JPH08148183A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PURPOSE: To effectively restrain abnormal current at the time of the occurrence of a shortcircuit or the like in a nonaqueous electrolyte lithium battery, and eliminate a current breaking element. CONSTITUTION: Lithium or a lithium compound is used as an active material for a negative electrode, and a metallic oxide or a conductive polymer to become an oxidized state due to p-type doping is used as an active material for a positive electrode. Regarding this nonaqueous electrolyte secondary battery, the surface of at least one of the positive and negative electrodes is covered with an ion conductive polymeric film.

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 using lithium or a lithium compound as an active material for a negative electrode. More specifically, the present invention relates to a non-aqueous electrolyte secondary battery in which an abnormal current at the time of short circuit is suppressed by forming a polymer film on the electrode surface.

[0002]

2. Description of the Related Art In recent years, with the rapid progress of various electronic devices, research on secondary batteries has been advanced as a portable power source that can be used stably and economically for a long time. Typical rechargeable batteries include lead storage batteries, alkaline storage batteries, lithium rechargeable batteries, etc. Among them, lithium rechargeable batteries have higher output and higher energy density than other conventional rechargeable batteries. Is being actively developed because it can achieve

For example, the negative electrode of a lithium secondary battery is generally composed of a material capable of reversibly doping and dedoping lithium (including lithium ions), metallic lithium or a lithium alloy, and as such a material. It has been proposed to use a conductive polymer or layered compound (carbon material, metal oxide, etc.) doped with lithium.

On the other hand, a metal oxide, a metal sulfide, or a specific polymer is used as the positive electrode active material forming the positive electrode, and more specifically, TiS ₂ , MoS ₂ , and NbS are used.
compounds containing no lithium such as e ₂ and V ₂ O ₅ , and L
It has been proposed to use a composite oxide containing lithium such as iMO ₂ (M = Co, Ni, Mn, Fe, etc.). It has also been proposed to use a mixture of a plurality of these compounds.

As a separator for the negative electrode and the positive electrode, a microporous polymer film such as polyethylene or polypropylene is used. In this case, from the viewpoint of lithium ion conductivity and energy density, the polymer film needs to be as thin as possible, and is practically set to 50 μm or less.

As the electrolytic solution, a solution prepared by dissolving a lithium salt in a non-aqueous solvent such as ethylene carbonate or propylene carbonate is used.

Further, in such a lithium secondary battery,
When the positive electrode and the negative electrode are short-circuited due to a failure of the battery charger or peripheral circuit or an error in the user's usage method, the battery itself has the characteristics of high output and high energy density, resulting in a large current. Flowing, rapid heat generation, internal pressure increase, and eventually battery damage may occur. Therefore, as a safety mechanism in such a case, a current interrupting element (for example, a PTC circuit, a fuse, etc.) that interrupts an instantaneous output current is provided in the battery body or peripheral equipment.

[0008]

However, in the conventional safety mechanism, once the current cut-off element such as the fuse is activated, the recovery procedure such as the replacement of the fuse is required. Therefore, the lithium battery is immediately reused. There is a problem that you cannot do it. Therefore, there has been a demand for the development of a new safety mechanism that replaces the conventional current interruption element.

The present invention is intended to solve such a problem of the prior art, and suppresses an abnormal current from flowing into the battery even when the positive electrode and the negative electrode are short-circuited to generate a sudden large current itself. The purpose is to prevent.

[0010]

Means for Solving the Problems As a result of intensive studies for achieving the above-mentioned object, the present inventor has found that forming a polymer film on the surface of the electrode that facilitates the conduction of lithium ions deteriorates the battery performance. In other words, the inventors have found that it is possible to effectively suppress an abnormal current when a battery is short-circuited, and have completed the present invention.

That is, according to the present invention, lithium or a lithium compound is used as an active material in the negative electrode, and a metal oxide, a metal sulfide, or a conductive polymer which is oxidized by p-type doping is used as the active material in the positive electrode. Provided is a nonaqueous electrolyte secondary battery in which an ion conductive polymer film is formed on the surface of at least one of the positive electrode and the negative electrode in the water electrolyte secondary battery.

