JP2000067861A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a large output capacity without lowering an energy density when a battery is used as a motor drive source of an electric vehicle by using LiTixMn2-xO4 in which one portion of Mn of LiMn2O4 having a cubic system spinel structure is substituted with Ti as a positive electrode active material and making a substituted amount in a specific range. SOLUTION: As a positive electrode active material of a lithium secondary battery, LiTixMn2-xO4 (x represents a substituted amount) in which one portion of Mn of LiMn2O4 having a cubic system spinel structure is substituted with Ti is used. A substituted amount x is preferably in a range of 0.01<=x<=0.5 and when it is in the range of 0.1<=x<=0.3, since an effect of a low resistance of the positive electrode active material particularly outstandingly appears, the range is more preferable. A synthesis of such LiTixMn2-xO4 is preferably carried out by calcinating a mixture of a salt and/or an oxide of each element prepared at a predetermined ratio at a range of 700-900 deg.C under an oxygen atmosphere for 5 to 50 hours.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a secondary battery used as an operating power source for a portable electronic device or a motor driving power source for an electric vehicle or a hybrid electric vehicle. The present invention relates to a lithium secondary battery used as a substance and having a small internal resistance and capable of performing high-power discharge stably.

[0002]

2. Description of the Related Art In recent years, portable electronic devices such as mobile phones, VTRs, and notebook computers have been rapidly reduced in size and weight. As a power supply battery, a lithium transition metal composite oxide is used as a positive electrode active material. A secondary battery using an organic electrolytic solution obtained by dissolving a carbonaceous material as an anode active material and a Li-ion electrolyte in an organic solvent as an electrolytic solution has been used.

[0003] Such a battery is generally called a lithium secondary battery or a lithium ion battery, and is characterized by a high energy density and a high open circuit voltage of a unit cell of about 4V. In addition to the portable electronic devices, an electric vehicle (EV) or a hybrid electric vehicle (HEV) has been actively promoted as a low-emission vehicle due to recent environmental problems.
Has also attracted attention as a motor drive power supply.

The charge and discharge characteristics of a lithium secondary battery largely depend on the material characteristics of a positive electrode active material to be used. As the lithium transition metal composite oxide as the positive electrode active material, specifically, LiCoO ₂ , LiMn ₂ O _{4 and the} like are used. However, with respect to LiCoO ₂ , the production area of Co is limited, and the production of LiCoO ₂ is limited. Since the amount is not large and expensive, there is a problem in using it for a general-purpose lithium secondary battery. In addition, there is a problem that the output density is lower than that of LiMn ₂ O ₄ . In particular, EV, HEV
When used in applications that require a large output when horsepower is required, such as when accelerating or climbing a slope, such as when using batteries, a low output density is a critical disadvantage. On the other hand, Li
Mn ₂ O ₄ is characterized in that the raw material is inexpensive, the output density is high, and the potential is high.

[0005]

However, the conductivity of LiMn ₂ O ₄ , which is a mixed conductor of electron conductivity (conductivity) and ionic conductivity, is not very large. Therefore, when LiMn ₂ O ₄ is used as the positive electrode active material, a means for improving the conductivity by mixing a fine powder such as acetylene black as a conductive aid has been adopted, but a conductive material that does not contribute to ionic conduction is employed. The addition of the auxiliary agent has the disadvantage that the amount of the positive electrode active material that can be filled in the battery is reduced, and the energy density (weight density) is reduced. Acetylene black also has a problem that it is difficult to uniformly mix acetylene black because of its low bulk density and poor dispersibility.

Therefore, it is considered preferable to reduce the resistance of LiMn ₂ O ₄ itself without relying on such a conductive assistant. A reduction in the resistance of the positive electrode active material itself leads to a reduction in the internal resistance when a battery is assembled. In particular, in a lithium secondary battery used as a power supply for driving a motor such as an EV, the energy density is low. This is very important in that a large current output required for acceleration, climbing a slope, or the like can be obtained without lowering the power, and the charge / discharge efficiency can be increased.

[0007]

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to provide LiMn ₂ O ₄ used as a positive electrode active material.
It is an object of the present invention to reduce the internal resistance of the manufactured lithium secondary battery and improve the charge / discharge characteristics by reducing the resistance.
That is, according to the present invention, LiTi in which a part of Mn of LiMn ₂ O ₄ having a cubic spinel structure is substituted with Ti.
_x Mn _2-x O ₄ (where, x is. representing the amount of substitution) lithium secondary battery, characterized by using as the positive electrode active material, is provided.

