JPH07288124A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE:To enhance performance of a nonaqueous electrolyte secondary battery. CONSTITUTION:In a nonaqueous secondary battery using a carbon material such as coke and graphite as a negative active material, a positive electrode 2 is prepared by using a mixture of a lithium containing composite oxide (for example, LiMn2O4 or LiCoO2) and a lithium-titanium composite oxide represented by a general formula, Li1+x Ti2-xO4 (0<=x<=0.17) as an active material. Elution of a negative current collector 1 in overdischarge is prevented, and the battery free from remarkable deterioration in performance is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the performance of a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art As electronic devices are becoming smaller and lighter, there is a growing demand for high energy density secondary batteries as their power sources. In order to meet the demand, non-aqueous electrolyte secondary batteries are being put into practical use because of their high potential as high energy density batteries. A non-aqueous electrolyte secondary battery that uses metallic lithium for the negative electrode and a lithium-containing manganese composite oxide for the positive electrode seemed to be quite promising, but the metallic lithium negative electrode was powdered by repeated charging and discharging, and its performance was remarkably high. It has been found that there is a problem in practical cycle life because it deteriorates or metallic lithium deposits on dendrites and causes an internal short circuit, and it is difficult to put it into practical use. So recently,
In place of the lithium metal negative electrode, a non-aqueous electrolyte secondary battery having a negative electrode of a carbon electrode that utilizes the inflow / outflow of lithium ions from / to carbon has become the mainstream of development. This battery is named as a lithium ion secondary battery by the present inventors,
1990 (magazine Progress in Batte
ries & Solar Cells, Vol. 9,
P. 209), which typically uses a lithium-containing composite oxide such as LiCoO ₂ or LiMn ₂ O _{4 for the} positive electrode material and coke or a graphite material for the negative electrode. It is a major feature that it exhibits the above high battery voltage. At present, it is recognized by the battery industry and academic societies under the name of lithium-ion secondary battery, and it is attracting attention as the next-generation secondary battery. Actually 240W
A small amount of lithium ion secondary batteries having an energy density of about h / l have already been put into practical use. The energy density of existing nickel-cadmium batteries is 100-150 Wh / l
The energy density of the lithium ion secondary battery is far higher than that of the existing battery. Another feature of lithium-ion secondary batteries is their long life. The carbon negative electrode is such that during charging, lithium ions are doped into the carbon in the electrode, and during discharging, lithium ions are only undoped, and the carbon itself does not undergo a large change in crystal structure during charging and discharging,
It exhibits extremely stable charge and discharge characteristics, has little characteristic deterioration due to charge and discharge, and specifically, can be repeatedly charged and discharged 1000 times or more. However, a major drawback of the lithium-ion secondary battery is that its performance is significantly deteriorated due to overdischarge. For this reason, for example, lithium-ion secondary batteries that are currently in practical use as power sources for video cameras incorporate an over-discharge prevention circuit in their battery packs and take measures against it. Therefore, the use of expensive cobalt as the positive electrode active material is a battery with a high material cost, and the cost of the overdischarge prevention circuit is added. This is a major obstacle to the widespread use of batteries. In terms of materials, inexpensive lithium-manganese composite oxide should be used in the future instead of lithium-cobalt composite oxide. However, replacing the existing nickel-cadmium battery,
In order for a lithium ion storage battery to be used in a wide range of applications, it must be completed as a battery that does not require an overdischarge prevention circuit, that is, a battery that does not have performance deterioration due to overdischarge.

[0003]

DISCLOSURE OF THE INVENTION The present invention intends to improve the characteristic deterioration due to over-discharge of a non-aqueous electrolyte secondary battery containing a lithium-containing composite oxide as a main positive electrode active material and a carbon material as a negative electrode active material. To do.

[0004]

[Means for Solving the Problems] Means for solving the problems are represented by the general formula Li _{1 + X} Ti _{2 -X} O ₄ (where 0 ≦ X ≦ 0.1
A positive electrode is prepared by using the lithium-containing composite oxide mixed with the lithium titanium composite oxide shown in 7) as an active material.