The present invention will be described in detail below.

The battery of the present invention can be constructed in the same manner as a conventional non-aqueous electrolyte lithium secondary battery except that an ion conductive polymer film is provided on the electrode surface. That is, in the battery of the present invention, the positive electrode comprises a metal oxide, a metal sulfide or a specific polymer as an active material. More specifically, such a positive electrode active material includes TiS ₂ and Mo.
Lithium-free metal sulfides or oxides such as S ₂ , NbSe ₂ , and V ₂ O ₅ , Li _x MO ₂ (wherein M
Represents one or more kinds of transition metals, and usually 0.05 ≦ x ≦ 1.
It is possible to use a lithium composite oxide mainly composed of 10). As the transition metal M constituting the lithium composite oxide, Co, Ni, Mn and the like are preferable.
Specific examples of such a lithium composite oxide include Li
_{CoO 2, LiNiO 2, Li x} Ni y Co 1-y O 2
(In the formula, x and y are different depending on the charge / discharge state of the battery, and are generally 0 <x <1 and 0.7 <y <1.02), LiM
Examples thereof include n ₂ O ₄ . These lithium composite oxides can generate a high voltage and become a positive electrode active material excellent in energy density.

As the specific polymer used as the positive electrode active material, a conductive polymer which is oxidized by p-type doping can be used. Specifically, for example, polyacetylene, polyaniline, polypyrrole and its derivatives, poly 2,5-thienylene and its derivative, polyparaphenylene and its derivative, polyacene and the like can be used.

A plurality of kinds of these compounds may be mixed and used for the positive electrode. In the present invention, when forming a positive electrode using the above positive electrode active material,
Known conductive agents, binders and the like can be added.

On the other hand, in the present invention, the negative electrode uses lithium or a lithium compound as an active material. Therefore, the negative electrode is made of a material that can be doped or dedoped with lithium, metallic lithium or a lithium alloy. Examples of the material constituting the negative electrode or the material capable of being doped with lithium out of the negative electrode active material include pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glass. Carbons, fired organic polymer compounds (carbonized by firing a phenol resin, furan resin, etc. at an appropriate temperature), carbon fibers, carbonaceous materials such as activated carbon, or polyacetylene,
Polymers such as polypyrrole can be used. Moreover, as the lithium alloy, a lithium-aluminum alloy, a lithium-indium alloy, or the like can be used.

When forming the negative electrode from such a material, a known binder or the like can be added.

The present invention is characterized in that at least one of the surfaces of the positive electrode and the negative electrode is coated with an ion conductive polymer film. As a result, without increasing the internal resistance when using a normal battery,
The internal resistance at the time of abnormality such as a short circuit can be greatly increased, and the abnormal current can be suppressed. On the other hand, in a conventional battery without such a coating, the internal resistance is reduced and a large abnormal current flows at the time of an abnormality such as a short circuit.

In the present invention, the polymer coating for coating the surface of the electrode is one having ion conductivity per se, and one having no ion conductivity per se, but the non-aqueous electrolyte is a polymer coating. Can be used as well as those that gain ionic conductivity by penetrating into the.

As the former polymer film, a film having a high donor property capable of coordinating lithium ions with polymer chains can be used. Further, those having good wettability with respect to the non-aqueous electrolyte are preferable. As such a film, for example, a film of polyether, polyphosphazene, polyester, polyamine, polysulfide, posisiloxane, or the like can be used. In this case, the thickness of the film is preferably 5 μm to 500 μm from the viewpoint of the charge / discharge efficiency of the secondary battery and the effect of suppressing the short circuit current.

