li _X MA Mn _YA O _Z substituted with two or more kinds of transition metal elements consisting of one or more kinds of elements selected from the group consisting of Sn, P, V, Sb, Nb, Ta, Mo and W.
(However, M is a transition metal element, A is a substitution amount, and X, Y, and Z are constituent ratios of each element in one molecule.) A salt of each element that constitutes the lithium transition metal oxide And / or oxides are preferably mixed in a predetermined ratio. In the present invention, it is preferable that the secondary firing and the cooling are each performed once or more.

Further, according to the present invention, there is provided a lithium secondary battery characterized by using any one of the above positive electrode active materials.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments. It should be understood that design changes, improvements, etc. can be appropriately made based on the knowledge of.

A first aspect of the present invention is a positive electrode active material used for a positive electrode of a lithium secondary battery, which comprises 220 to 40
The weight loss at 0 ° C. is in the range of 0 to 2 mass%, and the chemical formula is Li _X M _Y O _Z (provided that M
Represents a transition metal element, and X, Y, and Z represent constituent ratios of each element in one molecule. ) Is a lithium transition metal oxide represented by. The details will be described below.

The positive electrode active material of the present invention has zero weight loss under a predetermined high temperature condition, specifically, 220 to 400 ° C.
It has a characteristic such that it is within the range of 2% by mass.
Here, the “weight reduction” under the predetermined high temperature condition means a weight reduction caused by the release of oxygen atoms, and the room temperature of the lithium transition metal oxide represented by the general formula. It corresponds to the weight of oxygen released from the state under the conditions. In addition, the fact that this weight reduction corresponds to the weight of oxygen released is that DTA-
It is known from the analysis result by TG thermal analysis, and the positive electrode active material of the present invention is a lithium transition metal oxide which is extremely difficult to release oxygen even under the high temperature condition.

Therefore, according to the positive electrode active material of the present invention,
It is difficult to release oxygen even when the battery is overcharged and has a high temperature of, for example, 200 ° C. or higher, so that it is possible to provide a lithium secondary battery that is excellent in safety because problems such as rupture and ignition hardly occur. In addition, from the viewpoint that the lithium secondary battery produced is less likely to be ruptured or ignited, and further safety is given to the produced lithium secondary battery, the positive electrode active material of the present invention contains 220 to
The weight loss at 400 ° C. is preferably in the range of 0 to 1% by mass, more preferably 0 to 0.5% by mass.

Further, a general formula (Li _X M _Y O _Z (where M is a transition metal element, X, Y, and Z are constituents of each element in one molecule) is a chemical formula representing the positive electrode active material of the present invention. Ratio))), X, Y, and Z are X> 0, Y
> 0, and a value represented by a real number of Z> 0, in the present invention, a general formula Li _X M _Y O _Z (where M is a transition metal element, X, Y, and Z are each a molecule) When X is 1 and Y is 2, Z is preferably 3.6 to 3.8, and 3.6 to 3.7.
It is more preferably 5, and particularly preferably 3.6 to 3.7. This makes it possible to provide a lithium secondary battery with further safety, while avoiding the risk of rupture or ignition.

Here, in the present invention, the value of Z indicating the number of oxygen in one molecule of the general formula (Z = 3.6 to
3.8) means a value analyzed and calculated from the weight loss of the positive electrode active material by DTA-TG thermal analysis. When the value of Z is less than 3.6, the crystal structure of the positive electrode active material collapses and is likely to become amorphous, and the charge / discharge characteristics of the lithium secondary battery may deteriorate. Is more than 3.8, it is difficult to exert the effect of imparting safety, which is not preferable.

In addition, a general formula (Li _X M _Y O _Z (where M is a transition metal element, X, Y, and Z are constituents of each element in one molecule) is a chemical formula representing the positive electrode active material of the present invention. Although Mn, Co, Ni and the like can be mentioned as specific examples of the transition metal element M in the ratio))), the raw material is inexpensive and the output of the lithium secondary battery produced by using this positive electrode material. From the viewpoint of increasing the density and increasing the potential, Mn is preferable, and from the same viewpoint, the positive electrode active material of the present invention preferably has a cubic spinel structure.

