JP2002208441A - Nonaqueous electrolyte secondary cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary cell with big capacity and very high safety. SOLUTION: For the nonaqueous electrolyte secondary cell using a positive electrode containing one or more of 4 V class double oxide, chosen from 4 V class spinel type lithium.manganese double oxide, laminated LiNi double oxide, or laminated LiCo double oxide, a positive electrode is made to contain 5 V class double oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a lithium secondary battery or a lithium ion secondary battery, and relates to a non-aqueous electrolyte secondary battery having improved safety and improved cycle life at high temperatures, capacity storage characteristics, and self-discharge properties.

[0002]

2. Description of the Related Art In recent years, as devices have become more portable and cordless, expectations are growing for non-aqueous electrolyte secondary batteries that are small, lightweight, and have a high energy density. On the other hand, due to the problem of energy and the problem of global warming, expectations for automobile systems and the like using batteries have been greatly increased. In such an application, a battery used with a large capacity and a large current is required, and it is important to further enhance the safety of the battery under such conditions.

[0003] As a means for realizing a battery which can be used with a large capacity and a large current, by mixing a cathode material of 3V class with a cathode material of 4V class, it can be used in a wide voltage range and the capacity can be increased. The invention to the effect is described in JP-A-9-180718
No. is proposed.

[0004]

However, the above proposal has no effect on improving the safety of the battery and has a problem that the energy density is low due to a low average operating voltage.

[0005] Spinel-type lithium Mn oxide is said to be highly safe among 4V class materials. For example, when this spinel-type lithium manganate is used as a cathode material of a small battery for portable use, it is said that there is no problem in safety at all. On the other hand, in high-capacity applications such as electric vehicles and power storage, charging and discharging with a large current is expected to have higher safety than portable applications, but even with spinel-type lithium Mn oxide, safety is expected. However, there was no problem at all. Further 4V
Among the class materials, the LiNi layered composite oxide and the LiCo layered composite oxide have higher electric capacity than spinel-type lithium Mn oxide, and can improve the energy density of the battery. Therefore, it has been difficult to ensure safety in large-capacity applications.

The present invention has been made in view of the above circumstances, and has as its object to enhance the safety of a large battery. In a large battery, since there is a high possibility of being exposed to an overcharged state due to use at a high current or use at a high rate, the present invention provides a non-aqueous electrolyte having higher safety under such use conditions. It is intended to provide a secondary battery.

[0007]

In order to achieve the above object, the present invention employs the following constitution. The non-aqueous electrolyte secondary battery of the present invention comprises a 4V class spinel lithium-manganese composite oxide, a LiNi layered composite oxide, and a LiCo
In a non-aqueous electrolyte secondary battery using an electrode containing one or more 4V-class composite oxides selected from layered composite oxides, the electrode contains a 5V-class composite oxide. Further, the non-aqueous electrolyte secondary battery of the present invention is the non-aqueous electrolyte secondary battery described above, wherein the 5V-class composite oxide functions as a buffer in an overvoltage region of 4.2 V or more. It is characterized by. Further, the non-aqueous electrolyte secondary battery of the present invention is the non-aqueous electrolyte secondary battery described above, wherein the 5V-class composite oxide functions as a buffer in an overvoltage region of 4.3 V or more. It is characterized by.

Further, a non-aqueous electrolyte secondary battery according to the present invention is the non-aqueous electrolyte secondary battery described above, wherein the 5V-class composite oxide is a spinel type crystal material. I do. Furthermore, the non-aqueous electrolyte secondary battery of the present invention is the non-aqueous electrolyte secondary battery described above, wherein the 5V-class composite oxide is a spinel-type lithium-manganese composite oxide. Features.

A non-aqueous electrolyte secondary battery according to the present invention is the above-described non-aqueous electrolyte secondary battery, wherein the 5V-class composite oxide is represented by the following composition formula. It is characterized by the following. LiNi _x Mn _2-x O _{4 wherein x} indicating the above composition ratio is an atomic ratio of 0.2 ≦ x ≦
The range is 0.7.

According to the above nonaqueous electrolyte secondary battery, since the electrode contains the 5V-class composite oxide, even if the battery is overcharged, the 5V-class composite oxide participates in the charging reaction. The reaction between the positive electrode and the electrolyte does not occur, the heat generation of the battery is prevented, and the safety of the battery can be enhanced. In the present invention, a 5V-class composite oxide is defined as one having a plateau region of a charge / discharge curve at 4.5 to 5.2 V with respect to a metal lithium counter electrode.

