JPH07272758A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To extend a cycle life by using tungsten dioxide as a negative electrode active material holder, and using a mixed solvent as an electrolytic solvent, so as to provide high voltage, high energy density, large charge/discharge capacity and further high safety. CONSTITUTION:Tungsten dioxide (WO2) is used as a negative electrode active material holder, and further a mixed solvent, having ethylene carbonate (EC) as an inevitable component, is used as a nonaqueous electrolyte. Thus in electrode potential 0.0 to 1.0V relating to a lithium reference electrode conventionally called non-reversible in WO2, a large charge/discharge capacity and stable long cycle life are obtained. Consequently, even when the WO2 is used as the negative electrode active material holder, remarkably decreasing operating voltage of a battery is eliminated, and high voltage and high energy density can be attained. Further because of large specific gravity, in also a charge/ discharge capacity per negative electrode volume, a very large capacity can be obtained as compared with a graphite system negative electrode, and also a cycle life can be extended.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and particularly to a high voltage, a high energy density, a large charge / discharge capacity, and a long cycle life.
The present invention also relates to a lithium secondary battery that provides highly safe battery characteristics.

[0002]

2. Description of the Related Art In recent years, electronic devices have become smaller, lighter and more portable, and there has been a demand for the development of batteries having a high energy density as their power sources. As a battery that meets such a demand, a lithium secondary battery using lithium metal as a negative electrode active material is expected. The lithium secondary battery basically has higher voltage and higher energy density than various commercially available secondary batteries such as nickel cadmium battery and lead storage battery. However, in general, a lithium secondary battery using lithium metal as a negative electrode active material generates needle-shaped lithium (dendrites) during charging, and the needle-shaped lithium is cut off during discharging and is detached from the electrode substrate. Dead lithium is produced that does not contribute. Further, since the deposited metal lithium particles are very active, the lithium metal is consumed by the reaction with the electrolytic solution. For these reasons, batteries using lithium metal as the negative electrode active material are
There is a problem that the cycle life is shortened, and it is very difficult to secure the cycle life in a battery system using lithium metal or lithium alloy for the negative electrode. An intercalation compound utilizing the intercalation reaction of lithium has been attracting attention as a new negative electrode active material holder replacing lithium metal or lithium alloy. As a typical intercalation compound, molybdenum dioxide (Mo
Inorganic compounds such as O ₂ ), tungsten dioxide (WO ₂ ), titanium disulfide (TiS ₂ ), and carbonaceous materials such as natural graphite and artificial graphite have been investigated, but there are problems as described below. Since such an intercalation compound is held in the skeleton structure in the state where lithium is ionized, it is more stable than a chemically active lithium negative electrode, and dendrite formation seen in lithium metal is also generated. Cycle life is improved because it is not present. Among the above-mentioned intercalation compounds, the carbonaceous material is 0 relative to the lithium reference electrode (metal lithium).
In the range of the base electrode potential of ˜1 V, lithium ions can be occluded and released stably, and 150 to 350 mA.
It has a large charge / discharge capacity of h / g. In fact, some lithium ion secondary batteries using a carbonaceous material for the negative electrode active material holder have been put to practical use.

[0003]

However, when a carbonaceous material is used as the negative electrode, although the capacity per weight is large, the specific gravity of these carbonaceous materials is 1.6 to 2.2 g.
The specific gravity when processed into an actual electrode sheet is about 1 g / cc. Therefore, there is a problem that the capacity per volume of the negative electrode becomes small, and the energy density of the battery becomes smaller than that of a secondary battery using lithium metal for the negative electrode. On the other hand, the inorganic compound in the intercalation compound has a specific gravity much larger than that of the carbonaceous material, and although the capacity per volume of the negative electrode can be expected, generally, the electrode potential capable of stably occluding and releasing lithium ions is: Since it is as high as 0.5 to 2.0 V with respect to the lithium reference electrode, when these inorganic compounds are used as the negative electrode active material holder, the working potential of the lithium secondary battery decreases by 0.5 to 2.0 V, and There is a problem in that the energy density of Among these inorganic compounds, WO having a rutile type crystal structure
₂ has a large specific gravity of 12 g / cc, and is WO
₂ has an electrode potential of 0.5 to 0.5 with respect to metallic lithium.
It has a relatively base potential of 1.1 V, and is very promising as a negative electrode active material holder for a lithium secondary battery. However, it is widely known that charging / discharging 1 equivalent or more of lithium ion per 1 equivalent of WO ₂ causes a significant decrease in charging / discharging capacity along with a cycle, resulting in a very short cycle life. The potential when 1 equivalent of lithium ion is absorbed per equivalent of WO ₂ is about 0.5 V with respect to metallic lithium. Therefore, in order to achieve a lithium secondary battery having a higher voltage and a higher energy density, the charge / discharge capacity and reversibility of WO _{2 at} an electrode potential of 0.5 V or less as low as possible with respect to the lithium reference electrode are important. However, it has been reported so far that the occlusion / release of lithium ions of WO _{2 at} this electrode potential is not reversible. An object of the present invention is to solve the above problems of the prior art, to provide a lithium secondary battery having high voltage, high energy density, large charge / discharge capacity, safety, and long cycle life. To provide.