Here, when the transition metal element M is Mn, that is, when the positive electrode active material is lithium manganate, the stoichiometric composition of this lithium manganate is represented by the composition formula of LiMn ₂ O ₄ . As shown, the positive electrode active material of the present invention is in a state of lacking oxygen, specifically, LiMn ₂ O _Z.
(Z = 3.6 to 3.8). Therefore, it is difficult to release oxygen even under a high temperature condition of 200 ° C. or higher.

Taking the case where the positive electrode active material is lithium manganate as an example, LiMn ₂ O ₄ having a stoichiometric composition is used.
In comparison with the case where the positive electrode is used for the positive electrode, the positive electrode active material (LiMn ₂ O _Z (Z = 3.6 to 3.
When 8)) is used for the positive electrode, it is possible to provide a lithium secondary battery with safety, avoiding dangers such as bursting and ignition. Further, in terms of battery characteristics such as charge / discharge cycle characteristics of the lithium secondary battery,
It is possible to provide a lithium secondary battery that is comparable to the case where LiMn ₂ O ₄ having a stoichiometric composition is used for the positive electrode.

The lithium secondary battery is overcharged,
When the temperature is 200 ° C. or higher, it is assumed that a Li-desorbed compound (X = 0 state), that is, M _Y O _Z is formed. This M _Y O _Z has an unstable crystal structure as compared with the case where Li is contained in the molecule and is considered to easily release oxygen under a temperature condition of 200 ° C. or higher. Because the amount of oxygen in the crystal structure is smaller than that in the stoichiometric composition, Li is released and M _Y
Even in the case of O _Z , it is difficult to release oxygen.

When the transition metal element constituting the lithium transition metal oxide is Mn, that is, lithium manganate, in the present invention, a part of this Mn contains Ti, and in addition, Li, Fe, Ni, Mg, Zn, Co,
Cr, Al, B, Si, Sn, P, V, Sb, Nb, T
a, Mo, and consists of one or more elements selected from the group consisting of W, the general formula becomes substituted with 2 or more transition metal elements _{Li X M A Mn YA O Z} ( where, M is a transition metal element , A is the substitution amount, and X, Y, and Z are the constituent ratios of each element in one molecule.) Are also preferably used.

The positive electrode of the present invention represented by the general formula LiM _A Mn _2-A O _Z (where M is a transition metal element, A is a substitution amount, and Z is a constituent ratio of each element in one molecule). When the element substitution as described above is performed for lithium manganate, which is an example of the active material, the Li / Mn ratio (molar ratio) is
(1 + A) / in the case of excess Li in which Mn is replaced by Li
(2-A), and when substituted with a transition metal element M other than Li, it becomes 1 / (2-A), so in any case, the Li / Mn ratio is always 0.5. .

In the present invention, as described above, Li / M
A lithium transition metal oxide having an n ratio of more than 0.5 is preferable. As a result, the crystal structure is further stabilized as compared with the case of using a stoichiometric composition, so that a battery having excellent high temperature cycle characteristics can be provided.

In the transition metal element M, theoretically, Li has a valence of +1, Fe, Ni, Mg, and Zn have a valence of +2,
B, Al, Co and Cr are +3 valence, Si and Sn are +4 valence,
P, V, Sb, Nb, and Ta are +5 valence ions, and Mo and W are +6 valence ions, which are elements that form a solid solution in the lithium transition metal oxide.
+3 for e and Sb, +4 for Cr
There may be cases where the valence is +6. Similarly, Ti, which is also a transition metal element, is an element that theoretically becomes +4 valent ions and forms a solid solution in the lithium transition metal oxide, but may be +3 valent. Therefore, various transition metal elements M and Ti may exist in a state having mixed valences,
Further, as described above, the amount Z of oxygen is not the value represented by the theoretical chemical composition (Z = 4), but is deficient within the range for maintaining the crystal structure.

Next, the second aspect of the present invention will be described. A second aspect of the present invention is a method for producing a positive electrode active material, which has a general formula of Li _X M _Y O _Z (where M is a transition metal element,
X, Y, and Z represent the composition ratio of each element in one molecule. ), The salts and / or oxides of the respective elements constituting the lithium transition metal oxide represented by the formula (1) are mixed at a predetermined ratio to form a mixture, and the mixture is mixed with an oxidizing atmosphere, 650-1000.
In the temperature range of 50 ° C., primary firing is performed once or more over 5 to 50 hours, and the obtained primary firing is performed in a nitrogen atmosphere at 750 to 90.
It is characterized in that secondary calcination is performed in a temperature range of 0 ° C., and then the obtained secondary calcination product is cooled to a temperature of 600 ° C. at a cooling rate of 10 ° C./min or more to manufacture. The details will be described below.

