JP2011124074A - Thermal runaway inhibitor for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermal runaway inhibitor for a lithium ion secondary battery promptly reducing a battery voltage in case of a high temperature condition, and a safer lithium ion secondary battery containing the thermal runaway inhibitor. <P>SOLUTION: The thermal runaway inhibitor for the lithium ion secondary battery includes an aqueous solution of polyvinyl alcohol encapsulated in a container. The lithium ion secondary battery includes the thermal runaway inhibitor in it. Preferably, the thermal runaway inhibitor is contained in an amount no less than 0.4 mg per 1 mAh of a battery capacity as an amount of a unit part of vinyl alcohol. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a thermal runaway inhibitor for lithium ion secondary batteries and a lithium ion secondary battery.

  With recent advances in electronic technology, small portable electronic devices such as camera-integrated VTRs, mobile phones, laptop computers, etc. have been developed. There is a strong demand for the development of secondary batteries.

  As a secondary battery that meets such a demand, a non-aqueous electrolyte secondary battery that uses a light metal such as lithium, sodium, and aluminum as a negative electrode active material that can theoretically generate a high voltage and has a high energy density is expected. ing. In particular, lithium ion secondary batteries that charge and discharge lithium ions via non-aqueous electrolytes are higher in output and energy than nickel-cadmium batteries and lead-acid batteries, which are aqueous electrolyte secondary batteries. Research and development is actively underway to achieve density.

  In this lithium ion secondary battery, since the inherent energy is large, high safety is required in the event of an abnormality such as an internal short circuit or external short circuit. For this safety measure, a polyolefin microporous membrane is used as a separator. Has been. This is because the polyolefin-based microporous membrane is considered to have a function (shutdown function) that is made non-porous at the time of abnormal heat generation and does not flow electricity. However, even if such safety measures were taken, abnormal heat generation did not stop, and the polyolefin microporous membrane contracted or melted, and the electrodes contacted each other to cause a short circuit, resulting in ignition.

  Therefore, the applicant of the present application has proposed a “lithium ion secondary battery containing a vinyl alcohol polymer or its derivative of 0.3 mg or more per 1 mAh of battery capacity as the amount of vinyl alcohol unit portion” (Patent Document 1).

International Publication No. 2009/025332 Pamphlet

  The lithium ion secondary battery of Patent Document 1 is capable of lowering the battery voltage in a high temperature state, and is excellent in safety that cannot be recharged after being exposed to a high temperature state. However, since it is safer to quickly reduce the battery voltage when the temperature becomes high, such a lithium ion secondary battery has been desired.

  The present invention has been made in view of the situation as described above, and when it becomes a high temperature state, a thermal runaway inhibitor for a lithium ion secondary battery that can quickly lower the battery voltage, and its thermal runaway suppression. An object of the present invention is to provide a lithium ion secondary battery having a higher safety and containing an agent.

The present invention
[1] A thermal runaway inhibitor for lithium ion secondary batteries, wherein an aqueous solution of polyvinyl alcohol is enclosed in a container,
[2] A lithium ion secondary battery comprising the thermal runaway inhibitor according to [1],
[3] The lithium ion secondary battery according to [2], wherein the thermal runaway inhibitor is incorporated in an amount of 0.4 mg or more per 1 mAh of battery capacity as the amount of vinyl alcohol unit portion. About.

When the polyvinyl alcohol is in an aqueous solution state, the thermal runaway inhibitor for lithium ion secondary battery according to [1] of the present invention quickly lowers the battery voltage even when the lithium ion secondary battery is in a high temperature state. It has been found that it can be.
Since the lithium ion secondary battery of [2] of the present invention incorporates the thermal runaway inhibitor, even if it becomes a high temperature state, the battery voltage can be quickly lowered, and the safety is high. It is a lithium ion secondary battery.
In a preferred embodiment of the above [3] in the lithium ion secondary battery of the present invention, the amount of the thermal runaway inhibitor is not less than a specific amount, and even when the temperature becomes high, the battery voltage can be quickly reduced. It is a lithium ion secondary battery with particularly high safety.

  The thermal runaway inhibitor for lithium ion secondary batteries of the present invention (hereinafter sometimes simply referred to as “thermal runaway inhibitor”) may be represented as polyvinyl alcohol (hereinafter referred to as “PVA”) in the container. ) In an aqueous solution. The present invention has been found that PVA is not in a solid state but in an aqueous solution state, so that when it reaches a high temperature state, the battery voltage can be quickly lowered and the safety is excellent. Note that the inventors believe that when the battery is abnormally heated, the specific heat of water is high, so that the temperature rise inside the battery is moderated.

PVA constituting the PVA aqueous solution of the present invention is a polymer based on vinyl alcohol units [—CH (OH) —CH ₂ —]. The degree of polymerization and the degree of saponification are not particularly limited, but the degree of polymerization can be 100 to 10,000, preferably 300 to 5,000, more preferably 500 to 3,000. . The saponification degree of PVA can be 60 to 98%, preferably 70 to 98% PVA is used.

  In this invention, although the PVA aqueous solution which dissolved PVA in water is used, the density | concentration etc. are not specifically limited.

