JP2001210324A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery which is low in price and superior in a cycle characteristic, especially under high temperature. SOLUTION: The lithium secondary battery is composed of a positive electrode which contains lithium manganese oxide compound expressed by the composition formula as Li1+x, My, Mn2-x-yO4-z (M is one of the followings, Ti, V, Cr, Fe, Co, Ni, Zn, Cu, W, Mg, and Al; 0<=x<0.2, 0<=y<0.5, 0<=z<0.2), and the half band width of the (400) diffraction peak by the powder X ray diffraction method using CuK α beam is 0.02 θ to 0.1 θ (θ is a diffraction angle), and the form of its first particles is octahedron as a positive electrode active substance, and a negative electrode which contains lithium titanium oxide compound expressed by the composition formula as LiaTibO4, (0.5<=a<=3, 1<=b<=2.5) as a negative electrode active substance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for storing and storing lithium.
The present invention relates to a lithium secondary battery that charges and discharges using a desorption phenomenon.

[0002]

2. Description of the Related Art As mobile phones and personal computers become smaller,
Secondary batteries with high energy density are required, and lithium secondary batteries have come into widespread use in the fields of communication devices and information-related devices. In addition, demands for electric vehicles are increasing in the field of automobiles due to resource problems and environmental problems, and development of lithium secondary batteries that are inexpensive, have large capacities, and have good cycle characteristics is urgently required.

At present, as a positive electrode active material of a lithium secondary battery, LiCoO ₂ having a layered rock salt structure has been adopted as a material capable of constituting a 4 V class secondary battery. Li
Since CoO ₂ is easy to synthesize and relatively easy to handle and has excellent charge / discharge cycle characteristics, secondary batteries using LiCoO ₂ as a positive electrode active material are mainly used.

However, cobalt is scarce as a resource,
A secondary battery using LiCoO ₂ as a positive electrode active material is difficult to cope with future mass production and enlargement of an automotive battery, and is inevitably expensive in price. Therefore, an attempt has been made to use, as a positive electrode active material, LiMn ₂ O ₄ having a spinel structure and containing manganese, which is relatively abundant and inexpensive as a constituent element, as a constituent element instead of cobalt.

[0005] However, LiMn ₂ O ₄ is much more liable to cycle degradation than LiCoO ₂ , and is particularly remarkable at high temperatures, so that LiMn ₂ O ₄ is still in practical use as a battery for electric vehicles placed under severe use environments. Not reached. L
Although the mechanism of the cycle deterioration of iMn ₂ O ₄ is not yet clear, the elution of Mn due to the reaction between the positive electrode active material and the electrolyte,
Destruction of the crystal structure due to repetition of charging and discharging is considered.

As a means for solving the problem of the cycle deterioration of LiMn ₂ O ₄ , for example, as shown in Japanese Patent Application Laid-Open No. 9-147867, the Mn site of the LiMn ₂ O ₄ crystal is partially Co, Cr, Means for stabilizing the crystal structure by substituting with Fe or the like, and GGAmatucci et al., J. Powe
As shown in r Sources 69, 11 (1997), means for modifying the surface of the positive electrode with a boron compound to suppress the reaction with the electrolyte are being studied.

[0007] Conventionally, LiMn ₂ O ₄ is generally MnO ₂
Is synthesized by a solid-phase reaction method in which a mixture obtained by dry-mixing a powder of a manganese compound such as Li and a powder of a lithium compound such as Li ₂ CO ₃ is fired. However, in the synthesis by this method, it is difficult to synthesize LiMn ₂ O ₄ having excellent crystallinity. In particular, when the Mn site is replaced with another element, the replacement element remains unreacted. The phenomenon easily occurred, and it was more difficult to perform uniform substitution. The presence of such an unreacted phase causes a decrease in the capacity of the positive electrode active material, and also causes a large phase change at the time of charge / discharge, so that good cycle characteristics cannot be obtained.

On the other hand, the cycle deterioration of a lithium secondary battery largely depends not only on the positive electrode but also on other components such as a negative electrode and a non-aqueous electrolyte. Currently, the mainstream lithium secondary battery is a so-called lithium ion secondary battery that uses carbon materials such as graphite, coke, and hard carbon as the negative electrode active material.However, these carbon materials are irreversible during the first charge / discharge. In addition to the retention problem that a reaction occurs, the reduction potential is about 0.1 with respect to Li / Li ⁺ .
As low as around V, the non-aqueous electrolyte is likely to be decomposed on the negative electrode surface, and cycle deterioration due to this phenomenon is also a problem.