Raw Material for Synthesizing Positive Electrode Active Material of the Present Invention
As the charge, general formula Li _XM_YO_Z(However, M is a transition metal
Elements, X, Y, and Z are constituents of each element in one molecule
The ratio is shown. ) Constitutes a lithium transition metal oxide represented by
A salt and / or oxide of each element is used. In addition,
Mn can also be used as the transition metal element M, and
Contains Ti as a part of this Mn, and Li,
Fe, Ni, Mg, Zn, Co, Cr, Al, B, S
i, Sn, P, V, Sb, Nb, Ta, Mo, and W
Two kinds consisting of one or more elements selected from the group consisting of
In the case of performing substitution with a transition metal element of the above class, Ti and
Salt and / or acid of each element represented by the transition metal element M
Compounds can also be used. The salt of each element is not particularly limited
It is not a thing, but it has a high purity as a raw material and is inexpensive
It goes without saying that it is preferable to use
Yes. Specifically, carbonate, hydroxide, or organic acid salt is used.
It is preferable to use nitrates, hydrochlorides, sulfates, etc.
You can also

A mixture of the above-mentioned respective raw materials including a lithium salt in a predetermined ratio was first mixed in an oxidizing atmosphere at 650 ° C. to 100 ° C.
Firing (primary firing) is performed once or more in the range of 0 ° C. for 5 hours to 50 hours. Here, the oxidizing atmosphere generally means an atmosphere having an oxygen partial pressure that causes an in-furnace sample to undergo an oxidation reaction, and specifically, an atmospheric atmosphere, an oxygen atmosphere and the like are applicable.

After the first primary firing, the uniformity of the composition is not always good, but when the target positive electrode active material satisfies the Li / Mn ratio> 0.5, that is, the stoichiometric composition When elemental substitution of Mn is carried out for the lithium manganate, especially when the composition is in excess of Li formed by substituting a part of Mn with Li, Ti, Mg, etc., even if the primary firing is performed once, It was experimentally confirmed that the one exhibiting the thermal characteristics was easily obtained. Although the reason for this is not clear, it is presumed that the addition of the transition metal element M stabilizes the crystal structure.

As described above, with some compositions, it is possible to synthesize lithium manganate exhibiting predetermined thermal characteristics even by performing a single primary firing, but the synthesis conditions that are more independent of the composition are established. Therefore, it is preferable to perform the primary firing twice or more.

The number of primary firings mainly depends on the firing temperature and the firing time, and a large number of firings are required when the firing temperature is low and / or when the firing time is short. Also,
Depending on the type of the transition metal element M, it may be preferable to increase the number of firings from the viewpoint of uniform composition.
In this case, it is considered that it is difficult to form a phase atmosphere suitable for crystal growth by adding the transition metal element M.

However, since increasing the number of primary firings means that the production process is lengthened accordingly,
It is preferable to keep the number of firings to a necessary minimum. The sample (primary fired body) obtained by performing such primary firing a plurality of times is more than that obtained by performing the primary firing once.
Crystallinity is improved.

The firing temperature is lower than 600 ° C., and /
Or, when the firing time is less than 5 hours, X of the fired product
In the RD chart, a peak showing the residual of the raw material, for example, when lithium carbonate (Li ₂ CO ₃ ) is used as the lithium source, a peak of Li ₂ CO ₃ is observed and a single-phase product cannot be obtained. On the other hand, when the firing temperature is higher than 1000 ° C. and / or the firing time is longer than 50 hours, a high-temperature phase is generated in addition to the target crystal system compound, and a single-phase product cannot be obtained.

When the primary firing is performed in two or more times, it is preferable that the firing temperature in the next step is successively higher than the firing temperature in the previous step. For example, when performing the primary firing twice, the primary fired body obtained when the temperature of the second primary firing is equal to or higher than the temperature of the first time, the primary firing temperature and time of the second primary firing are used. The peak shape on the XRD chart is projected more sharply than that obtained, and the crystallinity can be improved.

