JP2003203678A - Lithium secondary cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary cell of which, manganese is prevented from flowing out into electrolyte liquid, and having excellent cycle property at high temperature. <P>SOLUTION: A positive electrode activator of the lithium secondary cell is composed of lithium double oxide, and the lithium double oxide is composed of lithium manganate having a cubic system spinel structure including titanium, and carbonate is mixed to the lithium double oxide. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a lithium secondary battery having excellent cycle characteristics, especially at high temperatures.

[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 unit cell voltage of about 4V.
Due to its high degree of features, not only the portable electronic device but also an electric vehicle (hereinafter referred to as “EV”), which has been widely popularized as a low-pollution vehicle due to recent environmental problems. Or as a motor drive power source for a hybrid electric vehicle (hereinafter referred to as “HEV”).

In such a lithium secondary battery, the battery capacity and charge / discharge cycle characteristics (hereinafter referred to as “cycle characteristics”) largely depend 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 ), lithium nickel oxide (LiNiO ₂ ), or lithium manganate (Li
Mn ₂ O ₄ ) and the like.

[0005]

Problems to be Solved by the Invention Among such lithium composite oxides, lithium manganate (having a cubic spinel structure) is used because the raw material is inexpensive, the power density of the battery is large, and the potential is high. Stoichiometric composition: L
iMn ₂ O ₄ ) is often used. However, this lithium manganate has a problem that it is difficult to obtain good cycle characteristics because the discharge capacity gradually decreases with repeated charging and discharging at high temperature.

As one of the causes of decrease in discharge capacity,
The following phenomena are expected to occur in the battery system. That is, in the case of a lithium secondary battery produced by using, for example, a LiPF ₆ type electrolytic solution as an electrolytic solution, HF is generated in the system particularly under high temperature conditions, whereby a part of manganese is electrolyzed from the lithium manganate spinel. It is considered to be eluted in the liquid, and the deterioration of the positive electrode active material and the adverse effect on the negative electrode active material are caused, so that the cycle characteristics at high temperatures as described above (hereinafter, referred to as “high temperature cycle characteristics”) are deteriorated. It is assumed that a malfunction such as a job will be caused.

As a related technique for solving such a problem, a positive electrode active material obtained by substituting a part of manganese (Mn) of lithium manganate (LiMn ₂ O ₄ ) used as a positive electrode active material with a different element is used. In order to suppress the elution of manganese into the electrolytic solution, lithium manganate has been generally surface-treated, and other measures have been taken. Further, in JP-A-2000-67869, a non-aqueous electrolyte secondary battery using a lithium-manganese composite oxide for a positive electrode, wherein the positive electrode contains manganese carbonate or the electrolyte contains manganese carbonate A non-aqueous electrolyte secondary battery that is dissolved and has improved cycle characteristics is disclosed.

However, it cannot be said that the effect of suppressing the elution of manganese into the electrolytic solution is still sufficient by any of the above-mentioned measures, and further, JP-A-2000-67869.
Even with the non-aqueous electrolyte secondary battery described in the publication, it cannot be said that the discharge capacity retention rate, which is an index for evaluating the cycle characteristics of the battery, is sufficient, and further improvement in high temperature cycle characteristics is achieved. It has been requested by industry to develop a lithium secondary battery. The "discharge capacity maintenance rate" means
It is a value obtained by dividing the discharge capacity of the battery after a lapse of a predetermined number of charge / discharge cycles by the initial discharge capacity.

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 suppress the elution of manganese into the electrolytic solution and to improve the cycle characteristics at high temperature. It is to provide an excellent lithium secondary battery.

[0010]

Means for Solving the Problems That is, according to the present invention, there is provided a lithium secondary battery in which the positive electrode active material is a lithium composite oxide, wherein the lithium composite oxide has a cubic spinel structure. Provided is a lithium secondary battery characterized in that lithium is contained in Ti and that the lithium composite oxide is mixed with a carbonate.

In the present invention, the lithium composite oxide is a LiMn _2-X Ti _X O ₄ obtained by substituting Ti for part of the transition element Mn in lithium manganate (LiMn ₂ O ₄ ).
(However, X represents a substitution amount.)