[0004]

The present invention will be described in brief. The present invention relates to a lithium secondary battery, which comprises a negative electrode active material holder which occludes lithium ions by charging and releases lithium ions by discharging. A negative electrode as a main component, a positive electrode capable of reversible electrochemical reaction with lithium ions,
In a lithium secondary battery comprising a lithium ion conductive non-aqueous electrolytic solution or an electrolytic solution-impregnated polymer electrolyte, WO ₂ is used as the negative electrode active material holder, and the non-aqueous electrolytic solution or the impregnating electrolytic solution is used. A mixed solvent containing ethylene carbonate as an essential component is used as the solvent.

The essential points of the lithium secondary battery according to the present invention are:
WO ₂ is used as the negative electrode active material holder, and a mixed solvent containing ethylene carbonate (EC) as an essential component is used as the solvent of the non-aqueous electrolyte. By combining a mixed solvent containing WO ₂ and EC, an electrode potential of 0.0 to 1.0 V with respect to a lithium reference electrode which has been said to be irreversible in WO ₂ up to now.
In the above, it was found by experiments that a large charge / discharge capacity and stable and long cycle life can be obtained.

The present invention will be described in more detail below. As the negative electrode active material holder used in the present invention, WO ₂ which is a commercially available product is used. As the solvent of the non-aqueous electrolyte used in the present invention, a mixed solvent containing EC as an essential component is used. As a solvent to be mixed with EC, a solvent used in this type of battery other than propylene carbonate (PC) can be used, and for example, dimethyl carbonate (DM
C), chain esters such as diethyl carbonate (DEC) and methyl ethyl carbonate (MEC), γ-lactones such as γ-butyrolactone, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane ( DE
E), at least one selected from chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran, nitriles such as acetonitrile, etc.
More than one type of solvent can be used. The solute of the non-aqueous electrolyte solution is LiAsF ₆ , LiBF ₄ , LiP.
F ₆ , LiAlCl ₄ , LiClO ₄ , LiCF ₃ SO
₃ , LiSbF ₆ , LiSCN, LiCl, LiC ₆ H
₅ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃
Lithium salts such as SO ₂ ) ₃ , C ₄ F ₉ SO ₃ Li and mixtures thereof can be used.

Furthermore, when the negative electrode material of the present invention is used in a lithium secondary battery, the positive electrode active material contains lithium, such as titanium, molybdenum, tungsten, niobium, vanadium, manganese, iron, chromium, nickel and cobalt. It is possible to use a complex oxide or complex sulfide of the above transition metal. In particular, the electrode potential with respect to the lithium metal electrode is 3
LiMn ₂ O ₄ , LiCoO ₂ , and LiNiO _{2 which} are higher than V and are expected to have high voltage and high energy density are
It is suitable as a positive electrode active material.

According to the present invention, when WO ₂ is used as a negative electrode active material holder and a mixed solvent containing EC as an essential component is used as a non-aqueous electrolyte, WO ₂ is compared with a lithium reference electrode (metal lithium). In the range of the base electrode potential of 0 to 1 V, lithium ions can be occluded and released stably, and a high capacity charge / discharge region is provided. Also, the specific gravity is 1
Since it is as large as 2 g / cc, the charging / discharging capacity per unit volume of the negative electrode is remarkably large and the polarization of charging / discharging is small compared to graphite or the like which has been conventionally used as a negative electrode active material holder of this type of battery. It can be charged and discharged with electric current, and almost no deterioration such as generation of irreversible substances due to repeated charging and discharging can be seen, and extremely stable battery characteristics with long cycle life can be obtained, so high voltage, high energy It is possible to provide a lithium secondary battery that has a high density, a large charge / discharge capacity, safety is ensured, and a long cycle life.

