JP2002033128A - Lithium polymer secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve a problem that heat stability is insufficient due to large heat generation when overcharging beyond the normal charging termination voltage is performed in addition to a problem that a large quantity of gas is generated when a lithium polymer secondary battery is stored at a high temperature. SOLUTION: Six-silane lithium fluoride (Li2SiF6) is formed on the surface of a negative electrode active material during preservation to suppress generation of gas in high temperature preservation by arranging vinylidene fluoride system copolymer film adding fine powder of silicon dioxide on a surface facing a negative electrode as an isolation member. Heat generation from a positive electrode is absorbed and excellent heat stability and safety of a battery can be secured, even when a battery charger fails and charging is not controlled by arranging a microporous polyethylene film on a surface facing the positive electrode.

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.

[0003] For example, the battery disclosed in the above-mentioned US patent and developed and mass-produced by many companies has a positive electrode of LiCoO _2.
Alternatively, LiMn ₂ O ₄ is used, the negative electrode is made of a carbon material such as graphite, and the electrolyte also serving as a separator is made of a vinylidene fluoride-based polymer made of a non-aqueous electrolyte absorbed and gelled. 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 liquid leakage by fixing the electrolyte. ing.

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 A method has been proposed in which a battery is formed by sandwiching a polymer electrolyte membrane between a positive electrode and a negative electrode.

[0005]

In the battery system comprising only the gel polymer electrolyte described above, the flammability is improved by reducing the vapor pressure of the flammable organic solvent because the electrolyte is fixed by the polymer material. Then, since the reactivity between the charged positive and negative electrode materials and the organic solvent is suppressed, some improvement in heat resistance in terms of the heat generation rate and amount is recognized. However, there remains a problem in the safety when the charger as the first-stage safety device or the safety circuit in the second-stage safety device in the electronic circuit outside the battery breaks down. I have. For example, a battery system consisting only of a gel-like polymer electrolyte does not have a current interruption function. In addition, when the temperature of the battery increases due to overcharging, the hardness of the gel decreases, so that ions can be mobilized and current flows easily. As a result, it becomes more difficult to secure safety. 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.

[0006] Lithium-ion batteries have a problem in that, in addition to ensuring safety in the event of overcharging, reliability, especially battery swelling due to gas generation during long-term storage and ultimately venting (opening) of the battery case. ) Problem. In particular, the charged negative electrode is only as reactive as metallic lithium, and the electrolyte in contact with the negative electrode is reduced and decomposed, resulting in a large amount of gas generation. In particular, when batteries are going to be thinner, this type of gas generation is fatal because commonly used aluminum laminate outer bodies lack mechanical strength and are more likely to expand and deform than metal outer cans. Has become.

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

[0008]

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 was also found that if the heat generated by the first reaction between the positive electrode and the electrolytic solution was dissipated or absorbed in some way, the subsequent reaction between the negative electrode and the electrolytic solution could be prevented, and fatal thermal runaway could 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, gas generation during storage, the components and composition of the generated gas are analyzed and
Each of the charged positive electrode and negative electrode was separately stored in an aluminum laminate bag together with the electrolytic solution, and the amounts and components were measured and analyzed while observing the state of gas generation. As a result, it was found that gas generation was mainly due to the reaction between the charged negative electrode and the electrolyte. In order to suppress gas generation on the surface of the negative electrode, it is effective to cut off or isolate the electrolyte from the negative electrode. One of the solutions is to form a film on the surface of the negative electrode. However, this film needs to be able to transmit lithium ions coming in and out of the negative electrode during charge and discharge. That is, it must be lithium ion conductive. At present, it has been reported that the addition of vinylene carbonate, ethylene sulfite, or the like to the electrolyte is effective in forming this film. However, it has now been found that forming lithium hexafluorosilane (Li ₂ SiF ₆ ) on the negative electrode surface is very effective. In this case, the coexistence of a very small amount of water inside the battery is an important factor.
The outline of the reaction mechanism inferred from the analysis results is shown below.

LiPF ₆ + H ₂ O → LiF + 2HF + POF ₃ 4HF + SiO ₂ → SiF ₄ + 2H ₂ O SiF ₄ + 2LiF → Li ₂ SiF ₆ As a result of studying the method of forming this film, it was found that a fine powder of silicon dioxide was added to the negative electrode mixture. It is the simplest to do this, but the effects varied. Rather, it is better to arrange the fine powder on the opposite surface of the negative electrode in advance, generate silane tetrafluoride gas here, and react on the negative electrode surface charged by diffusion to form a film without variation and greater effect. found. In order to realize this mechanism, it is necessary to provide a film to which fine powder of silicon dioxide is added on the opposite surface of the negative electrode, but a simple film has little effect. The present inventors have found a method of dispersing a fine powder of silicon dioxide in a film made of a vinylidene fluoride copolymer having a gelling function, preferably a material that absorbs an electrolyte and gels. Although the manufacturing process and the purpose are different, the finally obtained film itself is disclosed in US Pat.
It is similar to the specification of Japanese Patent No. 0,741 or Japanese Patent Application Laid-Open No. 10-511216.