Further, since the 5V-class composite oxide functions as a buffer in an overvoltage region of 4.2V or more or 4.3V or more, a 4V-class spinel-type lithium-manganese composite oxide, a LiNi layered composite oxide, LiCo
In an overcharge region of 4.2 to 4.3 V or more, which is the upper limit of the charging voltage of a 4V-class composite oxide such as a layered composite oxide, 5
The V-class composite oxide can function as a buffer, suppressing damage to the crystal structure of the 4V-class material,
The safety of the battery can be further improved.

Further, the 5V-class composite oxide is a spinel-type crystal material, which is relatively stable even in a charged state, so that the safety of the battery can be further improved. Further, the 5V-class composite oxide is a spinel-type lithium-manganese composite oxide, and since the spinel-type lithium-manganese composite oxide is relatively stable even in a charged state, the safety of the battery can be further improved.

As the above 5V class composite oxide, LiNi is used.
The use of _x Mn _2-x O _4, it is possible to further improve the safety of the battery.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. Non-aqueous electrolyte secondary battery of the present invention,
A 5V-class composite oxide is placed in an electrode containing at least one 4V-class composite oxide selected from among a 4V-class spinel-type lithium-manganese composite oxide, a LiNi layered composite oxide, and a LiCo layered composite oxide as a positive electrode. Including. here,
The 5V-class composite oxide is defined as 4.
In the present invention, the charge / discharge curve has a plateau region at 5 to 5.2 V. In the present invention, a 4V-class composite oxide is defined as one having a plateau region of a charge / discharge curve at 3.5 to 4.3 V with respect to a metal lithium counter electrode.

In the non-aqueous electrolyte secondary battery of the present invention using the above-described positive electrode, lithium ions are usually released from the 4V-class composite oxide during charging, and the lithium ions are inserted into the negative electrode active material. The reaction proceeds. And
The charging is completed when the potential of the positive electrode rises to about 4.1 to 4.3 V with respect to the metal lithium.

At this time, if the charging is not completed even when the voltage exceeds 4.3 V for some reason, the 5V-class composite oxide contained in the electrode participates in the charging reaction, and lithium ions are generated from the 5V-class composite oxide. The charge reaction proceeds by desorption.
Thus, the 4V-class composite oxide does not participate in the charge / discharge reaction, so that the 4V-class composite oxide does not reach an overcharged state and the crystal structure is not destroyed.

The 5V-class composite oxide functions as a buffer in an overvoltage region of 4.2 V or more or 4.3 V or more. The upper limit of the charging voltage of a 4V-class composite oxide such as a 4V-class spinel-type lithium-manganese composite oxide is approximately 4.2 to 4.3 V, and when a voltage higher than this is applied, the battery is overcharged. The 5V class composite oxide of the present invention can function as a buffer in an overcharge region of 4.2 to 4.3 V or more. That is, it is involved in the charging reaction in place of the 4V-class composite oxide. Thereby, damage to the crystal structure of the 4V-class composite oxide can be suppressed, and the safety of the battery can be further improved.

As the above 5V-class composite oxide, a spinel-type crystal material can be exemplified, and among the spinel-type crystal materials, a spinel-type lithium-manganese composite oxide is particularly preferable. This spinel-type lithium-manganese composite oxide is relatively stable in a state in which lithium is separated from the crystal, that is, in a charged state, so that the safety of the battery can be further improved.

In particular, among the spinel-type lithium-manganese composite oxides, those represented by the following composition formula are preferable in that the safety of the battery can be further improved. LiNi _x Mn _2-x O ₄ However, x indicating the above composition ratio is in the range of 0.2 ≦ x ≦ 0.7 in atomic ratio.

Examples of the 4V class spinel type lithium-manganese composite oxide include Li _{1 + y1} Mn _2-y1 O ₄
(Y1 representing the above composition ratio is 0.05 ≦ y1 ≦
0.182). Examples of the LiNi layered composite oxide include, for example, Li _z1 Ni _{2 -z 2} M _z2 O ₂
(The above composition ratios z1 and z2 are atomic ratios of 0 ≦ z1 ≦
1, 0 <z2 ≦ 0.4, M is Co, Mn,
Al). Furthermore, L
As the iCo layered composite oxide, LiCoO ₂ can be exemplified. The above composition formula is merely an example, and the above 4V
Class spinel type lithium-manganese composite oxide, LiNi
The layer composite oxide and the LiCo layer composite oxide are not limited to those represented by the above composition formula.