[0009]

SUMMARY OF THE INVENTION The present inventor has attempted to reduce the cost of a lithium secondary battery by using a lithium manganese composite oxide having high crystallinity as a positive electrode active material, and to reduce the cost of the lithium secondary battery. It has been found that cycle characteristics, especially cycle characteristics at high temperatures, can be improved. In addition, it has been found that the use of a lithium-titanium composite oxide as a negative electrode active material can also suppress cycle deterioration caused by the negative electrode and the non-aqueous electrolyte. That is, the lithium-titanium composite oxide has a reduction potential of Li / L
It is as high as about 1.5 V with respect to i ⁺ , and in addition to being able to suppress the decomposition of the nonaqueous electrolyte on the negative electrode surface, it is also possible to suppress the cycle deterioration caused by the negative electrode and the nonaqueous electrolyte from the stability of the crystal structure It is a finding that.

The present invention has been made based on the above findings, and has as its object to provide a lithium secondary battery which is inexpensive and has excellent cycle characteristics, particularly excellent cycle characteristics at high temperatures.

[0011]

The lithium secondary battery of the present invention According to an aspect of the composition formula _{Li 1 + x M y Mn 2} -xy O 4-z (M is Ti,
V, Cr, Fe, Co, Ni, Zn, Cu, W, Mg,
At least one of Al, 0 ≦ x <0.2, 0 ≦ y <0.
5, 0 ≦ z <0.2, and the half-width of the (400) diffraction peak by the powder X-ray diffraction method using CuKα radiation is 0.02θ or more and 0.1θ or less (θ is a diffraction angle). , 1
A positive electrode containing, as a positive electrode active material, a lithium manganese composite oxide in which the shape of secondary particles forms an octahedron; and a composition formula Li _a Ti _b O
₄ (0.5 ≦ a ≦ 3, 1 ≦ b ≦ 2.5) and a negative electrode containing a lithium titanium composite oxide as a negative electrode active material.

In other words, for the positive electrode, the use of a lithium manganese composite oxide having a spinel structure and having a very good crystallinity as the positive electrode active material allows the crystal structure to be destroyed by repeated charging and discharging. Therefore, the cycle characteristics of the lithium secondary battery are improved.

A scanning electron microscope (SEM) photograph showing the primary particle shape of an octahedron is shown as an example.
It is shown in FIG. This photograph is at a magnification of 70,000, and the size of the octahedral primary particles photographed in the center is about 6 μm. The effect of having octahedral primary particles is not clear at present, but the expansion and contraction of the crystal lattice due to charge and discharge can be easily absorbed between the particles, and it is one of the elements constituting the positive electrode. It is considered that this acts to suppress a decrease in conductivity due to poor contact with a certain conductive material.

As for the negative electrode, for example, the composition formulas Li _0.8 Ti _2.2 O ₄ , LiTi ₂ O ₄ , and Li _1.33 Ti _1.67
By using a lithium-titanium composite oxide represented by O ₄ , Li _1.14 Ti _1.71 O ₄ or the like as the negative electrode active material, the stability of the crystal structure and the reduction potential are relatively high, and the nonaqueous electrolysis on the negative electrode surface is performed. From the effect that the decomposition of the liquid can be suppressed, the cycle characteristics of the lithium secondary battery are improved.

Therefore, the lithium secondary battery of the present invention in which the positive electrode and the negative electrode are opposed to each other is inexpensive by not using a Co-based material for the positive electrode material, and has low cycle characteristics, especially lithium manganese. A lithium secondary battery with improved cycle characteristics when used in a high-temperature environment, which has been a problem with composite oxides, and having excellent durability.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of a lithium secondary battery according to the present invention will be described with reference to a lithium manganese composite oxide serving as a positive electrode active material, a lithium titanium composite oxide serving as a negative electrode active material, and an overall configuration of a lithium secondary battery. Will be described in this order.

<Lithium-manganese composite oxide> A lithium-manganese composite oxide serving as a positive electrode active material of the lithium secondary battery of the present invention (hereinafter referred to as “the lithium-manganese composite oxide”)
Is a spinel-structured lithium manganese composite oxide. The basic composition formula of the spinel-structured lithium manganese composite oxide is represented by LiMn ₂ O ₄ , and this stoichiometric LiMn ₂ O ₄ can be used as the present positive electrode active material.

In addition, a material in which an excess of lithium is present in the crystal, a material in which a part of the Mn site is substituted with another metal in order to further stabilize the crystal structure, or a material in which an O site has a defect is also used. You can also. Other metals that can replace the Mn site include Ti, V, C
Examples thereof include r, Fe, Co, Ni, Zn, Cu, W, Mg, and Al, and one or more of these can be substituted. Of these replaceable metal elements, it is desirable to use Ni as the replacement element, when comprehensively judged from the ease of synthesis of the active material, cost, and the like.

[0019] Expressed all lithium manganese oxide of above general _{formula, Li 1 + x M y Mn} 2-xy O
_4-z (M is the other metal described above), but there is a range of substitution and deficiency that satisfies the characteristics as a positive electrode active material of a lithium secondary battery. As the present lithium manganese composite oxide, 0 ≦ x <0.2, 0 ≦ y <0.5, 0 ≦ z <0.
A range of 2 can be used. This is because x ≧
In the case of 0.2 or y ≧ 0.5, it is difficult to produce by the solid phase method, and an impurity phase other than the spinel phase is generated, which may lower crystallinity and deteriorate cycle characteristics. This is because the capacity per unit weight is excessively reduced. If z ≧ 0.2, the spinel structure is destroyed, the crystal structure becomes unstable, and the cycle deterioration may increase. Note that a more desirable range is 0.01 ≦ x ≦ 0.1, 0.05 ≦ y ≦ 0.3, and the value of z is preferably as close to 0 as possible.