As the latter polymer coating, one having a large affinity for the non-aqueous electrolyte and having particularly good wettability is preferable. For example, polystyrene, polybutadiene and copolymers thereof, polyvinylidene fluoride, polyacrylonitrile, polyvinylpyrrolidone, polyvinylidene carbonate and the like can be used. In this case, the thickness of the film is preferably 5 μm to 500 μm from the viewpoint of the charge / discharge efficiency of the secondary battery and the effect of suppressing the short circuit current.

Further, in the present invention, it is preferable that the polymer coatings for the electrodes all have a crosslinked structure.
As a result, even when the temperature rises or the pressure changes in the battery, the polymer coating can be prevented from falling off the electrode surface.

The method for forming the polymer coating is not particularly limited, but for example, the coating can be formed by applying a coating for forming a polymer coating on the electrode surface and crosslinking the coating. As a result, the coating material enters into the fine irregularities on the electrode surface, and the irregularities on the electrode surface serve as an anchor effect for fixing the polymer coating to the electrode surface, so that a coating having good adhesion can be formed.

In the present invention, the non-aqueous electrolyte may be the same as the conventional non-aqueous lithium secondary battery. That is, as the non-aqueous solvent of the non-aqueous electrolytic solution, for example, propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane,
1,2-diethoxyethane, 2-methyltetrahydrofuran, 3-methyl-1,3-dioxolane, methyl propionate, methyl butyrate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate and the like can be used. In particular, cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, vinylene carbonate, etc. from the viewpoint of stable voltage,
Alternatively, it is preferable to use chain carbonates such as dimethyl carbonate, diethyl carbonate and dipropyl carbonate. Further, such non-aqueous solvent may be used alone or in combination of two or more kinds.

As the electrolyte to be dissolved in the non-aqueous solvent, for example, LiClO ₄ , LiAsF ₆ and LiP can be used.
F ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃
SO ₂ ) ₂ or the like can be used, among which LiPF ₆ or L
It is preferred to use iBF ₄ .

The battery shape of the battery of the present invention is not particularly limited. Various shapes such as a cylindrical shape, a square shape, a coin shape, and a button shape can be used.

[0027]

In the non-aqueous electrolyte secondary battery of the present invention, the ion-conductive polymer film is formed on the surface of the electrode, so that rapid movement of ions in the battery is suppressed. Therefore, in the event of an abnormality such as a short circuit, the internal resistance is greatly increased and the abnormal current is suppressed. On the other hand, the polymer coating used in the present invention does not suppress the movement of ions during normal battery use. Therefore, in the battery of the present invention, the internal resistance does not increase during normal use, and the battery characteristics such as cycle characteristics are not impaired.

[0028]

EXAMPLES The present invention will be specifically described below with reference to examples.

Example 1 In preparing a positive electrode active material, first, Ni (OH) ₂ and LiOH.H ₂ O were mixed in equimolar amounts, and the mixture was mixed in an oxygen atmosphere at 60 ° C.
By sintering at 0-800 ° C for about 10 hours, LiNi
To obtain a powder of O _2. 85 parts by weight of this LiNiO ₂ powder,
9 parts by weight of graphite and 6 of fluoropolymer binder
Part by weight was mixed with dimethylformamide (DMF) as a solvent, and the mixture was thoroughly dried to completely volatilize DMF to obtain a positive electrode mixture powder.

60 mg of the positive electrode mixture powder was weighed and pressed into a disk-shaped electrode having a surface area of about 2 cm ² to obtain a positive electrode pellet.

Further, in order to coat the positive electrode pellet with a polymer film, a polyethylene oxide triol oligomer (number average molecular weight: 5000) prepared by adding ethylene oxide to glycerin was prepared, dried, and purged with argon. In a box, 3 g of dried oligomer and 0.16 g of toluene 2,4-diisocyanate (TDI) were mixed in dry propylene carbonate. Next, this was cast onto a positive electrode pellet so that the film thickness was 20 μm, and a cross-linking reaction was performed at 80 ° C. for 48 hours to coat the electrode surface.

