JP2002042872A - Lithium polymer secondary battery - Google Patents

Abstract

(57) [Summary] [PROBLEMS] A lithium polymer secondary battery ignites due to lack of thermal stability when it is overcharged due to a failure of a charger or a safety circuit, or when it reaches the end of a cycle. It has the problem that it is relatively easy to ignite when exposed to high temperatures. SOLUTION: Even if metal lithium is deposited at the end of the cycle by disposing a gel electrolyte membrane made of an acrylonitrile copolymer opposite to a negative electrode, the thermal safety of the battery is ensured and the positive electrode is formed. By arranging a microporous polyethylene membrane having an endothermic effect facing the battery, even if the charger breaks down and charging is no longer regulated, battery safety can be ensured in terms of thermal stability. This two-layer separating member has excellent thermal stability even at the end of overcharging and the end of the cycle, and can ensure the safety of the battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium polymer secondary battery in which a gel polymer electrolyte is disposed between a positive electrode and a negative electrode as an isolating member. More specifically, the present invention relates to an improvement in the configuration and arrangement of the isolation member.

[0002]

2. Description of the Related Art A lithium-containing metal oxide capable of reversibly storing and releasing lithium ions is used as a positive electrode material, and a non-aqueous material comprising a negative electrode material capable of reversibly storing lithium ions released from the positive electrode material during charging. As measures to make electrolyte secondary batteries, so-called lithium secondary batteries thinner and improve safety,
It has already been proposed to use a polymer material capable of absorbing and holding and fixing a non-aqueous electrolyte solution, for example, a gel polymer electrolyte in which an electrolyte solution is absorbed and held in polyvinylidene fluoride (PVDF) instead of a general separator ( For example, U.S. Pat. No. 5,296,318 or Japanese Unexamined Patent Publication No.
507407). A battery having this configuration is typically a battery in which a power generation element is sealed using a simple exterior body, that is, a laminated laminate film of aluminum foil and a resin film. Currently, active efforts are being made to commercialize the battery. Have been done.

For example, the battery disclosed in the above-mentioned US patent and developed or mass-produced by many companies has a positive electrode of LiC
Using oO ₂ or LiMn ₂ O ₄ , using a carbon material such as graphite for the negative electrode, and using a material obtained by absorbing and gelling a non-aqueous electrolyte solution to a vinylidene fluoride-based polymer for the electrolyte also serving as a separator I have. The gel-like polymer electrolyte holds the electrolyte in a three-dimensional network structure of the polymer formed by gelation, and has the characteristics of ensuring ionic conductivity and eliminating leakage by fixing the electrolyte. are doing.

Conventionally, as a method for producing such a lithium polymer secondary battery, an electrolyte solution (pregel electrolyte solution) containing an electrolyte solution, a polymerizable compound (monomer) and a polymerization initiator is used as a positive electrode and / or a negative electrode. After coating or impregnating, a method of forming a gel-like polymer electrolyte membrane on at least the electrode surface by heating or irradiation of ultraviolet rays, electron beams, etc., or a gel prepared by applying the gel-like polymer electrolyte membrane or porous body prepared in advance Polymer electrolyte membrane,
A method of sandwiching a battery between a positive electrode and a negative electrode to constitute a battery has been proposed.

[0005]

In the battery comprising only the gelled polymer electrolyte described above, since the electrolyte is fixed by the polymer material, the vapor pressure of the flammable organic solvent is reduced and the flammability is reduced. Since the reactivity between the improved positive and negative electrode materials and the organic solvent is suppressed, some improvement in heat resistance in terms of heat generation rate and amount is recognized.

[0006] However, the first in terms of the electronic circuit outside the battery.
There remains a problem in safety when a safety circuit in the charger as the second-stage safety device or a charger in the second-stage safety device breaks down. For example, a battery system consisting only of a gel polymer electrolyte does not have a current interrupting function, and if the temperature of the battery rises due to overcharging, the hardness of the gel decreases, ions are mobilized, and current easily flows,
Ensuring safety is even more difficult. On the other hand, in the case of a battery using a normal electrolytic solution, the above-mentioned problem is solved by disposing a single-layer or multilayer polyolefin separator having a blocking effect.