In the present invention, a lithium composite oxide
But LiMn_2-XTi_XO _Four(However, X indicates the substitution amount
You ), A part of the transition element Mn is further added to Li, Fe,
Ni, Mg, Zn, Co, Cr, Al, B, V, Si,
Is a group consisting of Sn, P, Sb, Nb, Ta, Mo and W
LiM formed by substituting one or more elements selected from_ZM
n_2-XZTi_XO_Four(However, M is a substitution element and X and Z are
Indicates the exchange amount. ) Is preferred, and the substitution amount X is
Is preferably in the range of 0.01 ≦ X ≦ 0.5.

Further, in the present invention, the carbonate is preferably manganese carbonate, and the lithium composite oxide is mixed with manganese carbonate at a ratio of 0.1 to 15 mass% with respect to the amount of the lithium composite oxide. It is preferable that manganese carbonate is mixed in a ratio of 1 to 10% by mass.

In the present invention, the Li / Mn ratio in the lithium composite oxide is preferably more than 0.5. Furthermore, in the present invention, lithium manganate is
A mixture of salts and / or oxides of each element adjusted to a predetermined ratio is added in an oxidizing atmosphere at 650 to 1000 ° C. for 5 to 5
It is preferably obtained by firing for 50 hours. The firing is more preferably performed twice or more, and it is preferable that the firing temperature be successively increased each time the firing is repeated.

[0015]

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.

The present invention is a lithium secondary battery in which the positive electrode active material is a lithium composite oxide, and the lithium composite oxide contains lithium manganate having a cubic spinel structure and Ti. A carbonate is mixed with the lithium composite oxide. The details will be described below.

The lithium composite oxide used as the positive electrode active material of the lithium secondary battery of the present invention is lithium manganate having a cubic spinel structure containing Ti, and is mixed with a carbonate. It is what Particularly, the carbonate neutralizes HF generated from the electrolytic solution at a high temperature, thereby exhibiting the effect of suppressing Mn elution from the positive electrode active material. Although the reason for this suppression effect is not clear, the suppression effect is most effective when the lithium composite oxide is one in which lithium manganate contains Ti. Neutralization of HF by such a carbonate is the main effect in the lithium secondary battery of the present invention, but in addition to this, the carbonate ion in the carbonate contributes to the formation of the Li ion permeable coating on the negative electrode surface. It is thought that this is the case, and it also has the function of suppressing the rise in battery impedance. Therefore, in the lithium secondary battery of the present invention, reduction in discharge capacity retention rate is suppressed, and particularly high temperature cycle characteristics are improved.

Furthermore, the lithium secondary battery according to the present invention using lithium manganate containing Ti as a positive electrode active material is a lithium secondary battery using lithium manganate containing no Ti as a positive electrode active material. In comparison, since the elution of Mn into the electrolytic solution is suppressed especially at high temperatures, the discharge capacity retention rate is high and the high temperature cycle characteristics are excellent.

The term “lithium manganate to T
“I is included” means that Ti is not substituted with the transition element Mn and is mixed as an independent chemical substance in addition to a state in which a part of the transition element Mn in lithium manganate is substituted with Ti. Or a state in which the Ti element is simply mixed as an impurity or the like in the positive electrode active material, and the effect of the present invention is exhibited in any of the contained states. . Here, the stoichiometric composition of lithium manganate is represented by LiMn ₂ O ₄ , but in the present invention, the lithium composite oxide used as the positive electrode active material is a lithium manganate (LiMn ₂ O ₄ ). LiMn _2−x Ti _x O formed by substituting Ti for part of the transition element Mn
₄ (however, X represents the amount of substitution). Although the reason for this is not clear, by using a lithium composite oxide in which a part of Mn is replaced by Ti, the elution of Mn into the electrolytic solution is suppressed more effectively, and the high temperature cycle characteristics are improved. Is done.

Further, in the present invention, the lithium composite oxide used as the positive electrode active material has the above stoichiometric composition, that is, LiMn _2-X Ti _X O ₄ (where X represents the substitution amount). Without being limited, a part of the transition element Mn in LiMn _2−x Ti _x O ₄ is further added to Li, Fe, Ni, Mg, Z
n, Co, Cr, Al, B, V, Si, Sn, P, S
1 selected from the group consisting of b, Nb, Ta, Mo and W
LiM _Z Mn _2-XZ Ti _X formed by substituting more than one kind of element
O ₄ (where M is a substitution element and X and Z are substitution amounts) is also preferably used.