Here, the replacement amount x is 0.01 ≦ x ≦
It is preferable to be in the range of 0.5, and 0.1 ≦ x ≦
More preferably, it is in the range of 0.3. Such a synthesis of LiTi _x Mn _2-x O ₄ is preferably carried out by preparing a mixture of salts and / or oxides of the respective elements adjusted to a predetermined ratio.
Oxidizing atmosphere, 700 ° C to 900 ° C, 5 hours to 5 hours
This is performed by firing for 0 hours. Such a lithium secondary battery of the present invention is particularly suitably used as a battery for driving a motor of an electric vehicle or a hybrid electric vehicle.

Japanese Patent Application Laid-Open No. Hei 6-275265 discloses that as an active material of at least one of a negative electrode and a positive electrode, at least one kind of metal M selected from Mn and V (vanadium) or Ti and lithium. Composite oxide Li
invention the effect of using _{x M y Mn 1-y O} 2 is disclosed. The formula _{Li x M y Mn 1-y} O 2 , depending on the way of taking the values of x and y, the same chemical formula as LiTi _x Mn _2-x O ₄ of the present invention. However, Japanese Patent Application Laid-Open No. Hei 6-275265 discloses that LiMn ₂ O ₄
To mention its advantages and disadvantages, LiMn ₂ O ₄
And the _{Li x M y Mn 1-y} O 2 thought because it is described to be differentiated, and the present invention and the invention of JP-A-6-275265 discloses disclosure, Formula entirely different but similar what Obviously, this is another invention using a substance.

Further, in Japanese Laid-6-275265 _{discloses, Li x M y Mn 1-} y O cites 0.25 <y <0.75 as a preferred composition range in _2, but the LiMn ₂ When it is applied to O ₄ , 0.5 <y <1.5, which indicates that it is out of the range of substitution of Ti in the present invention described above. In the present invention, as described below,
LiTi _x Mn _2-x O ₄ in 0.5 <from the fact that it is impossible to perform the substitution in the range of x, different from the cubic spinel LiMn ₂ O ₄ and _{Li x M y Mn 1-y} O 2 Considered to be a substance.

Furthermore, in JP-A 6-275265 discloses, although the description of the the _{Li x M y Mn 1-y} O 2 can be used as the positive electrode active material it is, used as a negative electrode active material, known in this case Positive electrode active materials LiCoO ₂ , LiM
There is disclosed only a usage example in which a large electromotive force is obtained using a material having a high electrode potential such as n ₂ O ₄ or metal Li. Material capable of lithium ion occlusion / release from the difference between the electrode potential of the counter electrode to be used, can be used in any of the cathode active material and the anode active material, the electrode potential is less noble Li _x M _y When Mn _1-y O ₂ is used as the positive electrode active material, a battery configuration capable of obtaining a practical electromotive force, that is,
No mention is made of the form of the combination with the negative electrode active material.

From the above, the present invention and Japanese Patent Laid-Open No. 6-2
The invention disclosed in Japanese Patent No. 75265 is a completely different invention although the appearances are similar, and from the viewpoint of its use and effects, the present invention is not easily made based on the invention disclosed in Japanese Patent Application Laid-Open No. 6-275265. It is clear.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In a lithium secondary battery of the present invention, LiTi _x Mn _2-x O _{4 in} which a part of Mn of LiMn ₂ O ₄ is substituted by Ti is used as a positive electrode active material. Here, x represents the substitution amount, and LiMn ₂ O ₄ has a cubic crystal system and a spinel structure as a crystal structure.

Here, the substitution amount x is 0.01 ≦ x ≦ 0.
It is preferably within the range of 5. Replacement amount x is 0.01
Below, no reduction in the resistance of the positive electrode active material was observed.
On the other hand, when the substitution amount x exceeds 0.5, the formation of a foreign phase (Ti compound) in the synthesis of the positive electrode active material is reduced by the powder X-ray diffraction method (XR
D), no single phase material was obtained. In a battery, such a different phase only increases the weight of the positive electrode active material and does not contribute to the battery reaction. Therefore, it is needless to say that it is preferable to avoid generation and inclusion of the different phase. When the substitution amount x is in the range of 0.1 ≦ x ≦ 0.3, the effect of lowering the resistance of the positive electrode active material is particularly remarkably exhibited, as shown in Examples described later, which is preferable.