[0007] Another problem of the lithium ion battery is to ensure safety in the event of overcharging and to reduce the safety at the end of the cycle, particularly the heat resistance of the battery at the end of the cycle. When a battery at the end of the cycle is subjected to a heat resistance test at 150 ° C. or the like, it is relatively easy to cause battery ignition.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to secure safety of a lithium polymer secondary battery.

[0009]

Means for Solving the Problems In order to achieve the above object, a lithium polymer secondary battery of the present invention secures safety when charging is no longer restricted due to failure of a charger and a safety circuit. As a goal, the factors that induce the thermal runaway reaction were investigated and analyzed. As a result, it was clarified that the reaction between the charged positive electrode and the electrolytic solution was the cause of the first large heat generation around 110 ° C. Following this heat generation, it was found that the charged negative electrode and the electrolytic solution reacted, leading to an uncontrollable thermal runaway reaction.

At this time, it has also been found that if the heat generated by the first reaction between the positive electrode and the electrolytic solution is dissipated or absorbed in some way, the subsequent reaction between the negative electrode and the electrolytic solution can be prevented, and fatal thermal runaway can be prevented. . Therefore, as a result of repeated studies to suppress heat generation in the positive electrode, it has been found that it is effective to arrange a polyethylene film or fine particles that start melting at around 100 ° C. in or near the positive electrode. In consideration of battery production, a microporous polyethylene membrane is particularly simple and effective.

Regarding the second problem, ensuring the safety of the battery at the end of the cycle, the battery is disassembled and observed as the charge / discharge cycle progresses, and the positive and negative electrodes of the disassembled battery are subjected to thermal analysis to determine the situation. Was analyzed. As a result, the deterioration of the negative electrode was progressing at the end of the cycle, and the negative electrode could not occlude lithium ions even during charging. In other words, lithium ions that have not been absorbed precipitate in the form of dendrites (lithium dendrite) on the surface of the negative electrode particles, and the deposited lithium easily reacts with the electrolytic solution because of its large surface area. It was found that when exposed to a high temperature, the reaction with the electrolyte proceeded very rapidly, and the heat of reaction was extremely large, causing thermal runaway and leading to ignition.

To prevent ignition of the battery, it is most effective to remove metallic lithium deposited on the surface of the negative electrode. One of the solutions is to chemically consume lithium deposited on the negative electrode surface. As a specific method, a material that reacts with lithium is added to the electrolytic solution, or a material that also reacts with lithium is disposed near the surface of the negative electrode. As a result of various studies, the simplest and most reliable method newly found this time is to cover the negative electrode surface with a polyacrylonitrile film. It was found that polyacrylonitrile, when gelled with an electrolyte, not only exhibits ionic conductivity but also easily reacts with lithium metal.

However, batteries are often exposed to a high temperature of about 90 ° C. in the summer, for example, in a car.
Up to this point, it is necessary to operate as a battery without any trouble. For this purpose, the electrolyte needs to maintain a gel state even at the above high temperature. In order to maintain the shape as an electrolyte at a high temperature, it was achieved by adding a filler such as silicon dioxide powder into the electrolyte. The addition of a filler to the electrolyte membrane is disclosed in U.S. Pat. No. 5,540,741 or Japanese Patent Publication No. H10-511216.

In the present invention, a microporous polyethylene film is disposed opposite to the positive electrode, and a polyacrylonitrile film which gels on the surface of the negative electrode is disposed as an isolated portion of a new lithium polymer battery. It is configured.