In the present invention, as a new battery configuration, a microporous polyethylene film is disposed near the positive electrode, and lithium hexafluorosilane (Li ₂ SiF ₆ ) is formed on the surface of the negative electrode active material.

As a result, the safety of the battery can be ensured even if the charger or safety circuit breaks down and charging is no longer regulated, and gas generation can be suppressed to a level that does not cause any problem even during long-term storage. As a result, it is possible to provide a lithium polymer secondary battery having excellent performance.

[0013]

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 thermal adhesive seal portions, respectively, and 10 is a lead insulating protective film. The microporous membrane 3, the gel-like polymer electrolyte membrane 4, the negative electrode 2, and the positive electrode 1 are finally housed in the outer package 7 in a laminated and wound form.

A method for manufacturing a 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 the dispersion binder is mixed and made into a paste, the paste is uniformly applied to both surfaces of an Al foil current collector with a die coater.
After drying and rolling, it was obtained by cutting to a predetermined size. A positive electrode lead 5 was welded to the positive electrode 1 at the end of the current collector. Negative electrode 2
Is prepared by mixing a conductive material and an aqueous binder into a graphite negative electrode active material, forming a paste, applying the paste uniformly to both surfaces of a Cu foil current collector with a die coater, and drying and rolling to a predetermined size. And obtained by cutting. A negative electrode lead 6 was welded to the negative electrode 2 at the end of the current collector. The separator 3 is a commercially available polyethylene microporous membrane. Gel-like polymer electrolyte membrane 4
Is obtained by adding a copolymer of vinylidene fluoride and propylene hexafluoride and silicon dioxide fine particles to an N-methylpyrrolidone solvent, stirring, dissolving and dispersing the paste into a film, and drying the paste. Depending on the type of the separator 3, a gel polymer can be applied and developed on this surface,
There is an effect that the gel polymer electrolyte membrane and the polyethylene microporous membrane can be used as they are without being laminated. The electrolytic solution is a 6 volume mixed solvent of ethylene carbonate and diethyl carbonate.
A solution prepared by adjusting the concentration of lithium fluorophosphate to 1 M / l 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 that houses 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 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 a portion of the lead that contacts the upper seal portion 8. The insulating protective film 10 is provided with a positive electrode lead 5,
This is a member provided to ensure airtightness in the negative electrode lead 6 portion.

As the battery, a predetermined amount of electrolyte is injected from the lower seal 9 which is still open, with the upper seal 8 already closing the outer package 7 accommodating the power generating element as described above. After that, the lower seal portion 9 is sealed by heat welding to complete the battery.

[0017]

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

(Embodiment 1) 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 3 generally has a porosity of about 40% which is commercially available,
A 15 μm thick polyethylene microporous membrane was used.

4. Gel Polymer Electrolyte Membrane The gel polymer electrolyte membrane 4 is composed of a copolymer 10 composed of 88 mol% of vinylidene fluoride and 12% of propylene hexafluoride.
A paste prepared by adding 0 parts by weight and 50 parts of silicon dioxide having a primary particle diameter of 20 nm to an N-methylpyrrolidone solvent, stirring and dissolving and dispersing the mixture into a film, and drying the paste to a thickness of 10 μm was used. . To be precise, the dried membrane is impregnated with an electrolytic solution and held at a high temperature for a predetermined period of 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 diethyl 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 electrolyte 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. Thereafter, a part of the lower seal portion 9 having a space is opened to release ethylene gas generated during the initial charging, and then the battery is heat-sealed immediately inside the lower seal portion 9 under reduced pressure. Then, sealing as a battery was completed. 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 and a size of 5
mm, width 34 mm, length 50 mm. This battery is referred to as battery A.

[7] High-temperature storage characteristics The prepared battery was subjected to constant current of 560 mA (0.7 C) at room temperature.
Then, the battery was charged at a constant voltage of 4.2 V until the charging current dropped to 50 mA. Then 90
C. for 4 days, and the amount of gas generated at that temperature was measured.

8. Thermal stability characteristics The study 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 room temperature with a constant current of 560 mA (0.7 C) to exceed the voltage regulation value of the charger to 4.35 V which is the upper limit value of the safety circuit. Transfer the charged battery to a constant temperature heating tank,
The entire vessel was heated to 150 ° C. at a heating rate of 150 ° C.
The battery was tested for heat resistance by standing at 2 ° C. for 2 hours. For the test, a 5-cell battery was used.

The manufactured lithium polymer secondary battery 90
Table 1 shows the characteristic results of the high-temperature storage stability at 150 ° C. and the thermal stability at 150 ° C.