In order to produce the lithium transition metal oxide that is the positive electrode active material of the present invention, a secondary fired body is obtained by performing the above-mentioned one or more times of primary firing and then secondary firing. Specifically, the primary fired body is placed in a nitrogen atmosphere at 750 to 750.
It is necessary to perform firing (secondary firing) in the temperature range of 900 ° C., and it is preferable to perform the second firing in the temperature range of 780 to 840 ° C. Then, the positive electrode active material can be manufactured by cooling to 600 ° C. at a cooling rate of 10 ° C./min or more.

In the conventional method for producing a lithium transition metal oxide, the cooling rate after firing is set to, for example, 10 described above.
C./min, which was slower than the cooling rate, that is, slow cooling was performed to suppress the release of oxygen. On the other hand, in the temperature range of 600 ° C or higher where oxygen is easily released,
By cooling at a cooling rate of ℃ / min or more,
Oxygen that has once departed becomes difficult to return, so that the desired positive electrode active material in an oxygen-deficient state can be obtained.

When the temperature of the secondary firing is lower than 750 ° C., the temperature is too low, so that the weight loss of the obtained positive electrode active material at 220 to 400 ° C. is controlled within the range of 0 to 2 mass%. However, when the temperature is higher than 900 ° C., the amount of oxygen released from the positive electrode active material increases, which reduces the crystallinity of the positive electrode active material, which is not preferable. Further, in order to more efficiently produce the desired positive electrode active material, 600
It is more preferable to cool to 20 ° C. at a cooling rate of 20 ° C./min or more, and particularly preferable to cool at a cooling rate of 30 ° C./min or more.

In the present invention, it is preferable to perform the above-mentioned secondary firing and cooling once or more, respectively, because the oxygen content can be adjusted more accurately. However, increasing the number of times of secondary firing and cooling means that the production process becomes longer, and therefore it is preferable to keep the number of times to a necessary minimum.

Next, the third aspect of the present invention will be described. A third aspect of the present invention relates to a lithium secondary battery that exhibits an excellent effect on safety, and is characterized by using any one of the positive electrode active materials described above. That is, since the above-described lithium composite oxide that is the positive electrode active material has a characteristic that it is extremely difficult to release oxygen even under a high temperature condition, the lithium secondary battery according to the present invention produced using the lithium composite oxide is particularly poor. Even when an abnormality such as a charge state occurs, the risk of explosion and fire is extremely low, and safety is excellent. Such improvement in safety is particularly remarkable in a large-capacity battery using a large amount of electrode active material, and therefore its application can be, for example, an EV or HEV motor drive power source. However, the present invention can of course be used for a small capacity battery such as a coin battery.

As the other members (materials) constituting the lithium secondary battery of the present invention used as the positive electrode active material, various conventionally known materials can be used.
For example, as the negative electrode active material, an amorphous carbonaceous material such as soft carbon or hard carbon, or a highly graphitized carbon material such as artificial graphite or natural graphite can be used. Above all, it is preferable to use a highly graphitized carbon material having a large lithium capacity.

Examples of the organic solvent used in the non-aqueous electrolytic solution include carbonate ester solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and propylene carbonate (PC), γ-butyrolactone, and tetrahydrofuran. A single solvent or a mixed solvent such as acetonitrile or acetonitrile is preferably used.

Further, as an electrolyte, lithium hexafluorophosphate (LiPF ₆ ) or lithium borofluoride (LiB) is used.
Examples thereof include lithium complex fluorine compounds such as F ₄ ), and lithium halides such as lithium perchlorate (LiClO ₄ ). One kind or two or more kinds are dissolved in the solvent and used. In particular, it is preferable to use LiPF ₆ which does not easily undergo oxidative decomposition and has high conductivity of the non-aqueous electrolyte.

The battery structure is a coin-type battery in which a separator is placed between a plate-shaped positive electrode active material and a negative electrode active material and filled with an electrolytic solution, or a surface of a metal foil is coated with the positive electrode active material. Cylindrical type or box using an electrode body formed by winding or laminating a positive electrode plate formed by processing and a negative electrode plate similarly formed by coating a negative electrode active material on the surface of a metal foil with a separator interposed therebetween. Examples include various types of batteries.