The reason why such stable and long cycle life battery characteristics are obtained is not necessarily clear, but is presumed as follows. So far, in a non-aqueous electrolyte secondary battery using WO ₂ as a negative electrode active material holder, WO ₂ undergoes a phase change due to intercalation of excess lithium,
It was thought that the charge / discharge capacity was significantly reduced due to the formation of an irreversible structure, but in the lithium secondary battery according to the present invention, lithium ions could be occluded and released stably, and no significant reduction in capacity was observed. From this, it is estimated that the cause of this capacity decrease is due to the non-aqueous electrolyte. In the conventional technology, PC is used as the electrolyte,
The PC reacts with WO _2, lithium ions on the surface of the WO ₂ capacity for forming a strong protective film can not pass through is estimated to have decreased. On the other hand, in the present invention, since EC is used, it is presumed that lithium ions can easily pass through the protective film formed on the surface of WO ₂ , and stable and long-cycle battery characteristics can be obtained.

[0010]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 FIG. 1 is a sectional view of a test cell used for performance evaluation of a negative electrode active material holder of a lithium secondary battery according to the present invention. Figure 1
In the above, reference numeral 1 is a counter case, which is formed by drawing a stainless steel plate. Reference numeral 2 is metallic lithium as a counter electrode, and a lithium metallic foil having a predetermined thickness has a diameter of 16 m.
It is the one punched out in m and crimped. Reference numeral 3 is a non-aqueous electrolytic solution, and EC is a solution in which 1 mol / liter of lithium perchlorate (LiClO ₄ ) is dissolved in a mixed solvent of DMC in a volume ratio of 1: 1. 4 is a separator made of a polypropylene or polyethylene porous film.
Reference numeral 5 is a working electrode case obtained by drawing a stainless steel plate. Reference numeral 6 is a working electrode composed of WO ₂ . This working electrode was prepared by mixing WO ₂ which is a general commercial product, acetylene black which is a conductive agent, and polytetrafluoroethylene which is a binder in a weight ratio of 93: 5: 2, and rolling the mixture to a thickness of 250.
A sheet with a diameter of 16 μm was prepared and punched into a diameter of 16 mm. Reference numeral 7 denotes a Ti net current collector, which is spot-welded to the working electrode case 5 while being covered with the working electrode 6. Reference numeral 8 denotes a gasket, which maintains the electrical insulation between the counter electrode case 1 and the working electrode case 5, and the opening edge of the working electrode case is bent inward and caulked to seal and seal the battery contents. There is.

The test cell was subjected to a charge / discharge test in a voltage range of 0.0 to 1.0 V and a current of 1 mA. The charge / discharge curve at the 10th cycle at this time is shown in FIG. In FIG. 2, the vertical axis represents voltage (V) and the horizontal axis represents charge / discharge capacity (mAh).
Further, FIG. 3 shows the relationship between the charge / discharge capacity (mAh, vertical axis) and the number of cycles (horizontal axis). As is clear from FIGS. 2 and 3, WO ₂ was capable of reversibly occluding and releasing lithium ions in the voltage range of 0.0 to 1.0 V, and the cycle life was 200 times or more. Moreover, the capacity during stable charge / discharge is 28 mAh, which is 560 mAh / cc when converted to the capacity per working electrode volume.
It was possible to obtain, and the capacity per volume was about twice that of the graphite electrode.

Comparative Example 1 For comparison, in the above test cell, EC of the non-aqueous electrolyte 3 was replaced with PC, and LiClO ₄ was added as a non-aqueous electrolyte in a mixed solvent of PC and DMC in a volume ratio of 1: 1. Mol / liter dissolved therein was used. This test cell is also the first embodiment.
Tested under the same charge and discharge conditions as in. The relationship between the charge / discharge capacity and the number of cycles is shown in FIG. As is clear from FIG. 3, EC
The capacity of the test cell in which PC was replaced with PC decreased sharply with the cycle, and it became impossible to charge and discharge in 10 cycles.

