JPH0513082A - Nonaqueous electrolytic liquid secondary battery - Google Patents

Abstract

PURPOSE:To provide a secondary battery having high output and energy density, and a long cycle life, while ensuring freedom from a drop in charge and discharge capacity and efficiency, and battery voltage due to the repetition of charge and discharge cycles. CONSTITUTION:In a nonaqueous electrolytic liquid secondary battery using an inorganic compound as a positive electrode active material, a compound containing titanium added to a lithium cobalt compound oxide in solid solution state is used as the positive electrode active material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chargeable / dischargeable non-aqueous electrolyte secondary battery used as a power source for various electronic devices, and more particularly to improvement of a positive electrode active material.

[0002]

2. Description of the Related Art In recent years, with the rapid progress of various electronic devices, researches on rechargeable secondary batteries as a power source that can be used conveniently and economically for a long time have been advanced. Lead storage batteries, alkaline storage batteries, lithium secondary batteries and the like are known as typical secondary batteries, and among them, lithium secondary batteries in particular have advantages such as high output and high energy density.

The lithium secondary battery comprises a positive electrode made of an active material that reversibly electrochemically reacts with lithium ions, a negative electrode containing lithium metal or lithium, and a non-aqueous electrolyte.

Generally, as the negative electrode active material constituting the negative electrode, metallic lithium, lithium alloy (for example, Li-Al alloy), conductive polymer doped with lithium (for example, polyacetylene, polypyrrole, etc.), or lithium ion in the crystal is used. The intercalation compound and the like incorporated in is used. Further, as the electrolytic solution, a solution in which a lithium salt is dissolved in an aprotic organic solvent is used.

On the other hand, as the positive electrode active material forming the positive electrode, a metal oxide, a metal sulfide, or a polymer is used. For example, TiS ₂ , MoS ₂ , NbSe ₂ , V ₂
O ₅ etc. are known.

The discharge reaction of a lithium secondary battery using these materials proceeds when lithium ions are eluted into the electrolytic solution at the negative electrode and lithium ions are intercalated between the layers of the active material at the positive electrode. vice versa,
When charging, the above reverse reaction proceeds and lithium deintercalates in the positive electrode. That is, charge and discharge can be repeated by repeating the reaction in which lithium ions from the negative electrode enter and leave the positive electrode active material.

[0007]

However, in the conventional lithium secondary battery, the charge and discharge capacity, charge and discharge efficiency, etc. gradually decrease with repeated charging and discharging, and there is a drawback that a sufficient cycle life cannot be obtained. is there. As a factor that reduces the discharge capacity and charge / discharge efficiency, there is an irreversible change of the active material such as a change in the crystal structure of the positive electrode active material,
The electrolytic solution decomposes when the positive electrode potential is higher than the potential window or when the positive electrode potential is lower than the potential window, and a protective film is formed by the reaction between lithium and the electrolytic solution, which can dissolve some of the deposited lithium. It is possible that some of the deposited lithium metal is deposited as needle-like (dendritic) crystals that are difficult to dissolve.

[0008] Therefore, in _{JP-B-63-59507, Li x M y O 2} (M is Ni or Co, x <
A non-aqueous electrolyte secondary battery using 0.8, y≈1) as a positive electrode active material and a lithium metal as a negative electrode has been proposed.
This non-aqueous electrolyte secondary battery has an electromotive force of 4 V or more,
Although it has advantages such as high energy density, it also has the problem of cycle deterioration as described above.

Various attempts have been made to improve the Li _x M _y O ₂ type positive electrode active material. For example, JP-A-62-9086
The 3 _{JP, A x M y N z O} 2 (A is 0.05 ≦ x ≦ 1.10 with an alkali metal, M is 0.85 ≦ y with transition metal
≦ 1.00, N is Al, In, or Sn and 0.001 ≦ z ≦
0.10), but also, in _{JP-A-62-90863, A x B y C z} D w O 2 (A is an alkali metal 0.0
5 ≦ x ≦ 1.10, B is a transition metal, and 0.85 ≦ y ≦ 1.
00 and C are Al, In, and Sn, and 0.001 ≦ z ≦ 0.1.
0, D are alkali metals other than A, transition metals other than B, I
0.001 ≦ w ≦ 0.10 for elements of Group Ia, Al, In, Sn, IIIb, IVb, Vb, and VIb, excluding carbon, nitrogen, nitrogen, or oxygen, in the second to sixth periods)
Etc. are used as the positive electrode active material.