As a result, the safety of the battery can be ensured even if the charger or the safety circuit breaks down and charging is no longer regulated, and the problem of lithium dendrite, which has a problem in heat resistance even at the end of the cycle, can be solved. As a result, a lithium polymer secondary battery having excellent performance can be provided.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a structure of a lithium polymer secondary battery sealed with an outer package made of a laminate material of the present invention. In FIG. 1, 1 is a positive electrode, 2 is a negative electrode, 3 is a microporous polyethylene membrane facing the positive electrode, 4 is a gel polymer electrolyte membrane facing the negative electrode, 5 is a positive electrode lead, 6 is a negative electrode lead, and 7 is a laminate material. 8 and 9 are upper and lower seal portions, respectively, and 10 is a lead insulating protective film. The microporous membrane 3, the gel polymer electrolyte membrane 4, the negative electrode 2 and the positive electrode 1 are ultimately housed in the outer package 7 in a state of being entirely wound.

A method for manufacturing the power generating element of this battery will be described. The positive electrode 1 is made of a positive active material LiCoO2 and acetylene black conductive material and polytetrafluoroethylene (PT
FE) After mixing and dispersing a binder of a dispersion system, the paste was uniformly applied to both surfaces of an Al foil current collector with a die coater, dried and rolled, and then cut into a predetermined size. . A positive electrode lead 5 was welded to the positive electrode 1 at the end of the current collector.

The negative electrode 2 is prepared by mixing a conductive material and an aqueous binder into a main graphite powder to form a paste, applying the paste uniformly to both surfaces of a Cu foil current collector with a die coater, and drying and rolling. It was obtained by cutting to a predetermined size. A negative electrode lead 6 was welded to the negative electrode 2 at the end of the current collector.

Reference numeral 3 denotes a generally commercially available polyethylene microporous membrane serving as a separator. The gel polymer electrolyte membrane 4 is obtained by adding a copolymer (copolymer) of acrylonitrile and methacrylic acid and fine particles of silicon dioxide to an N-methylpyrrolidone (NMP) solvent and stirring to dissolve and disperse the paste into a film. , Dried. The above-mentioned copolymer (copolymer) of acrylonitrile and methacrylic acid was previously neutralized with a fixed ratio of lithium hydroxide in an NMP solution, and the methacrylic acid group was subjected to stabilization treatment with lithium ions. Depending on the form and type of the separator, a gel polymer can be applied and developed on this surface, and the gel polymer electrolyte membrane and the microporous polyethylene membrane can be used without being laminated. As the electrolytic solution, a solution prepared by adjusting lithium hexafluorophosphate to a concentration of 1 M / l in a mixed solvent of equal volumes of ethylene carbonate and propylene carbonate was used. A microporous membrane 3 and a gel-like polymer electrolyte membrane 4 were placed together between the positive electrode 1 and the negative electrode 2 and wound into an ellipse as shown in FIG.

The exterior body 7 for housing the above-mentioned power generating element is
1 foil was used as an intermediate one layer, and a polypropylene film was disposed on the inner side and a polyethylene terephthalate film was disposed on the outer side. After storing the wound power generating element, the upper seal portion 8 of the exterior body 7 was sealed by thermal bonding with the tips of the positive electrode lead 5 and the negative electrode lead 6 protruding outside. An insulating protective film 10 is attached to portions of the leads 5 and 6 that are in contact with the upper seal portion 8. This insulating protective film 10
Is a member provided to ensure the airtightness of the positive electrode lead 5 and the negative electrode lead 6.

After injecting a predetermined amount of electrolytic solution from the lower seal 9 which is still open, with the upper seal 8 already closing the outer package 7 of the battery containing the power generating element as described above. Then, the lower seal portion 9 is sealed by heat welding to complete the battery.

[0022]

Next, the present invention will be described in detail with reference to examples.

(Embodiment 1) Preparation of Positive Electrode The positive electrode 1 was prepared by mixing 4 parts of an acetylene black conductive material and 5 parts of a water-based polytetrafluoroethylene (PTFE) dispersion binder with 100 parts by weight of a positive electrode active material LiCoO 2, and forming a paste. The product was uniformly coated on both surfaces of the body (Al foil) with a die coater so as to have a predetermined weight and thickness per unit area, dried and rolled, and then cut into a predetermined size. A positive electrode lead 5 was welded to the positive electrode 1 at the end of the current collector.