[0046]

[Examples] Specific results of the present invention will be described below.
It (Preparation of Positive Electrode Active Material) As a starting material, commercially available Li₂C
O₃, MnO₂Using the raw material powder,_1.1Mn _1.9O_Four"
Each was weighed and mixed so as to have the composition of. Then acid
Firing (primary firing) at 800 ° C for 24 hours in a chemical atmosphere
And then fired again in a nitrogen atmosphere at each temperature shown in Table 1.
(Secondary firing), 600 ℃ at a cooling rate of 10 ℃ / min
To obtain a positive electrode active material (Examples 1-3, comparison
Example 1). For Comparative Example 2, no secondary firing was performed,
The material obtained by primary firing was used as the positive electrode active material (comparison
Example 2).

(Production of Coin Cell) Each positive electrode active material, acetylene black as a conductive additive, and a binder as a binder were mixed in a weight ratio of 50: 2: 3. 0.02 g of the obtained mixture was applied to an aluminum foil (thickness 20 μm) as a positive electrode current collector so as to have a disk shape with a diameter of 20 mm, and press-molded at a pressure of 2940 Pa to obtain a positive electrode plate. This positive electrode plate (sample electrode plate) was mixed with an organic solvent prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at an equal volume ratio (1: 1) to prepare L as an electrolyte.
A non-aqueous electrolyte prepared by dissolving iPF ₆ to a concentration of 1 mol / l, a separator for separating the positive electrode plate and the negative electrode plate, metallic lithium serving as a negative electrode (the negative electrode current collector has a thickness of 10 μm).
m copper foil) was used to produce a coin cell.

(Preparation of Rolled Electrode Body) Acetylene black was added to each of the obtained positive electrode active materials as a conductive additive at an external ratio of 2 to 5.
A positive electrode material slurry prepared by adding a solvent and a binder to a material added in a range of 10% by mass, and having a thickness of 20 μm.
The aluminum foil is coated on both sides with a thickness of about 100 μm, and the fibrous highly graphitized carbon powder is used as a negative electrode active material. The copper foil with a thickness of 10 μm is coated with about 80 μm on each side. A negative electrode plate was prepared by coating so as to have a thickness of. Then, a strip of metal foil (positive electrode: aluminum foil, negative electrode: copper foil) having a thickness of 50 μm is provided on the uncoated portion of the positive and negative electrode plates where the electrode active material is not coated. A plurality of electric tabs were arranged at a predetermined interval, and attached by ultrasonic welding. Then, a separator with a thickness of 30 μm (polyolefin (PP) / polyethylene (PE) / with a three-layer structure of polyolefin)
A positive electrode plate and a negative electrode plate were laminated and wound via polypropylene (PP) to prepare a wound electrode body.

(Production of Battery) After the above-mentioned wound type electrode body was housed in a battery case, heating (100 Pa) was performed while depressurizing (1 Pa) the inside of the battery case through a predetermined electrolyte solution injection hole.
℃, 24 hours), at the same time, ethylene carbonate (E
C) and diethyl carbonate (DEC) in an equal volume ratio (1: 1) in an organic solvent, and LiP as an electrolyte.
A nonaqueous electrolytic solution prepared by dissolving F ₆ to a concentration of 1 mol / l was impregnated (vacuum impregnation). Then, the electrolyte injection holes were sealed to manufacture batteries (Examples 1 to 3 and Comparative Examples 1 and 2). The other members and the test environment were the same for all the samples, and the influence of water infiltration from the outside of the battery due to defective sealing of the battery was also eliminated. Further, the battery capacities of the respective batteries after the initial charging were all about 10 Ah.

(Measurement of Weight Reduction (Oxygen Desorption Amount) of Positive Electrode Active Material) With respect to each of the manufactured coin cells, room temperature, 0.2C
L at constant current-constant voltage charge up to 4.2V with an equivalent current value
i was released from the positive electrode active material. Then, the coin cell was disassembled in a glove box in an argon gas atmosphere to take out the positive electrode plate, washed with diethyl carbonate (DEC), and vacuum dried. The aluminum foil, which is the positive electrode current collector, is removed from the cleaned and dried positive electrode plate, and "positive electrode active material +
By performing a differential thermal analysis on the "binder + conductive auxiliary agent" under the differential thermal analyzer shown in Table 1 and the measurement conditions using the same, the positive electrode active material is reduced in weight by 220 to 400 ° C (oxygen desorption amount, Mass%) was measured. The results are shown in Table 2.