Example 2 FIG. 4 is a sectional view of a lithium secondary battery according to the present invention.
In FIG. 4, 9 is a negative electrode case. Reference numeral 10 is a negative electrode using WO ₂ as a negative electrode active material holder. This negative electrode was produced as follows. A mixture of WO ₂ which is a negative electrode active material holder and acetylene black which is a conductive agent was added to cyclohexane in which ethylene propylene terpolymer which was a binder was dissolved, and the mixture was sufficiently stirred, and the resulting slurry was made into stainless steel. The foil was applied and dried to prepare a negative electrode sheet. The weight ratio of the negative electrode active material holder, the conductive agent, and the binder was 93: 5: 2. Negative electrode 1
No. 0 was produced by spot-welding the negative electrode sheet punched out to a diameter of 16 mm to the negative electrode case 9. Reference numeral 3 is a non-aqueous electrolyte solution, which is prepared by dissolving 1 mol / liter of LiClO ₄ in a mixed solvent of EC and DMC in a volume ratio of 1: 1. 4 is a separator made of a polypropylene or polyethylene porous film. Reference numeral 11 is a positive electrode case. Reference numeral 12 is a positive electrode using a lithium manganese composite oxide (LiMn ₂ O ₄ ) as a positive electrode active material. This positive electrode was manufactured as follows. A mixture of LiMn ₂ O ₄ that is the positive electrode active material and acetylene black that is the conductive agent was added to cyclohexane in which the ethylene propylene terpolymer that was the binder was dissolved, and the mixture was thoroughly stirred and the resulting slurry was made into stainless steel. The foil was applied and dried to prepare a positive electrode sheet. The weight ratio of the positive electrode active material, the conductive agent, and the binder was 93: 5: 2. The positive electrode 12 was produced by spot-welding the positive electrode sheet punched into a diameter of 16 mm to the positive electrode case 11. Reference numeral 8 denotes a gasket, which maintains electrical insulation between the negative electrode case 9 and the positive electrode case 11, and the working electrode case opening edge is bent inward and caulked to seal the battery contents,
It is sealed.

This lithium secondary battery is operated at 3.0 to 4.2V.
The charge / discharge test was conducted in the voltage range of 1 mA. FIG. 5 shows the relationship between the charge / discharge capacity (mAh, vertical axis) and the number of cycles (horizontal axis), and Table 1 below shows the relationship between the charge / discharge capacity and the cycle life. The cycle life is the number of cycles when the capacity becomes half of the capacity during repeated stable charging and discharging. As is clear from FIG. 5, this lithium secondary battery was repeatedly charged and discharged very stably and had a very long cycle life of 900 times. After the charge / discharge test was completed, the battery was disassembled and the surface of the negative electrode was observed by SEM. No precipitation of lithium metal or growth of dendrite was observed on the surface of the negative electrode. Further, the negative electrode was analyzed by an X-ray diffractometer, but an X-ray diffraction pattern of lithium metal could not be recognized.

Example 3 In the lithium secondary battery of Example 2, the DMC of the non-aqueous electrolyte 3 was changed to DEE, and EC and DEE were used as the non-aqueous electrolyte.
LiClO ₄ in a mixed solvent having a volume ratio of 1: 1 of 1 mol / mol
What was melt | dissolved in 1 liter was used. The same thing as Example 2 was used except the nonaqueous electrolytic solution. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This lithium secondary battery is also repeatedly charged and discharged stably and has a cycle life of 1000.
It was once.

Example 4 In the lithium secondary battery of Example 2, the nonaqueous electrolytic solution 3 was replaced with DEC, and EC and DEC were used as the nonaqueous electrolytic solution.
LiClO ₄ in a mixed solvent having a volume ratio of 1: 1 of 1 mol / mol
What was melt | dissolved in 1 liter was used. The same thing as Example 2 was used except the nonaqueous electrolytic solution. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This lithium secondary battery was also repeatedly charged and discharged stably and had a cycle life of 800 times.

Example 5 In the lithium secondary battery of Example 2, the solute of the non-aqueous electrolyte solution 3, LiClO _4, was replaced with lithium hexafluorophosphate (LiP).
A non-aqueous electrolyte solution was used instead of F ₆ ). Solute concentration is 1
Mol / liter. The same thing as Example 2 was used except the nonaqueous electrolytic solution. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This lithium secondary battery was also repeatedly charged and discharged stably and had a cycle life of 1000 times.

Example 6 In the lithium secondary battery of Example 2, the positive electrode active material of the positive electrode 12 was changed from LiMn ₂ O ₄ to lithium nickel composite oxide (LiNiO ₂ ). The method for producing the positive electrode was the same as in Example 2, and the same method as in Example 2 was used except for the positive electrode. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below
Shows the relationship between charge and discharge capacity and cycle life. This lithium secondary battery had a large charge and discharge capacity, and was repeatedly charged and discharged stably, and had a cycle life of 750 times.

Example 7 In the lithium secondary battery of Example 6, DMC of the nonaqueous electrolytic solution 3 was replaced with MEC, and EC and MEC were used as the nonaqueous electrolytic solution.
LiClO ₄ in a mixed solvent having a volume ratio of 1: 1 of 1 mol / mol
What was melt | dissolved in 1 liter was used. The same thing as Example 6 was used except the nonaqueous electrolytic solution. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This lithium secondary battery was also repeatedly charged and discharged stably and had a cycle life of 880 times.