2. Preparation of Negative Electrode The negative electrode 2 was prepared by mixing 5 parts of a fibrous graphite conductive material and 5 parts of a styrene-butadiene rubber-based (SBR) aqueous binder with 100 parts by weight of spherical graphite powder, and forming a paste.
The foil was uniformly applied to both surfaces of the foil so as to have a predetermined weight and thickness per unit area by a die coater, dried and rolled, and then cut into a predetermined size. A negative electrode lead 6 was welded to the negative electrode 2 at the end of the current collector.

3. Separator The separator has a porosity of about 40% which is usually commercially available,
A polyethylene microporous membrane having a thickness of 16 μm was used.

4. Gel-like Polymer Electrolyte Membrane The gel-like polymer electrolyte membrane 4 is prepared by dissolving 100 parts by weight of a copolymer composed of 95 mol% of acrylonitrile and 5 mol% of methacrylic acid in an NMP solvent, and then dissolving lithium hydroxide at a quantitative ratio to give methacrylic acid. The acid was neutralized, and 100 parts by weight of a fine powder of silicon dioxide having a primary particle diameter of 20 nm was added thereto. The resulting mixture was stirred, dissolved and dispersed, developed into a film, and dried to a thickness of 10 μm. Strictly speaking, the dried membrane is impregnated with a non-aqueous electrolyte and kept at a high temperature for a predetermined time to become a gel electrolyte membrane.

5. Preparation of Electrolyte Solution The electrolyte solution was prepared by adjusting the concentration of lithium hexafluorophosphate to a concentration of 1 M / l in an equal volume mixed solvent of ethylene carbonate and propylene carbonate.

6. Preparation and Initialization of Lithium Polymer Secondary Battery Between the positive electrode 1 and the negative electrode 2 prepared by the above method, a microporous polyethylene film 3 and a gel polymer electrolyte film 4 are arranged, and the whole is overlapped and wound. Thus, an elliptical power generating element was manufactured. At the time of winding, the gel polymer electrolyte membrane 4 was made to face the negative electrode 2 and the polyethylene microporous membrane 3 was made to face the positive electrode 1. This power generating element was inserted into an exterior body 7 made of an Al laminate material, and the upper seal portion 8 of the exterior body 7 was sealed with the tips of the positive electrode lead 5 and the negative electrode lead 6 protruding outside.

Next, 2.5 g of the above-mentioned electrolytic solution was injected into the exterior body 7 with the sealed upper seal portion 8 facing down.
The lower seal portion 9 of the outer package body at the position directly opposite to the seal portion 8 where the positive and negative electrode leads protruded to the outside was sealed by heat welding with leaving a margin. After this, the battery was charged at room temperature
(Current value at which the battery can be charged in 10 hours) was charged for 2.5 hours, and then discharged at 1 C (current value at which discharge was completed in 1 hour) to 3.0 V.

After that, the lower seal portion 9 having the above-mentioned clearance is provided.
Was opened to release ethylene gas generated during the initial charging, and then the battery was heat-sealed immediately inside the lower seal portion 9 under reduced pressure to complete the sealing of the battery. Subsequently, the mixture was heated at 90 ° C. for 1 hour to gel the electrolyte membrane, thereby producing a lithium polymer secondary battery. This battery has a capacity of 800 mAh, a size of 5 mm in thickness and a width of 3
4 mm, length 50 mm. This battery is referred to as battery A.

7. Thermal stability test of overcharged battery Investigation was performed under the assumption that the charger failed and the battery was charged to the upper limit of the safety circuit. The produced battery was charged at a constant current of 560 mA (0.7 C) at room temperature, exceeding the voltage upper limit regulation value of the charger to 4.35 V, which is the upper limit value at which the safety circuit operates. The charged battery is transferred to a high-temperature test tank, and the entire tank is heated at a heating rate of 5 ° C. for 1 minute to a temperature of 150 ° C.
C., and left at 150.degree. C. for 2 hours to test the heat resistance of the battery. For the test, a 5-cell battery was used.