The lithium manganese composite oxide needs to have a spinel structure with high crystallinity. As a method of defining the crystallinity height, if the half width of the peak by the powder X-ray diffraction method is used, the peak due to the reflection of the (400) plane unique to the spinel structure is 0.02θ or more and 0.1θ or less (θ is Diffraction angle). In the case of exceeding 0.1θ, the crystallinity is low, and when used as a positive electrode active material,
The crystal structure of the spinel structure collapses due to repeated charging and discharging, resulting in severe deterioration of the cycle characteristics. In the case of less than 0.02θ, the production time is extremely long, and on the contrary, the cathode active material This will increase the cost. Incidentally, on the X-ray diffraction chart, the diffraction peak of the (400) plane is 42 to 4 at 2θ.
Appears at the 5 ° position. For example, when a peak appears at 2θ = 44 °, the appropriate half width is 0.44 ° or more and 2.2 ° or less. From the viewpoint of a lithium manganese composite oxide having better crystallinity, it is more preferable to set the thickness to 0.05θ or less. In addition, when expressing by lattice distortion by the Wilson method, it is desirable to set it as 0.035% or less.

In the present lithium manganese composite oxide, the primary particles have an octahedral shape. As described above, a scanning electron microscope (SEM) photograph of the primary particles is illustrated in FIG. This photograph is at a magnification of 70,000, and the size of the octahedral primary particles photographed in the center is about 6 μm. Further, as the lithium manganese composite oxide, it is desirable to use a powdery one in which the octahedral primary particles are aggregated to form secondary particles. FIG. 2 illustrates an SEM photograph showing a state in which the primary particles are aggregated to form secondary particles. The average particle size of the secondary particles in this photograph (sphere-converted average particle size) is about 10
μm.

For comparison with the particle shape of the present lithium manganese composite oxide, a commercially available composition formula L
S of lithium manganese composite oxide represented by iMn ₂ O ₄
The EM photograph is shown in FIG. The lithium manganese composite oxide in this photograph is in a state where very small irregularly shaped primary particles are aggregated. Compared to the one in this state, it can be easily confirmed that the present lithium manganese composite oxide having octahedral primary particles has good crystallinity.

In the present lithium manganese composite oxide, the particle diameter of the secondary particles also affects the cycle characteristics of the battery. If the particle size is too large, the ion diffusivity and the electron conductivity in the active material are reduced, and the internal short circuit due to the precipitation of dendrites is caused. On the other hand, if the particle size is too large, the filling property is poor, a high-density positive electrode cannot be obtained, and it is inferior in that a high-capacity battery is formed. On the other hand, when the particle size is too small, the reactivity with the electrolyte increases, which causes the decomposition of the active material and the electrolyte. Therefore, it is desirable to use a powder having an average particle diameter of secondary particles of not less than 5 μm and not more than 25 μm in sphere equivalent average particle diameter. The sphere-converted average particle diameter is a value determined by a laser diffraction / scattering particle size distribution analyzer.

The specific surface area of the powder also affects the cycle characteristics. Therefore, in order to further suppress the cycle degradation at high temperature, is desirable to use a specific surface area is relatively small, in the lithium manganese composite oxide, BET specific surface area of 0.2 m ² / g or more 2m ² / It is desirable to use one having g or less. Similar to the relationship with the secondary particle diameter, 0.2
If it is less than m ² / g, it becomes difficult to form a battery having a large capacity, and if it exceeds 2 m ² / g, decomposition of the electrolyte solution is likely to occur, resulting in inferior cycle characteristics. is there. The BET specific surface area is a value determined by the N ₂ adsorption one-point method.

The production method of the present lithium manganese composite oxide is not particularly limited, but it can be easily produced by the following method. The production method includes a pulverization-mixing step of wet-pulverizing and mixing a raw material containing Li, a raw material containing Mn, and a raw material containing the metal M, if necessary, to obtain a mixture. And a firing step of firing the mixture obtained in the step to obtain a lithium manganese composite oxide. In other words, in this manufacturing method, unlike the conventional solid-phase reaction method, the pulverization-mixing step is performed in a wet manner before the firing step, and a uniform mixture is produced in this step. The product is uniform and very crystalline.