[0005]

The cause of the excessive deterioration of the performance of the lithium ion secondary battery due to over-discharging is that copper, which is the negative electrode current collector, begins to dissolve in the electrolytic solution when over-discharging. This is a positive electrode active material containing a lithium-containing composite oxide (such as LiCoO ₂ or LiM).
When using n ₂ O _4, etc.), even reaching zero voltage of the battery is almost the potential of the positive electrode remains 3.5V (vsLi ⁺ / Li)
Due to the above, the potential of the negative electrode approaches the potential of the positive electrode, and the battery voltage has only reached 0. Therefore, if over-discharge is continued, 3.5 V (vsLi ⁺ / Li) is added to the copper of the negative electrode current collector.
When the potential is higher than the above, copper is dissolved into the electrolytic solution by the electrochemical oxidation reaction. The present inventor, when Li _{1 + X} Ti _{2 —X} O ₄ (where 0 ≦ X ≦ 0.17) is used as the positive electrode active material of the lithium ion secondary battery, is capable of removing copper of the negative electrode current collector even during overdischarge. It was found that there was no melting and no deterioration in battery performance. And further, the present inventor has added Li _{1 + X} Ti _{2 -X} to LiCoO ₂ LiMn ₂ O _4.
It has been found that when O ₄ (where 0 ≦ X ≦ 0.17) is mixed in a certain amount or more to form a positive electrode active material, the negative electrode current collector does not melt into the electrolytic solution due to overdischarging. I arrived.

[0006]

The present invention will be described in more detail with reference to the following examples.

Example 1 A specific battery of the present invention will be described with reference to FIG. A battery element which is a power generation element for carrying out the present invention was prepared as follows. First, mesocarbon microbeads (d002 = 3.
To 37 parts of 86), 4 parts by weight of acetylene black and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder are added, wet mixed with N-methyl-2-pyrrolidone as a solvent to form a slurry (paste form). did. Then, this slurry was uniformly applied to both surfaces of a 0.01 mm-thick copper foil serving as a current collector, dried, and then pressure-molded with a roller press to form a strip-shaped negative electrode (1).

Next, manganese dioxide (MnO ₂ ) and lithium carbonate (Li ₂ CO ₃ ) were mixed well at a ratio of 1 mol: 0.263 mol, and the mixture was calcined in air at 750 ° C. for 12 hours to prepare a positive electrode. Lithium manganese oxide as active material (Li
Mn ₂ O ₄ ) was prepared.

Next, as a sub-active material mixed with the positive electrode main active material, lithium titanium composite oxide Li _{1 + X} Ti _{2 -X} O
₄ was prepared as follows. However, as a result of preliminary experiments, L
The X value in i _{1 + X} Ti _{2 —X} O ₄ is 0 ≦ X ≦ 0.
It was confirmed to be more effective in the range of 17. Therefore, in this embodiment, X = 0.035 is used. Titanium dioxide (TiO ₂ : anatase) and lithium carbonate (Li ₂ CO
₃ ) was well mixed at a ratio of 1 mol: 1 mol, and this was calcined in air at 750 ° C. for 24 hours to prepare Li ₂ TiO ₃ . The adjusted Li ₂ TiO ₃ was further added with titanium dioxide (TiO ₂ : anatase) and metallic titanium powder in an amount of 2.0.
Mix well at a molar ratio of 7: 4.895: 0.895, put the mixture in a quartz tube, seal it under vacuum, install in an electric furnace, and heat up to 850 ° C at a rate of 150 ° C for 1 hour. Then 8
Lithium-titanium composite oxide (Li _1.035 Ti _1.965 O ₄ ) was maintained by holding it at a temperature of 50 ° C. for 16 hours.
Was adjusted. The production reaction is represented by the following formula (1). (Where _{X = 0.035) 2 (1 +} X) Li 2 TiO 3 + (5-3X) TiO 2 + (1-3X) Ti → 4Li 1 + X Ti 2-X O 4 ······ (1)

The lithium-manganese composite oxide and the lithium-titanium composite oxide prepared as described above were mixed well in a weight ratio of 9: 1, and 94 parts by weight of the mixture was used as a binder with 3 parts by weight of carbon black. N-methyl-2- which is a solvent together with 3 parts by weight of polyvinylidene fluoride
Wet mix with pyrrolidone to form a slurry (paste form). Next, this slurry is used as a positive electrode current collector in a thickness of 0.0
A 2 mm aluminum foil was evenly applied on both sides, dried, and pressure-molded with a roller press to form a strip-shaped positive electrode (2a).

Subsequently, the negative electrode (1) and the positive electrode (2a) were wound in a roll shape with a porous polypropylene separator (3) interposed therebetween to obtain a battery element having an average outer diameter of 15.7 mm. Next, the insulating plate (14) is placed on the bottom of the nickel-plated iron battery can (4) to house the battery element. Negative electrode lead taken out from battery element (5)
Is welded to the bottom of the battery can, and a 1 mol / liter LiPF ₆ dissolved ethylene carbonate (EC) and diethyl carbonate (DEC) mixed solution is injected into the battery can as an electrolyte. After that, the insulating plate (14) is also installed on the upper part of the battery element, the gasket (15) is fitted, and the explosion-proof valve (28) is installed inside the battery as shown in FIG. The positive electrode lead (7) taken out from the battery element is welded before injecting the electrolytic solution into this explosion-proof valve. On the explosion-proof valve, a ring-shaped PTC element (16) is sandwiched and a closing lid (29) serving as a positive electrode external terminal is overlaid, and the edge of the battery can is caulked, and the
A battery (A) having an outer diameter of 16.5 mm and a height of 65 mm was prepared with the battery structure shown in FIG.