8. Thermal stability test of end-of-cycle battery The fabricated battery was charged to 4.2 V at a constant current of 800 mA (1 C) at room temperature, followed by a 10-minute pause, and then to 3.0 V at a constant current of 800 mA (1 C). The battery was discharged and charged / discharged for 400 cycles under the condition of taking a rest time for 10 minutes. After charging the battery in the same manner up to 4.2 V, put it in a high-temperature test tank and heat the battery at a rate of 5 ° C./minute.
The battery was heated to 50 ° C., left at 150 ° C. for 2 hours, and the state of the battery during this time was observed. For the test, a 5-cell battery was used.

Table 1 shows the results of the thermal stability test of the overcharged battery of the produced lithium polymer secondary battery and the thermal stability test at 150 ° C. of the end-of-cycle battery.

[0034]

[Table 1]

Example 2 Example 1 was the same as Example 1 except that the gel polymer electrolyte membrane 4 was opposed to the positive electrode 1 and the microporous polyethylene film 3 was opposed to the negative electrode 2 (an arrangement opposite to that of Example 1). The battery was constructed exactly as described. This battery is referred to as battery B. The thermal stability test of the overcharged battery and the thermal stability test at 150 ° C. of the end-of-cycle battery were performed under the same conditions as in Example 1.

Example 3 A battery was constructed in the same manner as in Example 1, except that only the gel polymer electrolyte membrane having a thickness increased to 25 μm was used between the positive electrode and the negative electrode. . This battery is referred to as battery C. The thermal stability test of the overcharged battery and the thermal stability test at 150 ° C. of the end-of-cycle battery were performed under the same conditions as in Example 1.

Example 4 In Example 1, aluminum oxide (Al ₂ O ₃ ) having a primary particle diameter of 50 nm was provided between the positive electrode and the negative electrode instead of the silicon dioxide fine powder as an additive.
A battery was constructed in exactly the same manner as in Example 1, except that only a gel polymer electrolyte membrane having exactly the same specifications was used except that powder was used. This battery is referred to as Battery D. The thermal stability test of the overcharged battery and the thermal stability test at 150 ° C. of the end-of-cycle battery were performed under the same conditions as in Example 1.

Example 5 A battery was constructed in the same manner as in Example 1, except that only a 25 μm-thick polyethylene microporous membrane was used between the positive electrode and the negative electrode. This battery is referred to as battery E. The thermal stability test of the overcharged battery and the thermal stability test at 150 ° C. of the end-of-cycle battery were performed under the same conditions as in Example 1.

The characteristic results of Examples 2 to 5 are also shown in Table 1.
Are shown together.

As shown in Table 1, in a thermal stability test at 150 ° C. of a battery overcharged to 4.35 V, as originally thought, a microporous polyethylene membrane having an endothermic effect on the surface facing the positive electrode. No ignition was observed in the battery in which. On the other hand, a battery that does not take the countermeasure as in the present invention has a high probability of firing. From these, the effects of the measures according to the present invention can be evaluated. Also, in the thermal stability test of the battery at the end of the cycle at 150 ° C, as originally thought,
In the battery in which the gel electrolyte membrane made of the acrylonitrile copolymer reacting with the metal was arranged on the surface facing the negative electrode, no ignition was observed or smoke was emitted. On the other hand, in a battery in which a gel electrolyte membrane made of an acrylonitrile copolymer is not arranged on the negative electrode, ignition occurs with a high probability, and this clearly leaves a problem.

Therefore, the safety of a battery having a gel electrolyte membrane made of an acrylonitrile copolymer on the negative electrode and a microporous polyethylene membrane having an endothermic effect on the surface facing the positive electrode is extremely high. I understand.

In the examples, LiCoO was used as the positive electrode active material.
2, LiNiO2, LiCoO2 and LiNiO2
A transition metal oxide containing lithium such as Mn2O4 can be used. Further, spherical graphite powder is used as the negative electrode active material, but a carbon material or a lithium storage alloy that can store and release lithium ions can also be used.