Example 8 In the lithium secondary battery of Example 2, the positive electrode active material of the positive electrode 12 was changed from LiMn ₂ O ₄ to lithium cobalt composite oxide (LiCoO ₂ ). The method for producing the positive electrode was the same as in Example 2, and the same method as in Example 2 was used except for the positive electrode. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below
Shows the relationship between charge and discharge capacity and cycle life. This lithium secondary battery had a large charge / discharge capacity, was repeatedly charged and discharged stably, and had a cycle life of 1050 times.

Example 9 In the lithium secondary battery of Example 8, the DMC of the non-aqueous electrolyte 3 was changed to DME, and EC and DME were used as the non-aqueous electrolyte.
LiClO ₄ in a mixed solvent having a volume ratio of 1: 1 of 1 mol / mol
What was melt | dissolved in 1 liter was used. The same thing as Example 8 was used except the nonaqueous electrolytic solution. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This lithium secondary battery was also repeatedly charged and discharged stably and had a cycle life of 930 times.

Comparative Example 2 For comparison, a battery was prepared in which the negative electrode 10 of the lithium secondary battery of Example 2 was replaced with metallic lithium. The amount of metallic lithium was 3 to 4 times equivalent to the battery capacity.
The same as Example 2 except for the negative electrode. This lithium secondary battery was also tested under the same charge / discharge conditions as in Example 2. Table 1 below shows the relationship between charge / discharge capacity and cycle life. This non-aqueous electrolyte was stably charged and discharged repeatedly, but its cycle life was 120 times.

[0024]

[Table 1]

[0025]

As described above, when the lithium secondary battery according to the present invention is used, at an electrode potential of 0.0 to 1.0 V with respect to a lithium reference electrode which has been said to be irreversible in WO ₂ until now, Large charge / discharge capacity and stable and long cycle life can be obtained. Therefore, WO ₂
Even when is used as the negative electrode active material holder, a high voltage and a high energy density can be achieved without significantly lowering the operating voltage of the battery. Moreover, because of its large specific gravity,
The charge / discharge capacity per volume of the negative electrode can be much larger than that of the graphite-based negative electrode. Furthermore, no rapid decrease in capacity due to repeated charging / discharging is recognized, and a very long cycle life can be obtained. Moreover, since no deposition of lithium metal or generation of dendrites is observed on the surface of the negative electrode that has been repeatedly charged and discharged, there is no safety problem. Therefore, the present invention has an excellent effect that a lithium secondary battery having a high voltage, a high energy density, a large charge / discharge capacity, secured safety, and a long cycle life can be obtained.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a test cell of the present invention.

FIG. 2 is a diagram showing a charge / discharge curve at the 10th cycle of the test cell according to the present invention.

FIG. 3 is a diagram showing the relationship between the charge / discharge capacity and the number of cycles of a test cell of the present invention and a comparison.

FIG. 4 is a cross-sectional view of the battery of the present invention.

FIG. 5 is a diagram showing a relationship between charge / discharge capacity and cycle number of a battery according to the present invention.

[Explanation of symbols]

1: counter electrode case, 2: counter electrode, 3: non-aqueous electrolyte solution, 4: separator, 5: working electrode case, 6: working electrode, 7: current collector,
8: gasket, 9: negative electrode case, 10: negative electrode, 1
1: Positive case, 12: Positive electrode

Claims (4)

[Claims] 1. A negative electrode mainly composed of a negative electrode active material holder that occludes lithium ions by charging and releases lithium ions by discharging, a positive electrode capable of reversible electrochemical reaction with lithium ions, and lithium ion conductivity. In a lithium secondary battery comprising a water-soluble nonaqueous electrolytic solution or an electrolytic solution-impregnated polymer electrolyte, as the negative electrode active material holder,
A lithium secondary battery, wherein tungsten dioxide is used and a mixed solvent containing ethylene carbonate as an essential component is used as a solvent for the nonaqueous electrolytic solution or the impregnating electrolytic solution. 2. The lithium secondary battery according to claim 1, wherein a mixed solvent of ethylene carbonate and chain ester or chain ether is used as a solvent of the non-aqueous electrolyte. 3. The lithium secondary battery according to claim 2, wherein dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate is used as a solvent for the chain ester. 4. A solvent for the chain ethers is 1, 2
The lithium secondary battery according to claim 2, wherein -dimethoxyethane or 1,2-diethoxyethane is used.