However, even when these positive electrode active materials are used, a sufficiently satisfactory cycle life cannot be obtained, and in order to put a non-aqueous electrolyte secondary battery typified by a lithium secondary battery into practical use, There is a need for improvements in positive electrode active materials and negative electrode active materials that do not reduce charge / discharge capacity, charge / discharge efficiency, and electrode potential of active materials. Therefore, the present invention has been proposed in view of the above circumstances, and the charge / discharge capacity, charge / discharge efficiency, and battery voltage do not decrease due to repeated charge / discharge cycles, and non-aqueous electrolysis that provides a long cycle life. It is an object to provide a liquid secondary battery.

[0011]

Means for Solving the Problems As a result of earnest studies that the above-mentioned objects cannot be achieved, the present inventors have found that a new active material Li _X Ti _{y in} which Ti is solid-dissolved in a lithium cobalt composite oxide is obtained.
It has been found that Co _1-y O ₂ can be synthesized, and this Li _x Ti _y Co _1-y O ₂ has a discharge capacity and energy density equivalent to that of Li _x CoO ₂ , which has been conventionally used as a positive electrode active material. Furthermore, they have found that they have excellent characteristics in charge / discharge cycle life, and have completed the present invention.

The non-aqueous electrolyte secondary battery of the present invention has been proposed on the basis of such findings, and in the non-aqueous electrolyte secondary battery using an inorganic compound as the positive electrode active material, the positive electrode active material described above is used. In addition, a compound in which Ti is solid-dissolved in a lithium cobalt composite oxide is used.

The non-aqueous electrolyte secondary battery of the present invention comprises a positive electrode using an inorganic compound, a negative electrode, and an electrolyte for moving only specific ions, and is represented by the formula (1): The positive electrode is composed of the new positive electrode active material. Li _X Ti _y Co _1-y O ₂ (1) In the non-aqueous electrolyte secondary battery, the positive electrode active material is used to obtain high discharge capacity, high energy density and long cycle life. The atomic ratios x and y of Li and Ti are x ≦ 1.10 and y ≦ 0.05, preferably 0 <
y ≦ 0.05. For example, when the atomic ratio y of Ti exceeds 0.05, the sintered body undergoes phase separation, and sufficient performance as a positive electrode active material cannot be obtained. In addition, if Ti is not contained at all, long cycle life cannot be expected.

The method for synthesizing the composite oxide of Li, Ti, and Co, which is the above-mentioned new positive electrode active material, is not particularly limited, but Li, Ti, and Co may be used as respective oxides, carbonates, or the like. Are mixed in a predetermined ratio by metal atomic ratio,
It can be easily synthesized by heating at a temperature of about 900 ° C.

On the other hand, the material used as the negative electrode active material is not particularly limited, but metallic lithium, a lithium alloy (for example, Li-Al alloy, etc.), and a conductive polymer doped with lithium ions (for example, , Polyacetylene, polypyrrole, etc.), an intercalation compound in which lithium ions are incorporated into the crystal (for example, one containing lithium between layers such as TiS ₂ and MoS ₂ ), or a carbonaceous material that can be doped with lithium ions and dedoped Can be illustrated.

As the electrolytic solution, an aprotic organic electrolytic solution in which a lithium salt is used as an electrolyte and the electrolyte is dissolved in an organic solvent is used.

Here, as the organic solvent, esters,
Ethers, 3-substituted-2-oxazolidinones and mixed solvents obtained by mixing two or more of these organic solvents are used. Specific examples of the organic solvent include esters such as ethylene carbonate, propylene carbonate, γ-butyrolactone, and alkylene carbonate such as 2-methyl-γ-butyrolactone.

As the ethers, diethyl ether,
Dimethoxyethane, cyclic ether and the like can be mentioned. Examples of the ether having a 5-membered ring include tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2-ethyltetrahydrofuran,
Tetrahydrofuran substituted with an alkyl group such as 2-2′-dimethyltetrahydrofuran, tetrahydrofuran substituted with an alkoxy group such as 2-methoxytetrahydrofuran, 2,5-dimethoxytetrahydrofuran, and the like, and an ether having a 6-membered ring is 1 , 4-dioxolane, pyran, dihydropyran, tetrahydropyran and the like.