Further, an acrylonitrile copolymer film to which fine powder of silicon dioxide was added and a microporous polyethylene film were used in a laminated state. However, depending on the form and type of the separator, it can be directly applied and developed on this separator, and a gel polymer Even if the electrolyte membrane and the polyethylene microporous membrane are not laminated, they can be used as they are. The polyethylene microporous membrane can be used in a wide range from a conventional thickness of about 25 μm to a thin thickness of 8 μm because a gel polymer electrolyte membrane is laminated and has a reinforcing effect.

For the gel polymer electrolyte membrane, a copolymer having the most general composition consisting of 95 mol% of acrylonitrile and 5 mol% of methacrylic acid was used, but 90 mol of acrylonitrile having the function characteristic of the present invention was used. % Acrylonitrile 100%
There was no problem in the function of reacting with the lithium metal in the gelled electrolyte. Similar effects were also obtained when the copolymer with acrylonitrile was vinyl acetate, vinyl chloride, styrene, ether, or the like. Even in the case of these polyacrylonitrile-based copolymers, if acrylonitrile is 90 mol% or more, there is no problem in the function of consuming lithium by reacting with lithium metal even in a gelled electrolyte.

As the silicon dioxide fine powder added to the gel polymer electrolyte membrane, ordinary inorganic silicon dioxide was used.
A filler such as aluminum oxide also exhibited its function. The silicon dioxide powder used had a weight ratio of 50% to the polymer. However, in the range of the experiment, the content of 30% by weight or more was effective in maintaining the shape at a high temperature. The same was true for the aluminum oxide filler.

In the electrolyte, a lithium hexafluorophosphate salt having a concentration of 1 was mixed in an equal volume mixed solvent of ethylene carbonate and propylene carbonate.
M / l was used, but ethylene carbonate (E
C) as a base solvent and dimethyl carbonate (DMC)
And diethyl carbonate (DEC), ethyl methyl carbonate (EM
C) in a mixed organic solvent with a chain carbonate such as
A material in which an electrolyte such as iPF6, LiClO4, or LiBF4 is dissolved may be used.

[0047]

As described above, by arranging the gel electrolyte membrane made of the acrylonitrile copolymer opposite to the negative electrode, the thermal safety of the battery at the end of the cycle is secured and the surface facing the positive electrode is By disposing a microporous polyethylene membrane having an endothermic effect, the safety of the battery can be ensured in terms of thermal stability even if the charger breaks down and charging is no longer regulated. From these two points, the present invention can provide a lithium polymer secondary battery having excellent safety.

[Brief description of the drawings]

FIG. 1 is a schematic view of a lithium polymer secondary battery according to the present invention, in which a package made of a laminate material is cut open.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Polyethylene microporous membrane 4 Gel polymer electrolyte membrane 5 Positive electrode lead 6 Negative lead 7 Outer package 8 Upper seal part 9 Lower seal part 10 Insulation protection film

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Claims (5)

[Claims] 1. A lithium polymer secondary battery in which a power generating element having a gel polymer electrolyte disposed between a positive electrode and a negative electrode is sealed in an outer package, wherein the gel polymer electrolyte absorbs a non-aqueous electrolyte. A lithium polymer secondary battery formed of a two-layer structure of a polyacrylonitrile-based thin film or layer facing a negative electrode and a microporous polyolefin thin film facing a positive electrode. 2. The lithium polymer secondary battery according to claim 1, wherein the microporous polyolefin thin film is a polyethylene film having a thickness of 8 to 25 μm. 3. The lithium polymer secondary battery according to claim 1, wherein the polyacrylonitrile-based thin film or layer has at least 90 mol% of the acrylonitrile unit in the polymer material constituting the thin film or layer. 4. The lithium polymer secondary battery according to claim 3, wherein the polyacrylonitrile-based thin film or layer contains 30 to 50% by weight of an inorganic filler therein. 5. The lithium polymer secondary battery according to claim 4, wherein the inorganic filler is silicon dioxide fine powder or aluminum oxide powder.