As a raw material for the production, a lithium compound serving as a Li source, a manganese compound serving as a Mn source, and a compound containing the substituted metal when the Mn site is substituted with another metal are used. These compounds are not particularly limited, but are desirably compounds having a valence in which these metals are stably present. For example,
Manganese compounds include MnO ₂ , Mn ₃ O ₄ , Mn (COO
H) _{2 and the} like include lithium compounds such as Li ₂ CO ₃ and Li
(OH), Li ₂ O, LiI, LiNO _{3 and the} like. As the compound containing a substituted metal, an oxide, a hydroxide, or the like can be used. For example, when replacing with Ni,
Ni (OH) ₂ or the like can be used.

In the pulverizing and mixing step, the above compounds are mixed at a ratio according to the composition ratio of Li, Mn, and the substitution metal in the lithium-manganese composite oxide to be obtained. The mixing is performed by a wet method using a ball mill, a visco mill, an attritor, or the like. The reason for using a ball mill, a viscomiler, an attritor, etc. is that a mixture having an arbitrary particle size and particle size distribution can be obtained by changing conditions such as mixing and mixing time, which can be performed simultaneously with mixing. is there. The wet process is performed to obtain a uniform mixture. In addition,
Among ball mills, visco mills, attritors, etc.,
It is more preferable to use a ball mill because the mixing and grinding conditions can be easily changed by changing the size and type of the ball.

When pulverizing and mixing with a ball mill,
Ingredients for the mill pot and the balls in the pot
Must be heavy, hard, and resistant to wear
Use of ceramic materials is desirable
New Among them, Si which is hard _ThreeNi_Four, ZrO_TwoMaterials such as
Are more preferred. Also mixed in to make it wet
Liquids such as water, alcohol, hexane, etc.
Used. Among them, they do not react with the raw materials and after evaporation
It is desirable that it is hard to harden, and comprehensively considers costs etc.
If you do, ethyl alcohol, isopropyl alcohol, etc.
It is desirable to use industrial alcohols of the type described above.

When pulverizing and mixing by a ball mill,
The pulverization and mixing time needs to be changed depending on the particle size of the raw material compound, the particle size of the lithium-manganese composite oxide to be obtained, and the like, but is preferably 2 hours or more and 100 hours or less. This is because if less than 2 hours, uniform mixing cannot be achieved, and if more than 100 hours, the particle size of the resulting lithium manganese composite oxide becomes too small and the production cost is unnecessarily increased. Because it becomes. In consideration of the uniformity of the mixture, the production cost, and the like, it is more preferable to set the time to 4 hours to 24 hours.

The firing step is a step of firing the mixture obtained by the above-mentioned pulverizing and mixing step. The furnace used for firing is not particularly limited, and any furnace can be used as long as it can be used for synthesis by a usual solid phase reaction method. The firing temperature is 600 ° C
The temperature is desirably at least 1200 ° C. If the temperature is lower than 600 ° C., it takes too much time to grow crystal grains, and 12
If the temperature exceeds 00 ° C., crystals having a spinel structure will be decomposed. In order to obtain a lithium-manganese composite oxide having a higher crystallinity and a spinel structure, firing at a temperature of 900 ° C or more and 950 ° C or less is more preferable. The baking time depends on the baking temperature, but is preferably 5 hours or more and 50 hours or less.

[0031] <lithium-titanium composite oxide> lithium-titanium composite oxide having a negative electrode active material of a lithium secondary battery of the present invention (hereinafter, referred to as "the lithium-titanium composite oxide") the composition formula Li _a Ti _b O ₄ (0.5 ≦ a ≦ 3, 1 ≦
b ≦ 2.5). The lithium titanium composite oxide has a spinel structure or a structure similar thereto,
According to powder X-ray diffraction using CuKα ray, the interplanar spacing in the crystal structure was at least 4.84 °, 2.53 °, 2.0
It is characterized in that a diffraction peak is present on a diffraction surface (reflection surface) at 9 ° and 1.48 ° (± 0.1 ° between each surface).

The present lithium-titanium composite having this crystal structure
Oxide has a stable crystal structure, and lithium
The basic structure is also destroyed by the insertion and extraction of
Lithium secondary batteries that are hard to break and have good cycle characteristics
It can be a negative electrode active material that can be configured. Of various compositions
Among them, in terms of stability of the crystal structure, the composition formula Li_{0. 8}
Ti_2.2O_Four, LiTi_TwoO_Four, Li_1.33Ti_1.67O_Four, L
i_1.14Ti_1.71O_FourWhat is represented by is excellent,
One of them alone or two or more
It is desirable to use a mixture. Used as negative electrode active material
Is easy to synthesize, has a large capacity, and has a better crystal structure.
In terms of stability, the composition formula Li_1.33Ti
_1.67O_FourIt is more desirable to use the one represented by
By the way, the composition formula Li_0.8Ti_2.2O_Four, Li_1.33Ti
_1.67O_Four, Li_1.14Ti_1.71O_FourIs the composition formula Li
_FourTi₁₁O₂₀, Li_FourTi_FiveO₁₂, Li_TwoTi_ThreeO₇Express
Can also be.