Comparative Example 1 In order to compare the battery according to Example 1 with the overdischarge performance, a battery (Y) according to the prior art is prepared. That is, in the prior art, the lithium manganese composite oxide prepared in Example 1 was used.
Only (LiMnO ₄ ) is used as the positive electrode active material. To 91 parts by weight of the lithium manganese composite oxide prepared in Example 1, 6 parts by weight of graphite as a conductive agent and 3 parts by weight of polyvinylidene fluoride as a binder were used as a solvent, N-methyl-2.
Wet mix with pyrrolidone to form a slurry (paste). Next, this slurry was made to have a thickness of 0.
Evenly applied to both sides of 02 mm aluminum foil, dried and pressure-molded with a roller press machine to form a strip-shaped positive electrode (2y)
To create. After that, in exactly the same manner as in Example 1, a battery element was prepared using the negative electrode (1) and the positive electrode (2y) and housed in a nickel-plated iron battery can (4). The same electrolytic solution as in Example 1 was injected, and finally the edge of the battery can was caulked to prepare a battery (Y) with the same battery size and the same battery structure as in Example 1 shown in FIG.

Test Result 1 The batteries (A) and (Y) prepared in Example 1 and Comparative Example 1 were each charged with a charging voltage after an aging period of 12 hours for the purpose of stabilizing the inside of the battery. 4.2
The battery was set to V, each was charged for 8 hours, and discharged for all batteries by constant current discharge of 800 mA up to the final voltage of 3.0 V, and the initial discharge capacity of each battery was determined. Furthermore, the charging voltage is set to 4.2V again,
After charging for 8 hours in all cases, constant resistance discharge was performed for all batteries through a resistance of 5 ohms, and over discharge was continued even if the battery voltage dropped below the normal end voltage (3V). It was left for 2 weeks with the resistance of ohms connected. After that, the charging voltage was set to 4.2V again, and each battery was charged for 8 hours, and all the batteries were discharged to the final voltage of 3.0V by the constant current discharge of 800mA to recover after over-discharging each battery. The discharge capacity was determined. Further, after that, the battery was disassembled and the presence or absence of the dissolution of copper in the negative electrode current collector was examined. The results are compared with Table 1.

Up to now, it has been considered that a carbon material having good performance is a pseudo-graphite material having a certain degree of disordered structure, and a highly crystalline graphite material is decomposed by an electrolytic solution on the surface of graphite to cause an intercalation reaction of lithium ions. Was reported to be difficult to proceed. However, the negative electrode active material used in Example 1 was 2
The mesocarbon microbeads heat-treated at 800 ° C. were used to measure the interplanar spacing (d00
2) is a highly crystalline graphite material with 3.37Å. If the electrolytic solution of the mixed solvent of EC and DEC used in this example is used, the highly crystalline graphite material is rather flat and becomes a lithium ion secondary battery having a higher discharge voltage than the conventional pseudo-graphite material. As for the battery (A) and the battery (Y), in the initial capacity, as shown in the results of Table 1, even if the lithium manganese composite oxide is used as the positive electrode material, the energy density of 235 wh / l or more (currently commercialized) A lithium-ion secondary battery with almost the same energy density as a battery using cobalt as a positive electrode material can be obtained.

However, once the battery (Y) according to the prior art is over-discharged, the charge / discharge function is almost lost thereafter and the battery cannot be recovered. However, as can be seen from the results of the battery (A) in Table 1, the ratio of lithium titanium oxide (Li) to the lithium titanium oxide (Li) was 9 parts by weight to 9 parts by weight of the spinel-type lithium manganese composite oxide in which the positive electrode active material was the main active material. _{1 + X} Ti _2-X
A mixture of O ₄ , 0 ≦ X ≦ 0.17) as a sub-active material does not dissolve copper of the negative electrode current collector even when the battery is over-discharged, and the performance after the over-discharge is almost zero. It does not deteriorate. Lithium titanium oxide (Li _{1 + X} Ti
The mixture of _2-X O ₄ , 0 ≦ X ≦ 0.17) is extremely effective against overdischarge, and at the same time, an appropriate amount of the mixture increases the battery capacity.