In the present invention, LiMn _2-X Ti _X O
₄ (where X represents the amount of substitution), LiM _Z Mn _2-XZ T
In the positive electrode active material represented by i _X O ₄ (where M is a substitution element and X and Z are substitution amounts), X (replacement amount X) representing the substitution amount of Ti is 0.01 ≦ X ≦ The range of 0.5 is preferable, the range of 0.03 ≦ X ≦ 0.3 is more preferable, and the range of 0.05 ≦ X ≦ 0.2 is particularly preferable. When the substitution amount X is less than 0.01, the effect of improving the high temperature cycle characteristics due to substituting a part of Mn with Ti is not remarkably exhibited, which is not preferable. If it exceeds 5, it may cause deterioration of the high temperature cycle characteristics, which is not preferable.

Further, in the present invention, the fact that the carbonate mixed in the lithium composite oxide is manganese carbonate is higher than that in the case where another carbonate containing a metal cation other than Mn as a constituent element is mixed. It is preferable because the high temperature cycle characteristics are improved.

In the present invention, it is preferable that manganese carbonate is mixed with the lithium composite oxide that is the positive electrode active material at a ratio of 0.1 to 15 mass% with respect to the amount of the lithium composite oxide. It is more preferable that they are mixed in a ratio of 1 to 10% by mass, and it is particularly preferable that they are mixed in a ratio of 1 to 5% by mass. When the amount of manganese carbonate mixed is less than 0.1% by mass with respect to the amount of lithium composite oxide, the effect of improving the high temperature cycle characteristics due to the addition of manganese carbonate is not remarkably exhibited. Not preferable. If the amount of manganese carbonate mixed is more than 15% by mass with respect to the amount of the lithium composite oxide, the high temperature cycle characteristics may be deteriorated, which is not preferable.

In the present invention, lithium manganate (LiMn ₂ O ₄ ) is subjected to element substitution such as Ti as described above, and the lithium composite oxide used as the positive electrode active material has a Li / Mn ratio (molar ratio). ) Is 1 / (2-X) when Mn is replaced by only Ti, and 1 / (2-X- when it is replaced by Ti and the replacement element M.
Z), it is always Li / M in any case.
The n ratio is> 0.5.

In the present invention, as described above, Li / M
It is preferable to use a lithium composite oxide having an n ratio of more than 0.5. This further stabilizes the crystal structure as compared with the case of using a stoichiometric composition,
A battery having excellent high temperature cycle characteristics can be obtained.

The substitution element M is theoretically
Li is +1 valence, Fe, Ni, Mg, Zn is +2 valence, 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 solid-solution elements in LiMn ₂ O ₄ .
Co and Sn may be +2 valent, Fe and Sb may be +3 valent, and Cr may be +4 and +6 valent. Similarly, Ti, which is also a substitution element, is an element that theoretically becomes +4 valent ions and forms a solid solution in LiMn ₂ O ₄ , but it may be +3 valent. Therefore, the various substitution elements M and Ti may exist in a state having mixed valences, and the amount of oxygen does not necessarily need to be 4 as represented by the theoretical chemical composition, It may be deficient or excessively present within the range for maintaining the crystal structure.

As another synthetic raw material of the lithium composite oxide, a salt and / or oxide of each element (including the substitution elements M and Ti when element substitution is performed) is used. The salt of each element is not particularly limited, but it is needless to say that it is preferable to use an inexpensive salt having high purity as a raw material. Specifically, carbonates, hydroxides and organic acid salts are preferably used, but nitrates, hydrochlorides, sulfates and the like can also be used.

A mixture of the above-mentioned respective raw materials containing a lithium salt in a predetermined ratio was first prepared in an oxidizing atmosphere at 650 ° C.-100.
Baking is performed 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 firing, the compositional uniformity is not always good, but as in the positive electrode active material used in the lithium secondary battery of the present invention, the Li / Mn ratio> 0.
5 is satisfied, that is, when the elemental substitution of Mn is carried out with respect to the stoichiometric composition, particularly in a composition in which Li is excessive and a part of Mn is substituted by Li, Ti, Mg or the like,
It was experimentally confirmed that the one having a predetermined thermal characteristic can be easily obtained even by firing once. The reason for this is not clear, but it is presumed that the addition of the substitutional 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 one-time firing, but a synthesis condition that is more independent of composition is established. Therefore, it is preferable to perform firing twice or more.