The production method of the present lithium-titanium composite oxide is not particularly limited, but is easily synthesized by mixing a lithium compound serving as a lithium source and titanium oxide serving as a titanium source, and firing the mixture. be able to. As the lithium compound, Li ₂ CO ₃ , Li
(OH) or the like can be used. The firing is performed in an oxygen stream or in the atmosphere. The mixing ratio of each raw material may be a ratio according to the composition of the lithium titanium composite oxide to be synthesized. If the temperature is too low, it is not possible to obtain a particle having a particle size that has grown so as to have good characteristics as an active material. If the temperature is too high, a rutile-type titanium oxide phase (TiO ₂ ), The firing temperature is 500 to 1000 ° C.
It is desirable that More preferably, 700 to 900
℃ is good.

It is difficult to completely eliminate the titanium oxide phase generated as a subphase. Since this titanium oxide phase is generated in a mixed crystal state with the main phase of the lithium-titanium composite oxide, if present in a small amount, the charge / discharge characteristics and cycle characteristics when used as a negative electrode active material are extremely deteriorated. It does not make you. Therefore, the present lithium-titanium composite oxide may contain this titanium oxide in a mixed crystal state, and in the present specification, “lithium-titanium composite oxide” means containing the same. .

<Overall Configuration of Lithium Secondary Battery> The positive electrode of the lithium secondary battery is prepared by mixing the above-mentioned lithium manganese composite oxide as a positive electrode active material, a conductive material and a binder, A paste-like positive electrode mixture is formed by adding a solvent to the surface of a current collector made of a metal foil such as aluminum, dried, and then formed as necessary by increasing the density of the positive electrode mixture by pressing or the like. I do. The positive electrode active material can be composed of the present lithium manganese composite oxide alone. However, for the purpose of improving the characteristics of the lithium secondary battery, the present lithium manganese composite oxide may be provided with a known LiCoO ₂ , LiNiO _2, or the like. The positive electrode active material may be a mixture of another lithium composite oxide or a lithium manganese composite oxide having a low crystallinity and a spinel structure.

The conductive material is used to ensure the electrical conductivity of the positive electrode since the lithium manganese composite oxide itself has a large electric specific resistance.
One of powdered carbonaceous materials such as acetylene black and graphite
One kind or a mixture of two or more kinds can be used. The binder plays a role of binding the active material particles, and a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene can be used. Further, as a solvent for dispersing the active material, the conductive material, and the binder, an organic solvent such as N-methyl-2-pyrrolidone can be used.

The negative electrode was prepared by using the above-mentioned lithium-titanium composite oxide as a negative electrode active material, mixing a conductive material and a binder with the mixture, and adding an appropriate solvent if necessary, to form a paste-like negative electrode mixture. Is applied to the surface of a current collector made of a metal foil such as copper, dried, and then, if necessary, increased in density of the negative electrode mixture by pressing or the like. The negative electrode active material can be composed of the lithium titanium composite oxide alone. However, for the purpose of improving the characteristics of the lithium secondary battery, a known carbon material or the like is mixed with the lithium manganese composite oxide. May be used as the negative electrode active material.

The conductive material is carbon black, like the positive electrode.
One of powdered carbonaceous materials such as acetylene black and graphite
One kind or a mixture of two or more kinds can be used. Like the positive electrode, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used as the binder. Also, the solvent is N
Organic solvents such as -methyl-2-pyrrolidone can be used.

The lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte, and the like, similarly to a general lithium secondary battery. . The separator separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene, or the like can be used. The non-aqueous electrolyte is a solution in which a lithium salt as an electrolyte is dissolved in an organic solvent. As the organic solvent, an aprotic organic solvent,
For example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran, dioxolan, methylene chloride, or a mixture of two or more of these. Can be used. The electrolyte to be dissolved is LiI, LiClO ₄ , Li
AsF ₆ , LiBF ₄ , LiPF ₆ , LiN (CF ₃ S
Lithium salts such as O ₂ ) ₂ can be used.

The lithium secondary battery of the present invention configured as described above can have various shapes such as a cylindrical type, a stacked type, and a coin type. Regardless of the shape used, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and a current collecting lead extends from the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that lead to the outside. The electrode body is sealed together with the non-aqueous electrolyte in a battery case to complete the battery.

The lithium secondary battery of the present invention aims at improving the cycle characteristics. To this end, the capacity ratio between the positive electrode and the negative electrode (negative electrode capacity / positive electrode capacity) is 0.5 or more. It is desirably 5 or less. This capacity ratio is 0.
If it is less than 5, the battery capacity based on the positive electrode will be too low, and if the capacity ratio is more than 1.5, the positive electrode will have too high a potential and the decomposition reaction with the electrolytic solution will not occur. This is because the promotion promotes the deterioration of the cycle characteristics. In order to obtain a lithium secondary battery having a good balance between battery capacity and cycle characteristics, the capacity ratio should be 0.8 or more and 1.1 or more.
It is more desirable to set the following.