On the other hand, as the negative electrode, a metal lithium plate punched into a disk shape similar to the positive electrode was prepared. The amount of lithium in the negative electrode is in excess of the maximum charge capacity of the positive electrode, so it does not limit the electrochemical performance of the positive electrode.

A microporous polypropylene film was prepared as a separator, and an electrolyte prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) in propylene carbonate at a ratio of 1 mol / l was prepared.

The above-mentioned positive electrode, negative electrode, separator,
A test battery was prepared by storing the electrolytic solution in a coin cell (dimensions: diameter about 20 mm, height about 2.5 mm).

In order to evaluate the short-circuit current behavior of the batteries obtained, the batteries were charged at a current density of 100 μA / cm ² until a closed circuit voltage of 4.2 V was reached and the current curves when short-circuited for 1 msec were recorded. The result is shown in FIG.

Further, in order to evaluate the cycle characteristics of the obtained battery, the battery was subjected to constant current charging at a current density of 100 μA / cm ^{2 up} to a closed circuit voltage of 4.2 V, and after charging, a constant current up to a closed circuit voltage of 2.8 V. The charging / discharging cycle of discharging was repeated, and the charging / discharging efficiency (%) at that time was obtained. In this case, the charging / discharging efficiency was determined by the ratio of each charging / discharging capacity. Table 1 shows the results. It can be seen from Table 1 that the batteries of Examples in which the electrode surface is coated with a polymer film have excellent cycle characteristics.

[0037]

[Table 1] Comparative Example 1 A test battery was prepared in the same manner as in Example 1 except that the electrode surface was not subjected to any coating treatment, and its short-circuit current behavior was recorded in the same manner as in Example 1. The results are also shown in FIG.

From FIG. 1, in the battery of Comparative Example 1 having no surface coating, the instantaneous current of 400 mA flowed at the time of short circuit, while in the battery of Example 1 having the polymer coating on the positive electrode surface. Shows that the instantaneous current is greatly reduced to 300 mA.

Example 2 Co ₃ O ₄ and Li ₂ CO ₃ were mixed in an equimolar ratio, and the mixture was baked at 800 ° C. for 10 hours in an air atmosphere to obtain Li.
A powder of ₂ CoO ₂ was obtained. This powder was prepared as a positive electrode mixture powder in the same manner as in Example 1, pressure-molded into a positive electrode pellet, and further subjected to a polymer film treatment.

On the other hand, a non-graphitizable carbon pellet was formed as the negative electrode, and a coin-type secondary battery was prepared using this negative electrode, the above-mentioned positive electrode, the same electrolytic solution as in Example 1, and the separator.
The short circuit current behavior was recorded as in Example 1. The result is shown in FIG.

Comparative Example 2 A battery was prepared in the same manner as in Example 2 except that the electrode surface was not subjected to any coating treatment, and its short circuit current behavior was recorded in the same manner as in Example 2. The results are also shown in FIG.

From FIG. 2, it can be seen that in the battery of Comparative Example 2 having no surface coating, the instantaneous current of 400 mA flows at the time of short circuit, while in the battery of Example 2 having the polymer coating on the positive electrode surface. Shows that the instantaneous current is greatly reduced to 260 mA.

Example 3 In the same manner as in Example 2, the positive electrode pellet was made of lithium cobalt oxide and the negative electrode pellet was made of non-graphitizable carbon.

In order to coat this negative electrode pellet with a polymer film, polypropylenetriol oligomer (number average molecular weight: 5000) obtained by adding propylene oxide to glycerin and hexamethylene diisocyanate (H
MDI) and the molar ratio of both is H to the oligomer.
Mixed so that MDI was 3/2 times. Then, this was cast onto the negative electrode pellet so that the film thickness was 20 μm,
The surface of the electrode was coated by a crosslinking reaction at 80 ° C. for 48 hours.

A coin-type secondary battery was prepared using the negative electrode having the surface coated thereon, the positive electrode described above, and the same electrolytic solution and separator as in Example 1, and the short-circuit current behavior thereof was recorded in the same manner as in Example 1. did. The result is shown in FIG. The results of the battery of Comparative Example 2 are also shown in FIG. 3 for reference.