The number of firings mainly depends on the firing temperature and the firing time. When the firing temperature is low and / or the firing time is short, a large number of firings are required. Further, depending on the type of the substituting 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 substitution element M.

However, increasing the number of firings means that the production process becomes longer by that much, and therefore it is preferable to keep the number of firings to a necessary minimum. The sample obtained by performing such firing a plurality of times can have higher crystallinity than the sample obtained by performing the firing once.

The firing temperature is less 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 firing is performed twice or more, it is preferable that the firing temperature in the next step be successively higher than the firing temperature in the previous step. For example, in the case of the second firing, the product obtained when the synthesis is performed with the second firing temperature being equal to or higher than the first firing temperature is a product obtained by using the conditions of the second firing temperature and the second firing time. The peak shape on the XRD chart protrudes more sharply than the product, and the crystallinity can be improved.

Since the above-described lithium composite oxide has a stable crystal structure, the lithium secondary battery according to the present invention using the lithium composite oxide as a positive electrode active material is particularly improved in high temperature cycle characteristics. Has been. Such improvement in high-temperature cycle characteristics 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 member (material) for forming the lithium secondary battery of the present invention using the above-mentioned lithium composite oxide as the positive electrode active material, various conventionally known materials can be used. . For example, as the negative electrode active material,
Amorphous carbonaceous materials such as soft carbon and hard carbon, and highly graphitized carbon materials such as artificial graphite and 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.

Examples of the electrolyte include lithium complex fluoride compounds such as lithium hexafluorophosphate (LiPF ₆ ) and lithium borofluoride (LiBF ₄ ), or 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 metal foil surface is coated with the positive electrode active material. Cylindrical type or box type using an electrode body formed by winding or laminating a positive electrode plate formed by processing and a negative electrode plate formed by similarly applying a negative electrode active material on the surface of a metal foil with a separator interposed therebetween. Various batteries can be mentioned.

[0040]

EXAMPLES The concrete results of the present invention will be described below. (Synthesis of Lithium Composite Oxide) Commercially available Li ₂ CO ₃ , MnO ₂ , and TiO ₂ powders were used as starting materials, and LiM
Each was weighed and mixed so as to have a composition of n _1.9 Ti _0.1 O ₄ . Then, firing was performed in an oxidizing atmosphere at 800 ° C. for 24 hours to synthesize a lithium composite oxide having a spinel structure.

(Preparation of Positive Electrode Material) The synthesized lithium composite oxide and manganese carbonate were dry-mixed so that the mixing amount of manganese carbonate with respect to the lithium composite oxide was the ratio (mass%) shown in Table 1 to obtain a positive electrode. Materials were prepared (Example 1
5 and Comparative Examples 1 to 4 (however, Comparative Example 1 does not contain manganese carbonate).

(Soaking Treatment of Positive Electrode Material in Electrolyte Solution) 5 g each of the prepared positive electrode material (total 9 samples) was mixed with EC and DEC in an equal volume ratio (1: 1) in an organic solvent, and 1 mol of LiPF ₆ was used as an electrolyte. It was immersed for 400 hours in a 20 ml electrolytic solution at 80 ° C. prepared by dissolving it so that the concentration became 1 / l. Next, separate each sample from the electrolyte with a filter paper filter,
The mixed organic solvent of EC and DEC and DEC were washed in this order. Mn eluted in each electrolytic solution by inductively coupled high frequency plasma emission spectrometry (ICP) using the collected electrolytic solution.
Was quantified. The results are shown in Table 1. In addition, "Mn in Table 1
"Elution rate (%)" means "lithium composite oxide not mixed with manganese carbonate (LiMn _1.9 Ti) shown in Comparative Example 1.
_0.1 O ₄ only) ”is a numerical value calculated with the Mn elution amount being 100 (%).

(Production of Battery) Using the above-mentioned positive electrode material (total of 9 samples), acetylene black powder as a conductive material and polyvinylidene fluoride as a binder were mixed in a mass ratio of 50:
The mixture was added and mixed at a ratio of 2: 3. The resulting mixture 0.0
2 g was press-molded at a pressure of 300 kg / cm ² into a disk shape having a diameter of 20 mmφ to obtain a positive electrode. Next, EC and DEC
An electrolytic solution prepared by dissolving LiPF ₆ as an electrolyte in an organic solvent mixed at an equal volume ratio (1: 1) to a concentration of 1 mol / l, a negative electrode made of carbon, and a separator separating the positive electrode and the negative electrode. , And a total of 9 coin cells were produced using the positive electrode produced as described above.