Here, the “positive electrode capacity” and “negative electrode capacity” are obtained when an electrochemical cell in which the counter electrode is metallic lithium is configured to perform constant current constant voltage charge-constant current discharge. The term “positive electrode capacity” and “negative electrode capacity” as used herein mean the maximum capacity that can be used reversibly.
-3.5 V, 1.5 V-0 V, and the values obtained when the above-mentioned charge and discharge are performed with the current density at the time of constant current charging and constant current discharging being 0.1 mA / cm ² are evaluated.

The embodiment of the lithium secondary battery of the present invention has been described above. However, the above-described embodiment is merely an embodiment, and the lithium secondary battery of the present invention can be implemented by those skilled in the art. Can be implemented in various forms with various changes and improvements based on the knowledge of

[0044]

EXAMPLES Various lithium secondary batteries were manufactured as examples based on the above embodiment. Also, to compare with this,
A lithium secondary battery using a graphite material as a negative electrode active material and a lithium secondary battery using a lithium manganese composite oxide in which primary particles did not have an octahedral shape as a positive electrode active material were produced as comparative examples. . A charge / discharge cycle test was performed on the secondary batteries of these Examples and Comparative Examples, and the cycle characteristics of each secondary battery were evaluated. Hereinafter, typical secondary batteries of Examples and Comparative Examples will be described, and evaluation thereof will be described.

<Lithium Secondary Battery of Example 1> The positive electrode active material of this lithium secondary battery includes the composition formula Li _1.05 Ni _0.1
A lithium manganese composite oxide represented by Mn _1.85 O ₄ was used. This lithium manganese composite oxide uses Li ₂ CO ₃ as a Li source, MnO ₂ as a Mn source, and Ni (OH) ₂ as a Ni source, and wet crushes them by the method described in the above embodiment. It was synthesized by mixing and firing at 930 ° C. for 12 hours in an oxygen stream.

The synthesized Li _1.05 Ni _0.1 Mn _1.85 O ₄ is
It was confirmed by powder X-ray diffraction analysis using CuKα radiation that the powder had a spinel structure, and that the half width of the (400) diffraction peak was 0.08θ. Also,
The primary particles had an octahedral shape as shown in the photograph of FIG. 1, and the secondary particles formed by aggregation of the primary particles had an average particle size of about 10 μm. Further, the BET specific surface area of this lithium manganese composite oxide was 0.37 m ² / g.

As the negative electrode active material of the present lithium secondary battery, a lithium-titanium composite oxide represented by the composition formula Li _1.33 Ti _1.67 O ₄ was used. This lithium titanium composite oxide,
Li ₂ CO ₃ was used as a Li source and anatase type TiO ₂ was used as a titanium source. These were mixed at a predetermined ratio, and calcined at 800 ° C. for 12 hours in an oxygen stream to synthesize.
The synthesized lithium-titanium composite oxide was analyzed by powder X-ray diffraction analysis using CuKα radiation to have a spacing of 4.83 °,
It was confirmed that there were respective diffraction peaks obtained by the diffraction planes (reflection planes) of 2.52 °, 2.09 ° and 1.48 °.

The positive electrode was prepared by mixing 90 parts by weight of the above lithium manganese composite oxide, 7 parts by weight of carbon black as a conductive material, and 10 parts by weight of polyvinylidene fluoride as a binder. A paste-like positive electrode mixture was obtained by adding and kneading pyrrolidone, and this positive electrode mixture was applied to both sides of a 20-μm-thick aluminum foil positive electrode current collector, dried, passed through a pressing step, and then formed into a sheet. Was prepared.

The negative electrode was prepared by mixing 5 parts by weight of carbon black as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder with 90 parts by weight of the lithium-titanium composite oxide, and adding an appropriate amount of N-methyl-2- A paste-like negative electrode mixture was obtained by adding and kneading pyrrolidone, and this negative electrode mixture was applied to both sides of a 10-μm-thick Cu foil-made positive electrode current collector, dried, passed through a pressing step, and then formed into a sheet-like material. Was prepared.

Each of the positive electrode and the negative electrode is cut into a predetermined size, and the cut positive electrode and negative electrode are separated by a thickness of 2 mm therebetween.
5μm polyethylene separator sandwiched and wound,
A roll-shaped electrode body was formed. A current collecting lead was attached to the electrode body, inserted into a 18650 type battery case, and then a non-aqueous electrolyte was injected into the battery case. The non-aqueous electrolyte contains LiPF ₆ in a mixed solvent of ethylene carbonate and diethyl carbonate in a volume ratio of 1: 1.
M dissolved at a concentration of M was used. Finally, the battery case was sealed to complete the lithium secondary battery of Example 1.