The 3-substituted-2-oxazolidinones include 3-methyl-2-oxazolidinone and 3-ethyl-
3-alkyl-2-oxazolidinone such as 2-oxazolidinone, 3-cycloalkyl-2-oxazolidinone such as 3-cyclohexyl-2-oxazolidinone, 3-
3-Aralkyl such as benzyl-2-oxazolidinone
Examples thereof include 3-aryl-2-oxazolidinone such as 2-oxazolidinone and 3-phenyl-2-oxazolidinone.

Among them, propylene carbonate, dimethoxyethane, ether having a 5-membered ring, (particularly tetrahydrofuran, 2-methyltetrahydrofuran, 2-ethyltetrahydrofuran, 2-methoxytetrahydrofuran, 2,5-dimethoxytetrahydrofuran), 3-methyl-2. -Oxazolidinone and the like are desirable.

As the electrolyte, lithium perchlorate, lithium borofluoride, lithium phosphorofluoride, lithium chloroaluminate, lithium halide, lithium trifluoromethanesulfonate, LiAsF ₆ and LiB (C) are used.
₆ H ₅ ) _{4 and the} like can be used, and among them, lithium perchlorate, lithium borofluoride, lithium phosphorus fluoride and the like are preferable.

[0022]

Function: Li _x Ti _y Co _1-y O ₂ is Li _x CoO ₂
Compared with, the crystal structure during charging and discharging is stable, and it is chemically stable with respect to the electrolytic solution. Therefore,
When the positive electrode is formed by using such Li _X Ti _y Co _{1 -y} O ₂ as the positive electrode active material, the cycle life of the secondary battery is improved.

[0023]

EXAMPLES The present invention will be described below with reference to specific examples, but it goes without saying that the present invention is not limited to these examples.

In this embodiment, lithium carbonate (Li ₂ C
O ₃ ) powder and titanium oxide (TiO ₂ ) and cobalt carbonate (CoCO ₃ ) powder are used as raw materials, and they are
Li _X T synthesized by firing at 0 ° C for 5 hours
This is an example in which the i _y Co _{1 -y} O ₂ composite oxide is used as the positive electrode.

First, Li, Ti and Co are prepared as follows.
A composite oxide containing was synthesized. Commercially available lithium carbonate (L
A mixture was prepared by mixing i ₂ CO ₃ ) powder with titanium oxide (TiO ₂ ) and cobalt carbonate (CoCO ₃ ) powders such that Li, Ti, and Co had the composition ratios shown in Table 1, respectively.

[0026]

[Table 1]

Then, this mixture was fired in an electric furnace at 900 ° C. for 5 hours in an air atmosphere to obtain a composite oxide sintered body. At this time, the sintered body obtained by firing the mixture 3 having a Ti composition ratio y of 0.075 had phase separation.

Then, X-ray diffraction measurement by CoKα radiation was performed on each of the composite oxides thus obtained. The X-ray diffraction (XRD) patterns of the composite oxides 1, 2 and 3 are shown in FIGS. 1, 2 and 3, respectively. For comparison, FIG. 4 shows the XRD of Li _x CoO ₂ (x = 1.0).

Comparing FIG. 1, FIG. 2 and FIG. 3 with FIG.
The XRD pattern of the i _x Ti _y Co _{1 -y} O ₂ composite oxide is almost the same as the XRD pattern of Li _x CoO ₂ , and it can be seen that the crystal structure is similar. However, Ti
When the composition ratio y is 0.075 or more, phase separation occurs as described above, and a phase considered to be LiTiO ₂ is mixed in Li _X Ti _y Co _1-y O ₂ .

Therefore, a single layer structure of Li _X Ti _y Co
To obtain a _1-y O ₂ composite oxide, the composition ratio y of Ti is 0.0
It has been found that it is necessary to set it to 5 or less. In addition,
Regarding the crystal structure of Li _x CoO ₂ , see A. Mendi
Details of the report by Boure et al. (A. Mendibou
re, C.I. Delmasand P.M. Changmumu
ller, Mat. Res. Bull. 19,1383
(1984))

LiTi _0.025 Co _0.975 O ₂ and LiTi _0.05 Co, which are the positive electrode active materials thus obtained,
_0.95 O ₂ was mixed with graphite powder serving as a conductive agent and fluororesin powder serving as a binder, and pressure-molded to prepare a positive electrode pellet. Then, the positive electrode pellet was used as a positive electrode, metallic lithium was used as a negative electrode, and 1 M of LiPF ₆ was dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane as an electrolytic solution, and a coin type battery was used by a usual method. (Example battery 1, Example battery 2) was produced.