Note that L in LiTi _x Mn _2-x O ₄
Although the i amount is 1 in the chemical formula, it may be excessively present due to a lattice defect or the like, and on the contrary, it may contain only 1 or less. Further, Ti is affected by the occlusion / release of Li ions and lattice defects, and has a valence of +3 and +4.
It is thought to exist in a mixed valence state. Further, the oxygen content in LiTi _x Mn _2-x O ₄ is also 4 as a chemical composition formula, but the case where the oxygen content is missing or excessive within the range for maintaining the crystal structure is excluded. It does not do. That is, the chemical formula of LiTi _x Mn _2-x O ₄ shown in the present invention is expressed by the formula of LiMn ₂ O ₄ , which is the most typical chemical composition of lithium manganate having a cubic spinel structure.
It should be understood that this is a representation of a partially substituted state of n.

In the synthesis of LiTi _x Mn _2-x O ₄ of the present invention, preferably, a mixture of salts and / or oxides of the respective elements adjusted to a predetermined ratio is mixed in an oxidizing atmosphere at 700 ° C.
The calcination is performed in the range of 900 ° C. for 5 hours to 50 hours, and thus a single-phase product can be easily obtained. . The salt of each element is not particularly limited, but it goes without saying that it is preferable to use a high-purity and inexpensive raw material. Above all, it is preferable to use carbonates and acetates which do not generate harmful decomposition gases at the time of heating and firing, but nitrates, hydrochlorides, sulfates and the like can also be used. Note that an organometallic compound can also be used, and high-purity synthesis and composition uniformity can be obtained.However, since it is generally expensive for the inorganic reagent, it is used as a general-purpose battery material. hard.

By substituting a part of Mn with Ti as in the present invention, the resistance of the positive electrode active material itself can be reduced, and when incorporated in a battery, the internal resistance of the battery can be reduced. . In this way, high-power (high-current) discharge is possible, but since the internal resistance is small, the Joule heat generated due to the internal resistance is small, and the temperature rise of the battery is suppressed, and operation stability is secured. In addition, energy loss during charging and discharging can be reduced.

Further, it has been found that the solid solution of Ti suppresses a decrease in battery capacity due to repetition of charge and discharge, that is, an effect of improving charge and discharge cycle characteristics. Conventionally, in LiMn ₂ O ₄ , it has been known that the crystal structure changes irreversibly due to occlusion / release of lithium ions, and the battery capacity decreases accordingly. It is considered that the effect of stabilizing the crystal lattice against occlusion / release of lithium ions was obtained by dissolving Ti in LiMn ₂ O ₄ in a solid solution.

A battery having such a low internal resistance has a good traveling performance during acceleration or climbing a hill, which requires a large output, particularly when used as a power supply for driving an EV or HEV motor. Maintained and preferred. Further, it is preferable that the charge-discharge cycle characteristics are excellent and the capacity decrease is small from the viewpoint that the continuous running distance is prevented from becoming shorter with time.

The other materials used for producing the battery using the positive electrode active material of the present invention are not particularly limited, and various conventionally known materials can be used. For example, as the negative electrode active material, an amorphous carbon material such as soft carbon or hard carbon, artificial graphite such as highly graphitized carbon material, or a carbon material such as natural graphite is used.

As the organic electrolyte, ethylene carbonate (EC), diethyl carbonate (DE)
C), carbonates such as dimethyl carbonate (DMC), propylene carbonate (PC) and γ
- butyrolactone, dissolved in tetrahydrofuran, alone or a mixed solvent of an organic solvent such as acetonitrile, lithium complex fluorine compound such as LiPF ₆ and LiBF ₄ as an electrolyte, or one or two or more kinds of LiClO ₄ lithium halides such as Can be used.

The structure of the battery is not limited. Used for batteries having various structures, such as a thin-plate coin-type battery, a columnar battery formed by applying positive and negative electrode active materials to a current collector such as a metal foil or the like, and a box-type battery, and the like. It goes without saying that it can be done. Next, the present invention will be described in more detail with reference to examples.

[0023]

EXAMPLES (Production of Positive Electrode Active Material LiTi _x Mn _2-x O ₄ ) Commercially available Li ₂ CO ₃ , MnO ₂ , T
Using an iO ₂ powder reagent, the components were weighed and mixed so as to have the chemical compositions of Sample Nos. 1 to 5 in Table 1, and an oxidizing atmosphere was used.
Firing at 24 ° C. for 24 hours provided various positive electrode active materials. Also,
For comparison, LiMn without replacing Mn by Ti
Together to produce the same conditions ₂ O ₄ (Sample No. 6), Cr instead of Ti, Co, a cathode active material (sample No. 7-9 obtained by replacing a part of Mn by Fe, Cr ₂ as a raw material
O ₃ , Co ₃ O ₄ , and Fe ₂ O ₃ were used). Various positive electrode active materials described in Table 1 thus produced were confirmed to be single-phase by XRD. Note that T
In the case of substitution with i, synthesis of a positive electrode active material with a substitution amount x of 0.5 or more was also attempted. However, when the above synthesis method was used, a heterogeneous phase that was a Ti compound was obtained in the obtained positive electrode active material. Since formation was confirmed by XRD, it was not described in Table 1.