In this lithium secondary battery, the positive electrode and the negative electrode are manufactured such that the capacity ratio between the positive electrode and the negative electrode (negative electrode capacity / positive electrode capacity) is 1.1. The positive electrode capacity and the negative electrode capacity were based on the conditions described in the above embodiment, and the capacity ratio between the positive electrode and the negative electrode was determined by adjusting the amount of the positive electrode active material and the amount of the negative electrode active material included in each electrode. . By the way, the reference capacity at that time,
A value of 120 mAh / g per unit weight is adopted for the lithium manganese composite oxide as the positive electrode active material, and 160 mAh / g per unit weight for the lithium titanium composite oxide as the negative electrode active material.

<Lithium Secondary Battery of Embodiment 2> This lithium secondary battery is different from the secondary battery of Embodiment 1 in that the amount of the positive electrode active material and the amount of the negative electrode active material contained in each electrode are adjusted. The capacity ratio between the positive electrode and the negative electrode is changed to 0.7. Other configurations are the same as those of the secondary battery of the first embodiment.

<Lithium Secondary Battery of Comparative Example 1> This lithium secondary battery is a lithium secondary battery using graphitized mesophase spheres as the negative electrode active material. The negative electrode was formed into a paste by mixing 90 parts by weight of the graphitized mesophase microspheres with 10 parts by weight of polyvinylidene fluoride as a binder, adding an appropriate amount of N-methyl-2-pyrrolidone and kneading the paste. A negative electrode mixture was obtained, and the negative electrode mixture was coated with a 10 μm thick C
It is a sheet-like product produced by applying and drying both sides of a positive electrode current collector made of a u-foil and pressing. The capacity ratio between the positive electrode and the negative electrode was 1.2, and the other configuration was the same as that of the secondary battery of Example 1. By the way, the reference capacity for determining the capacity ratio between the positive electrode and the negative electrode is 330 g / unit weight for the graphitized mesophase spherules which are the negative electrode active material.
The value of mAh / g is adopted.

<Lithium Secondary Battery of Comparative Example 2> This lithium secondary battery uses graphitized mesophase microspheres as a negative electrode active material, and has primary particles in a lithium manganese composite oxide serving as a positive electrode active material. This is a secondary battery using one having no octahedral shape, in other words, having irregular primary particles as shown in FIG.

The lithium man used for the present lithium secondary battery
The cancer composite oxide has a composition formula of LiCo _0.1Mn_1.9O_FourIn table
Li as a Li source_TwoCO_ThreeIs a Mn source
MnO_TwoWith Co (NO) as the Co source_Three)_TwoAnd dry
Mix at 930 ° C for 12 hours in an oxygen stream
It was synthesized by firing. This lithium manganese
The composite oxide was obtained by powder X-ray diffraction using CuKα radiation.
Half width of the (400) diffraction peak is 0.19θ,
The average particle size of the secondary particles is 25 μm, and the BET specific surface area is
0.21m^Two/ G.

The present lithium secondary battery is the same as the secondary battery of Comparative Example 1 except for the negative electrode and the positive electrode active material except for the positive electrode active material. The capacity ratio between the positive electrode and the negative electrode is 1.2. By the way, as for the capacity as a reference when determining the capacity ratio between the positive electrode and the negative electrode, a value of 120 mAh / g per unit weight is adopted for this lithium manganese composite oxide.

<Evaluation of Cycle Characteristics> A charge / discharge cycle test was performed on the secondary batteries of the above Examples and Comparative Examples. The charge / discharge cycle test was performed in a high-temperature environment of 60 ° C., which is regarded as the upper limit temperature at which a lithium secondary battery is actually used. The conditions of the charge / discharge cycle for the secondary batteries of Example 1 and Example 2 were such that charging was performed at a constant current of 1 mA / cm ² up to a charge end voltage of 2.7 V, and then the current density was increased to a discharge end voltage of 1.5 V. Discharging at a constant current of 1 mA / cm ² is defined as one cycle. Also,
The conditions of the charge / discharge cycle for the secondary batteries of Comparative Example 1 and Comparative Example 2 were such that the current density was 1 m until the charge end voltage was 4.2 V.
Charging at a constant current of A / cm ² and then discharging at a constant current of 1 mA / cm ^{2 to} a discharge end voltage of 3.0 V constitute one cycle. These cycles were repeated for 300 cycles or more for all the secondary batteries.

FIG. 4 shows the discharge capacity per positive electrode active material weight in each cycle of each secondary battery as a result of the charge / discharge cycle test, and the capacity retention rate (cycles) of each secondary battery in each cycle. (Discharge capacity / discharge capacity at first cycle × 100%) in FIG. 5 is shown in FIG.

As is clear from FIGS. 4 and 5, in the secondary batteries of Comparative Examples 1 and 2 using a carbon material as the negative electrode active material, the discharge capacity was greatly reduced as charge / discharge cycles were repeated, It can be seen that the secondary battery is severely deteriorated. The secondary battery of Example 2 using the lithium manganese composite oxide having poor crystallinity has a small discharge capacity from the initial stage, and the crystallinity of the lithium manganese composite oxide serving as the positive electrode active material has a low cycle characteristic. It can be confirmed that the influence is exerted not only on the discharge capacity but also on the magnitude of the discharge capacity.