Example 2 A lithium cobalt composite oxide (Li
CoO ₂ ) is synthesized as follows. Add lithium carbonate (Li ₂ CO ₃ ) and cobalt carbonate (CoCO ₃ ) to L
i and Co were mixed so that the atomic ratio was 1.03: 1, and the mixture was baked in air at 900 ° C. for about 10 hours to obtain LiC.
Obtain oO ₂ . Since LiCoO ₂ after calcination is obtained as a very hard mass, it is crushed by a pulverizer to obtain an average particle size of 0.
The powder is adjusted to 01 mm.

Next, as a sub-active material to be mixed with the positive electrode main active material, lithium titanium composite oxide Li _{1 + X} Ti _{2 -X} O
₄ was prepared as follows. However, as a result of preliminary experiments, L
The X value in i _{1 + X} Ti _{2 —X} O ₄ is 0 ≦ X ≦ 0.
Since it has been confirmed that it is more effective in the range of 17, the present example was carried out at X = 0. Commercially available titanium dioxide (TiO ₂ : anatase) and lithium carbonate (Li ₂ CO
₃ ) was well mixed at a ratio of 1 mol: 1 mol, and this was calcined in air at 750 ° C. for 24 hours to prepare Li ₂ TiO ₃ . The adjusted Li ₂ TiO ₃ was further mixed with titanium dioxide (TiO ₂ : anatase) and metallic titanium powder 2: 5:
Mix well at a molar ratio of 1, the mixture is put in a quartz tube, sealed under vacuum, placed in an electric furnace, heated to 850 ° C at a rate of 150 ° C for 1 hour, and heated to 850 ° C at a temperature of 16 ° C. Hold for a period of time to cause a reaction, and a lithium titanium composite oxide (LiTi
₂ O ₄ ) was adjusted. The production reaction of Li _{1 + X} Ti _{2 —X} O ₄ is also represented by the above formula (1).

The lithium cobalt composite oxide (LiCoO ₂ ) and the lithium titanium composite oxide (LiTi ₂ O ₄ ) prepared as described above were well mixed at a weight ratio of 9: 1, and 94 parts by weight of the mixture was added. 3 parts by weight of carbon black and 3 parts by weight of polyvinylidene fluoride as a binder are wet-mixed with N-methyl-2-pyrrolidone which is a solvent to form a slurry (paste form). Next, the slurry was uniformly applied to both sides of a 0.02 mm-thick aluminum foil which was a positive electrode current collector, dried, and then pressure-molded with a roller press to form a strip-shaped positive electrode (2b). Positive electrode (2
b) is combined with the same negative electrode (1) created in Example 1,
A battery element was formed as a spiral wound body with a separator sandwiched between them. The battery element was housed in a nickel-plated iron battery can (4) as in Example 1, and the electrolytic solution was also used in Example 1.
Inject the same electrolyte as the above, and finally crimp the edge of the battery can,
Battery (B) with the same battery dimensions and structure as battery (A)
It was created.

Comparative Example 2 In order to compare the overdischarge performance with the battery according to Example 2, a conventional battery (Z) is prepared. That is, in the related art, only the lithium cobalt composite oxide ((LiCoO ₂ )) prepared in Example 2 is used as the positive electrode active material.
In 91 parts by weight of the lithium-cobalt composite oxide prepared in step 6, 6 parts by weight of graphite as a conductive agent and 3 parts by weight of polyvinylidene fluoride as a binder are used as a solvent in N-methyl-
Wet-mix with 2-pyrrolidone to form slurry (paste)
To Next, this slurry was evenly applied to both sides of a 0.02 mm-thick aluminum foil that serves as a positive electrode current collector,
After drying, it is pressure-molded with a roller press machine and strip-shaped positive electrode (2
z) is created. Thereafter, a battery element was prepared using the negative electrode (1) and the positive electrode (2z), and stored in a nickel-plated iron battery can (4) in the same manner as in Example 2. The same electrolytic solution as in Example 2 was injected, and finally, the edge of the battery can was caulked to prepare a battery (Z) having the same battery size and the same battery structure as those in Example 1 and Example 2 shown in FIG.