[0028]

[Table 1]

Example 2 Example 1 was the same as Example 1 except that the gelled polymer electrolyte membrane 4 was opposed to the positive electrode 1 and the microporous membrane 3 made of polyethylene 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. Both high-temperature storage characteristics and thermal stability tests were performed under the same conditions as in Example 1.

Example 3 A battery was prepared in the same manner as in Example 1 except that only a gel polymer electrolyte membrane having a thickness increased from 10 μm to 25 μm was used between the positive electrode and the negative electrode. Configured. This battery is referred to as battery C. Both high-temperature storage stability and thermal stability tests 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 as above was used. This battery is referred to as Battery D. Both high-temperature storage stability and thermal stability tests 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. Both high-temperature storage stability and thermal stability tests 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, when a 100% charged battery is stored at a high temperature of 90 ° C. for 4 days, a battery which has not taken measures against gas generation has a large swelling due to a large amount of gas generation. It was not already the shape of the battery. On the other hand, as described above, in the battery in which measures such as the present invention are taken, the gas generation amount is reduced and improved to a level that does not cause any problem. In the batteries in which measures were taken even in the analysis of the generated gas, most of the carbon dioxide gas (CO ₂ ) was caused by the reaction between the positive electrode and the electrolyte. Not only gas but also a large amount of carbon monoxide (CO) and methane gas (CH ₄ )
There was a gas generated from the negative electrode. From these, the effect of the measure according to the present invention can be greatly evaluated.

In a thermal stability test at 150 ° C., no ignition was observed in a battery in which a microporous polyethylene membrane having an endothermic effect was disposed on the surface facing the positive electrode, as originally thought.

On the other hand, in a battery in which the gel electrolyte membrane faces the positive electrode, ignition occurs, which clearly leaves a problem.

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, a vinylidene fluoride copolymer film to which fine powder of silicon dioxide is added and a microporous polyethylene film are used in a laminated state, but depending on the type of separator, it can be directly applied and developed on this separator to form a gel polymer. Even if the electrolyte membrane and the polyethylene microporous membrane are not laminated, they can be used as they are. Further, the polyethylene microporous membrane can be used from the conventional thickness of 25 μm to the thinnest 8 μm at present because the gel polymer electrolyte membrane is laminated and protected.

For the gel polymer electrolyte membrane, the most common copolymer consisting of 88 mol% of vinylidene fluoride and 12 mol% of propylene hexafluoride was used. If the amount is 15 mol% or less, there is no problem in both the complexity (man-hours) of the gelation process and the performance of the completed battery.

As the silicon dioxide added to the gel polymer electrolyte membrane, ordinary inorganic silicon dioxide was used. However, as shown by the mechanism for suppressing the gas generation, it is sufficient that silicon dioxide is present, and at least one of The effect did not change even if the part was a silicon dioxide powder modified with an organic functional group. The silicon dioxide powder has a dry weight of 5%.
Although 0% was used, the effect of suppressing gas generation was observed if the content was 20% by weight or more in the range of the experiment.

Further, as the electrolytic solution, a solution prepared by adjusting the concentration of lithium hexafluorophosphate to 1 M / l in a mixed solvent of equal volumes of ethylene carbonate and diethyl carbonate was used. In addition, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC) and chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) May be used in which an electrolyte such as LiPF6, LiClO4, or LiBF4 is dissolved in an organic solvent mixed with.

[0042]

As described above, by arranging a microporous polyethylene film on the surface facing the positive electrode, even if the charger breaks down and charging is no longer regulated, the safety of the battery can be improved in terms of thermal stability. Nature can be secured. In addition, by disposing a vinylidene fluoride copolymer film to which fine powder of silicon dioxide is added on the opposite surface of the negative electrode, lithium hexafluorosilane (Li ₂ SiF ₆ ) is formed on the surface of the negative electrode active material during storage. The generation of gas could be suppressed to a level at which there was no problem even in high-temperature storage. From these two points, the present invention can provide a lithium polymer secondary battery having excellent performance.

[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

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. It consists of a polyvinylidene fluoride thin film or layer in which silicon dioxide fine powder is dispersed inside, facing the negative electrode, and a microporous polyolefin thin film is arranged on the surface facing the positive electrode. A lithium polymer secondary battery formed from a two-layer structure. 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 polyvinylidene fluoride thin layer comprises at least 85 mol% of the polymer material constituting the thin layer made of polyvinylidene fluoride. 4. The lithium polymer secondary according to claim 1, wherein the silicon dioxide powder comprises at least one selected from ordinary inorganic silicon dioxide and silicon dioxide powder having at least a part of its surface modified with an organic functional group. battery. 5. The lithium polymer secondary battery according to claim 1, wherein the polyvinylidene fluoride thin layer contains 20 to 50% by weight of silicon dioxide fine powder inside.