Then, a charge / discharge cycle test was conducted on each of the produced batteries at a constant current (current density of 0.54 mA / cm ² ). The charge / discharge cycle test is performed by measuring the capacity retention ratio at each cycle, and the test conditions at this time are that the upper limit voltage during charging is 4.1V, the final voltage during discharging is 3.0V, The battery temperature was set to 23 ° C.

For comparison, Li _x CoO ₂ (x =
A coin-type battery (comparative battery) was prepared in the same manner as above except that 1.0) was used as the positive electrode active material, and a charge / discharge cycle test was performed.

Prior to the charge / discharge cycle test, the charge / discharge curves of the batteries of Examples 1 and 2 and the battery of Comparative Example were examined in order to compare the states of charge and discharge. The charge / discharge curves are almost similar to those of the comparative example battery, and the example batteries 1 and 2 have discharge capacities and energy densities comparable to those of the conventional non-aqueous electrolyte secondary battery. I understood it. Further, it was confirmed that the battery voltage and capacity were not decreased at the time of discharging, and sufficient reliability as a battery was obtained.

FIG. 2 shows the relationship between the cycle number and the capacity retention rate of the measured batteries of Examples 1 and 2 and the battery of Comparative Example.

As can be seen from FIG. 2, in the comparative batteries, the capacity retention rate drastically decreases with an increase in the number of cycles, whereas in the example batteries 1 and 2, the capacity retention rate is extremely low. Further, the decrease of the discharge capacity is caused not only by the deterioration of the positive electrode but also by the deterioration of the negative electrode, but in the case of all the batteries of the present example, the same metallic lithium was used as the negative electrode active material. can do. Therefore, when a relative evaluation is performed from the capacity retention rate at the 40th cycle, the decrease rate ΔDc of the discharge capacity in the comparative battery is about 2
On the other hand, the reduction rate ΔD _A of the discharge capacity in the example battery 1 in which the composition ratio y of Ti was 0.025 was about 1%, while it was about 1%
The reduction rate ΔD _B of the discharge capacity in the example battery 2 in which the composition ratio y of Ti was 2% and 0.05 was only about 8%, which indicates that the reduction rate of the discharge capacity is lower than that of the comparative example battery. ..

Therefore, from these results, the existing Li _x
Li _X Ti _y C, a new active material in which Ti is dissolved in CoO ₂
By using o _1-y O ₂ (x ≦ 1.10, y ≦ 0.05), it is apparent that the discharge capacity and energy density of the conventional battery can be maintained and the reduction of the cycle life of the battery can be suppressed. became.

[0038]

As is apparent from the above description, in the present invention, the composite oxide of the new positive electrode active material Li _X Ti _y Co _{1 -y} O _{2 in} which Ti is solid-dissolved in the lithium cobalt composite oxide is used in the positive electrode. Since the material is used, the chemical stability associated with changes in the crystal structure of the positive electrode is improved, without lowering the charge / discharge capacity and charge / discharge efficiency of the positive electrode active material and the electrode potential,
It is possible to prevent the cycle life of the battery from decreasing.

[Brief description of drawings]

FIG. 1 Li: Ti: Co = 1: 0.025: 0.97
5 is an X-ray diffraction spectrum of the complex oxide of No. 5.

FIG. 2 is an X-ray diffraction spectrum of a composite oxide with Li: Ti: Co = 1: 0.05: 0.95.

FIG. 3 Li: Ti: Co = 1: 0.075: 0.92
5 is an X-ray diffraction spectrum of a sintered body obtained by firing the mixture of No. 5.

FIG. 4 is an X-ray diffraction spectrum of a LiCoO ₂ composite oxide.

FIG. 5 is a characteristic diagram showing a change in capacity retention rate with respect to the number of cycles of a non-aqueous electrolyte secondary battery.

Claims (1)

Claim: What is claimed is: 1. In a non-aqueous electrolyte secondary battery using an inorganic compound as a positive electrode active material, a compound obtained by solid-solving lithium cobalt composite oxide with Ti is used as the positive electrode active material. A characteristic non-aqueous electrolyte secondary battery.