On the other hand, the secondary batteries of Examples 1 and 2, which are the lithium secondary batteries of the present invention, have a small decrease in discharge capacity even after a charge / discharge cycle, and have good cycle characteristics. It can be confirmed that it is a battery. If the capacity ratio between the positive electrode and the negative electrode is reduced, the cycle characteristics are improved. Instead, the discharge capacity itself is small, and the capacity ratio between the positive electrode and the negative electrode is required to obtain a well-balanced lithium secondary battery. , 0.8 to 1.1. Incidentally, in the case of the secondary battery of Example 2 in which the capacity ratio between the positive electrode and the negative electrode is relatively small, a phenomenon in which the discharge capacity increases with the progress of the cycle at the beginning of the charge / discharge cycle is observed. It is presumed that the use of the compound improved the “fit-in” of the conductive path and the wettability with the electrolytic solution.

[0061]

The lithium secondary battery of the present invention comprises a lithium manganese composite oxide having high crystallinity as a positive electrode active material and a lithium titanium composite oxide as a negative electrode active material. With such a configuration, the lithium secondary battery of the present invention can be a lithium secondary battery having good cycle characteristics, particularly, high-temperature cycle characteristics, while taking advantage of inexpensiveness.

[Brief description of the drawings]

FIG. 1 shows an SEM photograph of primary particles of a lithium manganese composite oxide used as a positive electrode active material in a lithium secondary battery of the present invention.

FIG. 2 is an SEM photograph showing a state where primary particles are aggregated to form secondary particles in a lithium manganese composite oxide used as a positive electrode active material in the lithium secondary battery of the present invention.

FIG. 3 is an SEM photograph showing a state in which irregular primary particles are aggregated in a lithium manganese composite oxide having poor crystallinity different from the lithium manganese composite oxide used for the lithium secondary battery of the present invention.

FIG. 4 shows the discharge capacity per positive electrode active material weight in each cycle of the lithium secondary batteries of Examples of the present invention and Comparative Examples as a result of a charge / discharge cycle test.

FIG. 5 shows the capacity retention ratio in each cycle of the lithium secondary batteries of Examples of the present invention and Comparative Examples as a result of a charge / discharge cycle test.

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[Procedure amendment]

[Submission date] December 6, 2000 (200.12.
6)

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Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H01M 4/02 H01M 4/02 CD 10/40 10/40 Z (72) Inventor Takuaki Okuda Nagakute, Aichi County, Aichi Prefecture (71) Inventor Takeshi Sasaki In the Toyota Central Research Institute, Inc. (72) Inventor Takeshi Sasaki Takeshiuchi, 41, Ogata Chochu, Yokomichi, Nagakute-cho, Aichi-gun, Aichi, Japan No. 1 Toyoda Central Research Institute, Inc. (No. 41), Toyota Chuo R & D Laboratories Co., Ltd. (72) Inventor Tetsuro Kobayashi, Toyota-Chuo Research Co., Ltd. In-house (72) Inventor Yoshio Ukyo 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture 1 Toyota Central Research Laboratory Co., Ltd.

Claims (5)

[Claims] 1. A composition formula _{Li 1 + x M y Mn 2} -xy O 4-z (M is Ti, V, Cr, Fe, Co, Ni, Zn, Cu, W,
One or more of Mg and Al, 0 ≦ x <0.2, 0 ≦ y
<0.5, 0 ≦ z <0.2), and the half value width of the (400) diffraction peak by the powder X-ray diffraction method using CuKα ray is 0.02θ or more and 0.1θ or less (θ is the diffraction angle ), and a positive electrode containing a lithium manganese composite oxide form of primary particles constituting the octahedral as the positive electrode active material, the composition formula _{Li a Ti b O 4 (0.5} ≦ a ≦ 3,1 ≦ b ≦ 2 .
A lithium secondary battery comprising: a negative electrode containing the lithium-titanium composite oxide represented by 5) as a negative electrode active material. 2. The lithium manganese composite oxide, wherein the primary particles are aggregated to form secondary particles.
4. The lithium secondary battery according to 1. 3. The lithium-manganese composite oxide has an average spherical equivalent particle diameter of the secondary particles of 5 μm or more and 25 μm or less, and a BET specific surface area of 0.2 m ² / g or more and 2 m ^2.
/ G or less. 4. The lithium-titanium composite oxide has a composition of Li _0.8 Ti _2.2 O ₄ , LiTi ₂ O ₄ , Li _1.33 T
The lithium secondary battery according to any one of claims 1 to 3, wherein the lithium secondary battery is at least one member selected from the group consisting of i _1.67 O ₄ and Li _1.14 Ti _1.71 O ₄ . 5. The capacity ratio between the positive electrode and the negative electrode (negative electrode capacity / positive electrode capacity) is 0.5 or more and 1.5 or less.
The lithium secondary battery according to claim 4.