Further, FIG. 1 shows a graph in which the weight reduction (% by mass) is plotted against the temperature (° C.) for Examples 1 and 3 and Comparative Example 2. It should be noted that each of the positive electrode active materials used in the above-mentioned “Measurement of Weight Reduction (Oxygen Desorption Amount) of Positive Electrode Active Material” is an electrochemically desorbed equivalent amount of Li. Each positive electrode active material whose decrease was measured was not a positive electrode active material as shown by the composition formula shown in Table 2.

[0052]

[Table 1]

(Overcharge test) The test conditions are: constant temperature charge at a room temperature and a current equivalent to 1 C (about 10 Ah).
An overcharge test was performed on each of the manufactured batteries. In addition, when the battery ruptured / ignited as the voltage increased, it was evaluated as “x”, and when not ruptured / ignited, it was evaluated as “◯”. The results are shown in Table 2.

[0054]

[Table 2]

As is clear from the results shown in Table 2, the positive electrode active materials of Examples 1 to 3 obtained by secondary firing at a predetermined temperature and cooling at a predetermined cooling rate were 220 to
It was found that the weight reduction (mass%) up to 400 ° C. was smaller than the weight reduction (mass%) of the positive electrode active materials of Comparative Examples 1 and 2. Here, it has been confirmed that the weight reduction of the binder and the conductive auxiliary agent is 0% by mass when heated to 220 to 400 ° C., and the weight reduction (mass%) due to heating in this temperature range is the same as that of the positive electrode active material. It is a value indicating the weight reduction, that is, the amount of oxygen released from the positive electrode active material.

Further, as shown in FIG. 1, it is clear that the weight reduction of each positive electrode active material starts at around 220 ° C. That is, the amount of water adsorbed on the positive electrode active material is 1000
Since it is below ppm, there is almost no weight loss due to adsorbed water up to 220 ° C, and the weight loss starts at around 220 ° C, which means that almost no adsorbed water was attached to the measured positive electrode active material. It is supposed to mean. Therefore, the desorption of oxygen from the positive electrode active material is 220
It is thought to start at ° C. From the above, it can be considered that the weight reduction due to the release of oxygen is suppressed in Examples 1 and 3 as compared with Comparative Example 2, and compared with lithium manganate having a stoichiometric composition. It was found that the weight loss from 220 ° C to 400 ° C was suppressed as the amount of oxygen was lost.

Further, it was found that the lithium secondary batteries using the positive electrode active materials of Examples 1 to 3 were safe because they did not burst or ignite. Usually, it is considered that EC and DEC used as the organic solvent constituting the electrolytic solution are decomposed as the temperature rises due to overcharging and generate combustible gases such as hydrogen and acetylene. It is assumed that these combustible gases burn when contacted and mixed with oxygen under a high temperature condition, which may cause the lithium secondary battery to burst or ignite. 400 ° C
As is clear from the results of the weight reduction (% by mass), it is considered that oxygen is hard to be released, and therefore the lithium secondary battery using this does not burst or ignite.

[0058]

As described above, since the positive electrode active material of the present invention is a lithium transition metal oxide whose weight loss within a predetermined range under a predetermined condition, it releases oxygen even under a high temperature condition. It is possible to provide a lithium secondary battery that is difficult to handle and does not cause problems such as rupture and ignition even under an overcharged condition and has excellent safety. Further, according to the method for producing a positive electrode active material of the present invention, a positive electrode active material having the above physical characteristics can be easily produced. Furthermore, since the lithium secondary battery of the present invention is manufactured by using the positive electrode active material having the characteristic that it is difficult to release oxygen even under high temperature conditions, it is difficult for problems such as rupture and ignition to occur.
Excellent safety is given.

[Brief description of drawings]

FIG. 1 is a graph showing the measurement results of the weight reduction of a positive electrode active material, in which the weight reduction (% by mass) is plotted against the temperature (° C.).