The compounding ratio of the 4V-class composite oxide to the 5V-class composite oxide is preferably in the range of 4V-class composite oxide: 5V-class composite oxide = 99.5: 0.5 to 90:10. If the compounding ratio of the 5V-class composite oxide falls below this range, the amount of the 5V-class composite oxide that functions as a buffer during overcharge becomes insufficient, and the safety of the nonaqueous electrolyte secondary battery is impaired. Further, when the compounding ratio of the 5V-class composite oxide is higher than the above range, the compounding ratio of the 4V-class composite oxide is relatively reduced, and the energy density of the nonaqueous electrolyte secondary battery is reduced. Not preferred.

The positive electrode according to the present invention may contain a conductive material such as a carbon material, a binder, and the like.

[0023]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples. (Examples 1 to 4) The synthesis of lithium manganate, which is a 4V-class composite oxide, was performed using lithium carbonate (Li ₂
CO ₃ ) and electrolytic manganese dioxide (EMD) were used. Prior to mixing the starting materials described above, Li ₂ CO ₃
And EMD classification. The purpose of this process is to improve the reactivity and secure lithium manganate having the desired particle size. In the ordinary solid-phase reaction synthesis method, the particle size of lithium manganate is substantially determined by the particle size of EMD before firing. That is, the synthesis of lithium manganate having the target particle size is ensured by classification of the EMD before firing at the target particle size. When lithium manganate is used as a positive electrode active material of a battery, it may be 5 to 5 due to the balance of ensuring reaction uniformity, ease of slurry preparation, and safety.
An average particle size of 30 μm is preferred. Therefore, the particle size of EMD is set to 5 to 30 μm, which is the same as the target particle size of lithium manganate.

Li ₂ CO ₃ is pulverized so that the average particle diameter D ₅₀ becomes 1.4 μm, and [Li] / [Mn] = 1.05.
/ 2 (molar ratio). This is because it is considered that a particle size of 5 μm or less is desirable for ensuring a uniform reaction.

The mixed powder is mixed under an oxygen flow atmosphere at 80
Baking at 0 ° C. Next, fine particles having a particle size of 1 μm or less in the obtained lithium manganate particles were removed by an air classifier. At this time, the specific surface area of the obtained lithium manganate was about 0.9 m ² / g. Further, a tap density of 2.17 g / cc, true density 4.09 g / cc, an average particle diameter D ₅₀ is 17.2Myuemu, lattice constant was powder characteristics of 8.236A.

The lithium manganate synthesized above and 5 V
Grade spinel LiNi _0.5 Mn _1.5 O ₄ with 100-α: α
(% By weight, where α = 0.1, 0.5, 1.5, 5.
A cylindrical cell having a diameter of 18 mm and a height of 65 mm was prototyped using the positive electrode mixed at a mixing ratio of 0). First, lithium manganate, LiNi _0.5 Mn _1.5 O ₄ and a conductive additive are dry-mixed, and the obtained mixed powder is uniformly dispersed in N-methyl-2-pyrrolidone (NMP) in which PVDF as a binder is dissolved. To prepare a slurry. Thick the slurry 2
After coating on a 5 μm aluminum metal foil, NMP was evaporated to form a positive electrode sheet. The solid content ratio in the positive electrode was lithium manganate: LiNi _0.5 Mn _1.5 O ₄ : conductive additive: PVDF = 80-α: α: 10: 10 (% by weight).

On the other hand, the negative electrode sheet is made of carbon: PVDF =
They were mixed at a ratio of 90:10 (% by weight), dispersed in NMP, and applied to a copper foil having a thickness of 20 μm. The electrode sheets of the positive electrode and the negative electrode prepared as described above were rolled up through a polyethylene porous membrane separator having a thickness of 25 μm to obtain a cylindrical battery.

The electrolyte uses 1M LiPF ₆ as a supporting salt,
Propylene carbonate (PC): diethyl carbonate (DEC) = 50: 50 (vol%) was used as a solvent. Thus, the non-aqueous electrolyte secondary batteries of Examples 1 to 4 were manufactured.

(Comparative Example 1) LiNi _0.5 Mn _1.5
30 mm in diameter and 120 m in height in the same manner as in Example 1 except that O ₄ was not contained and the solid content ratio was changed to lithium manganate: conductive additive: PVDF = 80: 10: 10 (% by weight).
m prototype cell was produced. Thus, a non-aqueous electrolyte secondary battery of Comparative Example 1 was manufactured.