From FIG. 3, it can be seen from the battery of Comparative Example 2 that the surface was not treated with a film, the instantaneous current of 400 mA flowed at the time of short circuit, while the battery of Example 3 with the polymer film was applied to the surface of the negative electrode. Shows that the instantaneous current is greatly reduced to 180 mA.

Example 4 In the same manner as in Example 2, the positive electrode pellet was formed from lithium cobalt oxide and the negative electrode pellet was formed from non-graphitizable carbon.

Further, both the positive electrode pellets and the negative electrode pellets were coated with a polymer coating having a film thickness of 20 μm in the same manner as in Example 3, and the obtained positive electrode and negative electrode and the same electrolytic solution and separator as in Example 1 were obtained. A coin-type secondary battery was prepared using and the behavior of the short-circuit current was recorded in the same manner as in Example 1. The result is shown in FIG. For reference, the results of the battery of Comparative Example 2 are also shown in FIG.

From FIG. 4, it can be seen that in the battery of Comparative Example 2 in which the surface is not coated, the instantaneous current of 400 mA flows at the time of short circuit, whereas in the battery of Example 4 in which the polymer surface is coated on the electrode surface. Shows that the instantaneous current is greatly reduced to 160 mA.

Example 5 In the same manner as in Example 2, the positive electrode pellet was formed from lithium cobalt oxide and the negative electrode pellet was formed from graphitizable carbon.

Further, both the positive electrode pellets and the negative electrode pellets were each coated with a polymer coating having a film thickness of 20 μm in the same manner as in Example 1, and the obtained positive electrode and negative electrode, the same electrolytic solution as in Example 1 and the separator were obtained. A coin-type secondary battery was prepared by using, and the short circuit current behavior thereof was recorded in the same manner as in Example 1. The result is shown in FIG.

Comparative Example 3 A battery was prepared in the same manner as in Example 5 except that the electrode surface was not subjected to any coating treatment, and its short circuit current behavior was recorded in the same manner as in Example 5. The results are also shown in FIG.

From FIG. 5, it can be seen that in the battery of Comparative Example 3 having no surface coating, an instantaneous current of 400 mA flows at the time of short circuit, whereas in the battery of Example 5 having polymer coating on the electrode surface. Shows that the instantaneous current is greatly reduced to 200 mA.

Example 6 In the same manner as in Example 2, the positive electrode pellet was formed from lithium cobalt oxide, and in the same manner as in Example 1, the negative electrode pellet was formed from metallic lithium.

Further, both the positive electrode pellets and the negative electrode pellets were each coated with a polymer film having a film thickness of 20 μm in the same manner as in Example 3, and the obtained positive electrode and negative electrode, the same electrolytic solution as in Example 1 and the separator were obtained. A coin-type secondary battery was prepared by using, and the short circuit current behavior thereof was recorded in the same manner as in Example 1. The result is shown in FIG.

Comparative Example 4 A battery was prepared in the same manner as in Example 6 except that the electrode surface was not subjected to any coating treatment, and the short circuit current behavior was recorded in the same manner as in Example 6. The results are also shown in FIG.

As shown in FIG. 6, in the battery of Comparative Example 4 having no surface coating, the instantaneous current of 400 mA flowed at the time of short circuit, while in the battery of Example 6 having polymer coating on the electrode surface. Shows that the instantaneous current is greatly reduced to 160 mA.

Example 7 A positive electrode pellet was formed of lithium cobalt oxide in the same manner as in Example 2, and a negative electrode was formed of metallic lithium in the same manner as in Example 1.

In order to coat both the positive electrode pellets and the negative electrode pellets with a polymer film, first, polyacrylonitrile (25 parts by weight) previously dried under reduced pressure at 60 ° C. in propylene carbonate (50 parts by weight) at 120 ° C. Was melted by heating, and the positive electrode pellets and the negative electrode pellets were coated with each other so as to have a film thickness of 20 μm to obtain a positive electrode and a negative electrode.