Continued front page    F-term (reference) 4G048 AA04 AB02 AC06 AD06 AE06                 5H029 AJ12 AJ14 AK03 AL07 AL08                       AL12 AM02 AM03 AM05 AM07                       CJ02 CJ08 CJ28 DJ17 EJ04                       HJ01 HJ02 HJ14                 5H050 AA03 AA15 AA19 BA16 BA17                       CA07 CA09 CB08 CB09 CB12                       FA19 GA02 GA10 HA01 HA02                       HA14 HA20

Claims (12)

[Claims] 1. A positive electrode active material used for a positive electrode of a lithium secondary battery, wherein the weight loss at 220 to 400 ° C. is in the range of 0 to 2 mass%, and its chemical formula is represented by the general formula Li _X M _Y O
A positive electrode active material, which is a lithium transition metal oxide represented by _Z (where M is a transition metal element, X, Y, and Z represent the composition ratio of each element in one molecule). 2. The positive electrode active material according to claim 1, wherein the weight loss at 220 to 400 ° C. is in the range of 0 to 1% by mass. 3. The _{X in the} general formula Li _X M _Y O _Z (where M is a transition metal element, X, Y, and Z represent the composition ratio of each element in one molecule), and the above X is 1, When Y is 2, the positive electrode active material according to claim 1 or 2, wherein Z is 3.6 to 3.8. 4. The transition metal element M in the general formula Li _X M _Y O _Z (where M is a transition metal element, and X, Y, and Z represent the composition ratio of each element in one molecule). Mn
The positive electrode active material according to any one of claims 1 to 3. 5. A cubic spinel structure according to any one of claims 1 to 4.
The positive electrode active material according to any one of 1. 6. The lithium transition metal oxide contains Ti as a part of the Mn, and Li, F
e, Ni, Mg, Zn, Co, Cr, Al, B, Si,
_A general formula Li _X M _A Mn formed by substituting two or more transition metal elements consisting of one or more elements selected from the group consisting of Sn, P, V, Sb, Nb, Ta, Mo, and W.
_YA O _Z (where M is the transition metal element, A is the substitution amount, X,
Y and Z represent the composition ratio of each element in one molecule. ] The positive electrode active material of Claim 4 or 5 represented by these. 7. The Li / Mn ratio is more than 0.5.
7. The positive electrode active material according to any one of items 6 to 6. 8. A lithium transition metal oxide represented by the general formula Li _X M _Y O _Z (where M is a transition metal element, and X, Y, and Z represent the composition ratio of each element in one molecule). After mixing the salts and / or oxides of each element constituting the product in a predetermined ratio to form a mixture, the mixture is mixed with an oxidizing atmosphere,
Primary firing is performed by performing primary firing at least once in the range of 1000 ° C. for 5 to 50 hours, and the primary firing is subjected to secondary firing in a nitrogen atmosphere and a temperature range of 750 to 900 ° C. to perform secondary firing. A method for producing a positive electrode active material, comprising: obtaining a body, and then cooling the secondary fired body up to 600 ° C. at a cooling rate of 10 ° C./min or more. 9. Lithium represented by the general formula Li _X Mn _Y O _Z in which the transition metal element M is Mn (where X, Y, and Z represent the composition ratio of each element in one molecule). The method for producing a positive electrode active material according to claim 8, wherein salts and / or oxides of respective elements constituting the transition metal oxide are mixed in a predetermined ratio. 10. A part of the Mn contains Ti, and in addition, Li, Fe, Ni, Mg, Zn, Co, Cr, A
1, B, Si, Sn, P, V, Sb, Nb, Ta, M
The general formula L substituted with two or more kinds of transition metal elements consisting of one or more kinds of elements selected from the group consisting of o and W
_{i X M A Mn YA O Z} ( where, M is a transition metal element, A is the substitution amount, X, Y, and Z are. showing the composition ratio of each element in the molecule) lithium transition metal oxide represented by The method for producing a positive electrode active material according to claim 9, wherein salts and / or oxides of respective elements constituting the product are mixed in a predetermined ratio. 11. The method for producing a positive electrode active material according to claim 8, wherein each of the secondary firing and the cooling is performed once or more. 12. A lithium secondary battery using the positive electrode active material according to any one of claims 1 to 7.