(Evaluation of high temperature cycle characteristics) The nine coin cells thus prepared were placed in a constant temperature bath having an internal temperature of 55 ° C., and a constant current of 1 C rate and a constant voltage of 4.1 V were set according to the capacity of the positive electrode active material. Charged up. Next, similarly, charging / discharging for discharging to 2.5 V with a constant current of 1 C rate was set as one cycle, and up to 100 cycles were performed, and the discharge capacity retention rate (%) after 100 cycles was measured. The "discharge capacity retention rate (%)" in this evaluation is a numerical value (%) obtained by dividing the discharge capacity after 100 cycles by the initial discharge capacity.

[0045]

[Table 1]

(Discussion) As is clear from the results shown in Table 1, the positive electrode materials in which the mixing amount of manganese carbonate is within the predetermined numerical range (Examples 1 to 5) are outside the predetermined numerical range (Comparative Examples 1 to 1). It was found that the elution rate of Mn in the electrolytic solution was lower than that of the positive electrode material of 4). Furthermore, regarding the discharge capacity retention ratios of the lithium secondary batteries manufactured using these positive electrode materials, the batteries of Examples 1 to 5 have higher discharge capacity retention ratios than the batteries of Comparative Examples 1 to 4. , And was found to have better high temperature cycle characteristics. Therefore, the excellent effect of the present invention could be confirmed.

[0047]

As described above, in the lithium secondary battery of the present invention, the lithium composite oxide that is the positive electrode active material contains lithium manganate having a predetermined structure and Ti. Since the lithium composite oxide is mixed with carbonate, elution of manganese into the electrolytic solution is suppressed even under high temperature conditions,
It has excellent high temperature cycle characteristics.

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Claims (11)

[Claims] 1. A lithium secondary battery in which the positive electrode active material is a lithium composite oxide, wherein the lithium composite oxide contains lithium manganate having a cubic spinel structure and Ti. A lithium secondary battery in which a carbonate is mixed with the lithium composite oxide. 2. The LiMn _2-X Ti _X O ₄ wherein the lithium composite oxide is obtained by substituting Ti for part of the transition element Mn in the lithium manganate (LiMn ₂ O ₄ ).
(However, X shows a substitution amount.) The lithium secondary battery according to claim 1. 3. The lithium composite oxide comprises: a part of the transition element Mn in the LiMn _2−x Ti _x O ₄ (where X represents a substitution amount);
Ni, Mg, Zn, Co, Cr, Al, B, V, Si,
LiM _Z M formed by substituting one or more elements selected from the group consisting of Sn, P, Sb, Nb, Ta, Mo and W
_{n 2-XZ Ti X O 4} ( where, M is a substituted element, X, Z represents a substituted amount.) The lithium secondary battery according to claim 2 which is. 4. The lithium secondary battery according to claim 3, wherein the substitution amount X is in the range of 0.01 ≦ X ≦ 0.5. 5. The lithium secondary battery according to claim 1, wherein the carbonate is manganese carbonate. 6. The lithium secondary battery according to claim 5, wherein manganese carbonate is mixed in the lithium composite oxide at a ratio of 0.1 to 15 mass% with respect to the amount of the lithium composite oxide. 7. The lithium secondary battery according to claim 5, wherein manganese carbonate is mixed in the lithium composite oxide at a ratio of 1 to 10 mass% with respect to the amount of the lithium composite oxide. 8. Li / in the lithium composite oxide
The lithium secondary battery according to claim 1, wherein the Mn ratio is more than 0.5. 9. The lithium composite oxide is calcined with a mixture of salts and / or oxides of each element adjusted to a predetermined ratio in an oxidizing atmosphere at 650 to 1000 ° C. for 5 to 50 hours. The lithium secondary battery according to any one of claims 1 to 8, which is obtained by the above. 10. The lithium secondary battery according to claim 9, wherein the lithium composite oxide is obtained by performing the firing twice or more. 11. The lithium secondary battery according to claim 10, wherein the lithium composite oxide is obtained by successively increasing the firing temperature each time the firing is repeated.