[0024]

[Table 1]

(Manufacture of Battery) For each of the prepared various positive electrode active materials, the positive electrode active material, acetylene black powder as a conductive material, and polyvinylidene fluoride as a binder were mixed in a weight ratio of 50: 2: 3. The mixture was mixed at a ratio to prepare a positive electrode material. 0.02 g of the positive electrode material was added to 300 kg / cm
^At a pressure of ^2, a disk was formed into a disk shape having a diameter of 20 mmφ to obtain a positive electrode. Next, an electrolytic solution prepared by dissolving LiPF ₆ as an electrolyte at a concentration of 1 mol / L in an organic solvent in which ethylene carbonate and diethyl carbonate are mixed at an equal volume ratio, a negative electrode made of carbon, and a positive electrode made of carbon A coin-type battery (coin cell) was fabricated using the separator separating the anode and the negative electrode.

(Measurement of Internal Resistance of Battery) The coin cell prepared as described above was charged to 4.1 V at a constant current of 1 C rate and a constant voltage according to the capacity of the positive electrode active material, and was similarly charged at a constant rate of 1 C rate. One cycle of a charge / discharge test for discharging to 2.5 V with a current was performed. Thereafter, the internal resistance of the coin cell was determined by dividing the difference (potential difference) between the potential in the rest state after charging and the potential immediately after the start of discharging by the discharging current. The results are shown in Table 1.

As shown in Table 1, a part of Mn is changed to T
It became clear that the substitution with i reduced the internal resistance as compared to LiMn ₂ O ₄ . It was confirmed that this effect was obtained even when the substitution amount x was 0.01. The internal resistance was smallest when the substitution amount x was 0.15, and was about 60% of LiMn ₂ O _4. It became clear that it was reduced. Further, the substitution amount x is 0.01 ≦ x
In the range of ≦ 0.5, the reduction of the internal resistance was recognized. In particular, when the substitution amount x was 0.1 ≦ x ≦ 0.3, a particularly remarkable effect of reducing the internal resistance was obtained.

On the other hand, a coin cell using a positive electrode active material in which a part of Mn is replaced with another transition metal Cr, Co, or Fe is compared with a case where LiMn ₂ O ₄ without element replacement is used. Although the internal resistance is reduced, Ti
The effect is small as compared with that of the above, and it is understood that partial replacement of Mn with Ti is most effective in reducing the internal resistance.

[0029]

As described above, according to the lithium secondary battery of the present invention, cubic spinel LiMn is used as the positive electrode active material.
_Since a low-resistance material obtained by substituting a part of Mn of ₂ O ₄ with Ti is used, the internal resistance of the battery is significantly reduced. Thereby, the lithium secondary battery of the present invention has an excellent effect that a large output discharge can be stably performed, and has particularly preferable characteristics as a battery for EV and HEV. In addition, LiTi _x Mn _2-x O
The crystal lattice of ₄ itself is stabilized, and the effect of improving cycle characteristics is also obtained. Furthermore, no conductive aid is added,
Alternatively, by reducing the amount of addition, the filling amount of the positive electrode active material can be increased, and the energy density can be increased.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G048 AA04 AB01 AC04 AC06 AD06 AE05 5H003 AA01 BA01 BB05 BD00 BD01 5H014 AA02 BB01 EE10 HH08 5H029 AJ02 AJ06 AK03 AL06 AM02 AM07 BJ02 BJ03 BJ14 CJ02 HJ02 HJ02

Claims (5)

[Claims] 1. LiMn ₂ having a cubic spinel structure
LiTi _x Mn _2-x O _{4 in} which part of Mn of O ₄ is substituted by Ti
(Where x represents a substitution amount) as a positive electrode active material. 2. The replacement amount x is 0.01 ≦ x ≦ 0.5.
The lithium secondary battery according to claim 1, wherein 3. The lithium secondary battery according to claim 2, wherein the replacement amount x is within a range of 0.1 ≦ x ≦ 0.3. 4. A mixture of salts and / or oxides of the respective elements, wherein the LiTi _x Mn _2-x O ₄ is adjusted to a predetermined ratio, in an oxidizing atmosphere at a temperature of 700 ° C. to 900 ° C. for 5 hours to 4 hours. The lithium secondary battery according to any one of claims 1 to 3, which is obtained by firing over 50 hours. 5. The lithium secondary battery according to claim 1, which is used as a battery for driving a motor of an electric vehicle or a hybrid electric vehicle.