A coin-type secondary battery was manufactured using the obtained positive electrode and negative electrode, the same electrolytic solution as in Example 1 and the separator, and the short circuit current behavior thereof was recorded as in Example 1. The result is shown in FIG. 7. For reference, the results of the battery of Comparative Example 4 are also shown in FIG.

From FIG. 7, in the battery of Comparative Example 4 in which the electrode surface was not coated, an instantaneous current of 400 mA flowed during a short circuit, while in Example 7 in which the electrode surface was coated with a polymer film. It can be seen that the instantaneous current of the battery is effectively reduced to 300 mA.

Modifications of Examples 1 to 7 In order to study the optimum value of the film thickness of the polymer film for coating the electrode surface, the film thickness of the polymer film in each of Examples 1 to 7 described above is shown in Table 2. , And changed batteries were prepared as shown in Table 3.

Then, the cycle characteristics of the battery were examined in the same manner as in Example 1, and the charge / discharge efficiency at the fifth cycle was determined.
The results are shown in Table 2. From Table 2, it is understood that in these battery systems, the film thickness on the electrode surface has an optimum value of 500 μm or less in terms of cycle characteristics.

The short-circuit current behavior of each battery was recorded in the same manner as in Example 1 to find the instantaneous current, and the obtained instantaneous current and
A ratio (attenuation rate of the instantaneous current) of the battery of the comparative example corresponding to the battery of the example (batteries of the respective examples in which the polymer film was not formed) to the instantaneous current was obtained. The results are shown in Table 3. From Table 3, it can be seen that in these battery systems, the film thickness on the electrode surface is preferably 5 μm or more from the viewpoint of suppressing the instantaneous current.

[0065]

[Table 2] Charge / Discharge Efficiency (%) Film Thickness Example (μm) 1 2 3 4 5 6 7 5 98 98 98 97 96 97 97 97 98 98 20 98 98 96 96 97 96 96 96 97 40 40 97 98 96 96 96 96 95 95 97 100 95 95 93 93 93 92 92 95 250 250 88 88 88 83 83 80 80 83 500 500 83 80 71 72 72 72 73 75

[0066]

[Table 3] Attenuation rate of instantaneous current (%) Film thickness Example (μm) 1 2 3 4 5 6 7 3 95 95 98 97 96 96 97 97 98 98 5 82 78 75 75 51 74 48 82 82 20 80 63 63 50 38 50 38 80

[0067]

According to the present invention, an abnormal current such as at the time of short circuit is effectively suppressed, so that it becomes unnecessary to provide a current cutoff element inside the battery or in the peripheral equipment.

[Brief description of drawings]

FIG. 1 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

FIG. 2 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

FIG. 3 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

FIG. 4 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

FIG. 5 is a short-circuit current behavior diagram of the batteries of Examples and Comparative Examples.

FIG. 6 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

FIG. 7 is a short-circuit current behavior diagram of batteries of Examples and Comparative Examples.

Claims (5)

[Claims] 1. A non-aqueous electrolyte solution which uses lithium or a lithium compound as an active material for a negative electrode and uses a metal oxide, a metal sulfide or a conductive polymer which is oxidized by p-type doping as an active material for a positive electrode. In the secondary battery, a non-aqueous electrolyte secondary battery in which an ion conductive polymer film is formed on at least one surface of the positive electrode and the negative electrode. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polymer film is a film having ion conductivity by itself. 3. The thickness of the polymer coating is 5 μm to 500 μm.
The non-aqueous electrolyte secondary battery according to claim 2, which is m. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polymer coating is a film that acquires ion conductivity by permeation of the non-aqueous electrolyte in the battery. 5. The thickness of the polymer coating is 5 μm to 500 μm.
The non-aqueous electrolyte secondary battery according to claim 4, which is m.