(Comparative Evaluation Example 1) Example 1 and Comparative Example 1
An overcharge test was performed using the cylindrical cell prepared in the above. In the overcharge test, overcharge was performed under the conditions of a charging voltage of 12 V, a charging current of 3 C, and room temperature. At that time, the surface temperature at the center in the battery length direction was measured. The initial charge is 4.2V at 1A
The discharge was performed up to 3.0 V at 5 A. Table 1 shows the maximum heat generation temperature of the cylindrical cells of Examples 1 to 4 and Comparative Example 1 during an overvoltage test at 20 ° C. It can be seen that the cylindrical cells of Examples 1 to 4 generate less heat during the overvoltage test than Comparative Example 1.

[0031]

FIG. 1 shows a charge-discharge curve of lithium manganate alone (Comparative Example 1) which is a 4V-class composite oxide.
FIG. 2 shows a charge / discharge curve of LiNi _0.5 Mn _1.5 O ₄ alone. FIG. 3 shows a charge / discharge curve of the battery of Example 1.
In the charging curve of FIG. 1A, the voltage gradually increases with the progress of charging, and there is no particular plateau portion that becomes flat when the charging curve has a certain voltage. This is the same for the discharge curve in FIG. On the other hand, FIG.
As is clear from FIG. 2B and FIG. 2B, each of the charge / discharge curves of LiNi _0.5 Mn _1.5 O ₄ , which is a 5V-class composite oxide, has a plateau around 4.5 to 4.8 V. Understand. This plateau of LiNi _0.5 Mn _1.5 O ₄ can also be found in the charge / discharge curves of the battery of Example 1 shown in FIGS. 3 (a) and 3 (b). Therefore, even when the charging voltage of the battery of the first embodiment increases due to overcharging, the rapid increase of the voltage of the battery of the first embodiment is suppressed by the presence of the plateau around 4.5 to 4.8 V. This indicates that the calorific value of the battery is reduced and safety is ensured.

[0033]

As is apparent from the above description, according to the present invention, the safety of the nonaqueous electrolyte secondary battery is improved by adding a 5V-class composite oxide to the positive electrode. Considering the safety of a large-capacity battery, it has been found that it is very effective to further improve the safety by mixing a 5V-class composite oxide, and its industrial value is extremely large.

[Brief description of the drawings]

FIG. 1 is a graph showing a charge / discharge curve of a non-aqueous electrolyte secondary battery of Comparative Example 1, in which (a) is a graph showing a charge curve, and (b) is a graph showing a discharge curve.

2 is a graph showing a charge / discharge curve of LiNi _0.5 Mn _1.5 O ₄ , wherein (a) is a graph showing a charge curve, and (b) is a graph showing a discharge curve.

3 is a graph showing a charge / discharge curve of the nonaqueous electrolyte secondary battery of Example 1, (a) is a graph showing a charge curve, and (b) is a graph showing a discharge curve.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masato Shirakata 5-7-1 Shiba, Minato-ku, Tokyo F-term in NEC Corporation (Reference) 5H029 AJ03 AJ04 AJ05 AJ12 AK03 AK18 AL08 AM03 AM05 AM07 HJ02 HJ18 5H050 AA07 AA08 AA09 AA15 BA17 CA08 CA09 CA29 HA02 HA18

Claims (6)

[Claims] 1. A 4 V class spinel type lithium-manganese composite oxide, LiNi layered composite oxide, LiCo
A non-aqueous electrolyte secondary battery using an electrode containing one or more 4V-class composite oxides selected from layered composite oxides, wherein the electrode contains a 5V-class composite oxide. Water electrolyte secondary battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the 5V-class composite oxide functions as a buffer in an overvoltage region of 4.2 V or more. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the 5V-class composite oxide functions as a buffer in an overvoltage region of 4.3 V or more. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the 5V-class composite oxide is a spinel-type crystal material. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the 5V-class composite oxide is a spinel-type lithium-manganese composite oxide. 6. The non-aqueous electrolyte secondary battery according to claim 1, wherein the 5V-class composite oxide is represented by the following composition formula. LiNi _x Mn _2-x O _{4 wherein x} indicating the above composition ratio is an atomic ratio of 0.2 ≦ x ≦
The range is 0.7.