Test Result 2 The batteries (B) and (Z) prepared in Example 2 and Comparative Example 2 were each charged with a charging voltage after an aging period of 12 hours for the purpose of stabilizing the inside of the battery. 4.1
The battery was set to V, each was charged for 8 hours, and discharged for all batteries by constant current discharge of 800 mA up to the final voltage of 3.0 V, and the initial discharge capacity of each battery was determined. Furthermore, the charging voltage is set to 4.1V again,
After charging for 8 hours in all cases, constant resistance discharge was performed for all batteries through a resistance of 5 ohms, and over discharge was continued even if the battery voltage dropped below the normal end voltage (3V). It was left for 2 weeks with the resistance of ohms connected. After that, the charging voltage was set to 4.1V again, and each battery was charged for 8 hours, and all batteries were discharged to the final voltage of 3.0V by constant current discharge of 800mA, and the recovery after over-discharge of each battery. The discharge capacity was determined. Further, after that, the battery was disassembled and the presence or absence of the dissolution of copper in the negative electrode current collector was examined. The results are compared in Table 2.

As can be seen from Table 2, once the battery (Z) according to the prior art is overdischarged, the charge / discharge function is almost lost thereafter and the battery cannot be recovered. However, even when the positive electrode main active material is the lithium cobalt composite oxide, as in the case of Example 1, when the lithium titanium composite oxide is mixed as the secondary active material in the ratio of 1 part by weight to 9 parts by weight of the main active material, ,
Even if the battery is over-discharged, the copper of the negative electrode current collector does not dissolve, and the performance after over-discharge is hardly deteriorated.

In both the case of Example 1 and the case of Example 2, the mixing of the lithium titanium composite oxide is extremely effective against over-discharging, and at the same time, the mixing of an appropriate amount also increases the battery capacity. However, if it is mixed with the main active material too much, the battery capacity tends to decrease again. The optimum mixing amount of the lithium-titanium composite oxide depends on the carbon material of the negative electrode used, but the mesocarbon microbeads heat-treated at 2800 ° C. used in this example are preferable for all positive electrode active materials. Is 3 to 30% by weight, more preferably 5 to 15% by weight.

As an example of the present invention, the case where LiMn ₂ O ₄ and LiCoO ₂ are used as the main active material of the positive electrode has been shown, but the present invention is not limited to this, and other lithium-containing composite oxides can also be used as lithium. Titanium oxide (Li
_{11X Ti 2-X O 4,} 0 ≦ X ≦ 0.17) similar results were mixed to obtain. Incidentally, the following materials are listed as other lithium-containing composite oxides useful as the positive electrode active material. LiNiO ₂ , LiCo _1-x M _x O ₂ (M
The metal element other than _{Co), LiMn 2-x M} x O 4 (M
Is Co, Ni, Fe, etc.), Li [Mn _2−X Li _X ] O
₄ (however, 0 ≦ X ≦ 0.081) and the like. Especially the general formula Li
[Mn _2−X Li _X ] O ₄ (however, 0 ≦ X ≦ 0.08
The spinel-based lithium manganese oxide represented by 1) is an inexpensive material and is attractive as a positive electrode material for future lithium-ion secondary batteries. LiMn _{2 of} Example 1
O _{4 is} also one of the spinel type lithium manganese oxides represented by the above general formula at x = 0, and as a result of the example, even if the lithium manganese composite oxide is used for the positive electrode material, it is 235 wh / l or more. A lithium-ion secondary battery with the energy density of (about the same energy density as the currently commercialized battery using cobalt as the positive electrode material) can be obtained. In addition, the present invention improves performance deterioration due to over-discharge, which is a major drawback of the lithium ion secondary battery, and thus the lithium ion secondary battery is widely used as an inexpensive power source.

[0024]

The lithium-containing composite oxide (for example, LiM)
With n ₂ O _4, LiCoO _2, etc.) LiTi ₂ O ₄ which the positive electrode active material by mixing typified by spinel type lithium-titanium composite oxides, over some big disadvantage of a lithium ion secondary battery Performance deterioration due to discharge can be eliminated. As a result, an over-discharge protection circuit is not needed, so it becomes possible to provide a lithium-ion secondary battery with an energy density sufficiently higher than that of existing secondary batteries as an inexpensive battery for a wide range of applications, and its industrial value. Is large.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structures of batteries in Examples and Comparative Examples.

[Explanation of symbols]

1 is a negative electrode, 2 is a positive electrode, 3 is a separator, 4 is a battery can, 5
Is a negative electrode lead, 7 is a positive electrode lead, 14 is an insulating plate, 15 is a gasket, 28 is an explosion-proof valve, and 29 is a battery lid.

Claims (1)

[Claims] 1. In a non-aqueous electrolyte secondary battery in which the negative electrode active material is a material capable of doping / dedoping lithium, the positive electrode active material is a mixture of two or more kinds of lithium-containing composite oxides, One type is the general formula Li _{1 + X} Ti _{2 -X}
A non-aqueous electrolyte secondary battery, which is a spinel-type lithium titanium oxide represented by O ₄ (where 0 ≦ X ≦ 0.17).