The thermal runaway suppressing agent of the present invention has various disadvantages (for example, generation of hydrogen fluoride and the like) caused by adding the PVA aqueous solution directly to the electrolytic solution because the PVA aqueous solution as described above is sealed in the container. It is possible to prevent deterioration of the electrolyte solution such as decomposition of the electrolyte solution. It should be noted that the PVA aqueous solution needs to leak from the container so that the battery voltage drop effect by the PVA aqueous solution is exerted during abnormal heat generation such as during a short circuit. Therefore, the container needs to be made of a material that does not react and soak with the electrolyte solution of the lithium ion secondary battery and that leaks the aqueous PVA solution by melting when abnormal heat is generated. Usually, as the electrolytic solution, a solution of LiPF ₆ dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate is used, and the battery operating temperature in daily life is about 60 ° C. or less. Those having a melting point of 80 ° C. or higher are preferable because they are not reacted and soaked with the electrolytic solution as described above, and for safety. In addition, since it may become difficult to suppress thermal runaway when the melting point of the container is too high, the melting point is preferably 150 ° C. or lower. More specifically, the material of the container includes low density polyethylene, linear low density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, ethylene copolymer (for example, ethylene-vinyl alcohol copolymer, ethylene-acrylic). And ethylene resins such as acid copolymers and ethylene-methacrylic acid copolymers.

In addition, as long as the container in the thermal runaway inhibitor of this invention has the said effect | action, the shape, a magnitude | size, etc. are not specifically limited.
In the present invention, “encapsulation” means that the PVA aqueous solution in the container does not leak at a temperature of less than 80 ° C.

  The method for producing the thermal runaway inhibitor of the present invention is not particularly limited. For example, after preparing a container of a desired size and injecting a PVA aqueous solution into the container, a method such as heat sealing Can be sealed. Moreover, it can also be microencapsulated by a conventional method.

  The lithium ion secondary battery of the present invention incorporates the thermal runaway inhibitor as described above. Therefore, even if the battery is abnormally heated and becomes a high temperature state, it is a highly safe lithium ion secondary battery that can quickly reduce the battery voltage.

The built-in amount of the thermal runaway inhibitor is preferably 0.4 mg or more per 1 mAh of battery capacity as the amount of vinyl alcohol unit portion. This is because the rate of dropping the battery voltage is high when it is 0.4 mg or more. More preferably, it is 0.5 mg or more, More preferably, it is 0.6 mg or more. The upper limit of the built-in amount of the thermal runaway inhibitor is not particularly limited as long as it can be built in the battery. For example, the lithium ion secondary of small portable electronic devices, hybrid vehicles (HEV) and electric vehicles (PEV) Considering that it is built in a battery with a limited volume like a battery, for example, it is 100 mg or less per 1 mAh of battery capacity.
The “amount of vinyl alcohol unit portion” can be calculated from the amount of PVA and the ratio of vinyl alcohol units in PVA. For example, the amount of PVA is M (g) and the degree of saponification is S (%). In this case, the amount of vinyl alcohol unit portion of the PVA is [M × (S / 100) = MS / 100].

  The lithium ion secondary battery of the present invention incorporates the thermal runaway inhibitor as described above, but when the PVA aqueous solution leaks from the thermal runaway inhibitor due to abnormal heat generation of the battery, the electrolytic solution and the PVA aqueous solution are in contact with each other. However, it can be placed on the inner wall of the battery, in the vicinity of the lid, on the tube bottom, or the like. In particular, it is preferable that the thermal runaway inhibitor is incorporated in a place where it comes into contact with the electrolytic solution before leakage because the electrolytic solution and the PVA aqueous solution can be efficiently contacted. In addition, when a thermal runaway inhibitor can be carried on a separator (for example, a nonwoven fabric separator) as in the case of microencapsulation, the PVA aqueous solution exists between the electrodes at the time of abnormal heat generation, which is more effective. high.

The lithium ion secondary battery of this invention can be set as the structure similar to the conventional lithium ion secondary battery except incorporating the thermal runaway inhibitor as mentioned above. For example, a material in which a paste of a lithium-containing metal compound is carried on a current collector is used as the positive electrode, and a material that becomes an alloy with lithium metal or lithium (for example, a tin alloy, a silicon alloy, SiO, etc. Material), and carbon materials that can store and release lithium or carbon materials containing graphite (for example, carbon materials such as coke, natural graphite, and artificial graphite) supported on a current collector, etc. An aqueous electrolytic solution (for example, an electrolytic solution in which LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate) or the like can be used. Further, the cell structure of the lithium ion secondary battery is not particularly limited, and may be, for example, a cylindrical shape, a square shape, a coin shape, or the like.

  EXAMPLES Hereinafter, the present invention will be specifically described by way of examples, but these do not limit the scope of the present invention.

Example 1
(1) Production of positive electrode;
90% by mass of lithium cobaltate (LiCoO ₂ ) powder, 5% by mass of acetylene black, and 5% by mass of polyvinylidene fluoride (PVdF) as a positive electrode active material are dispersed in N-methyl-2-pyrrolidone (NMP), A slurry was prepared. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried at a temperature of 140 ° C. for 30 minutes, and then pressed to prepare a positive electrode.

(2) Production of negative electrode;
As a negative electrode active material, 90% by mass of natural graphite powder and 10% by mass of PVdF were dispersed in NMP to prepare a slurry. The obtained slurry was coated on a copper foil having a thickness of 15 μm, dried under reduced pressure at a temperature of 140 ° C. for 30 minutes, and then pressed to prepare a negative electrode.

(3) non-aqueous electrolyte;
In a mixed solvent of ethylene Cabo sulfonate (EC) and diethyl Cabo sulfonate (DEC) (50:50), non-aqueous electrolytic solution obtained by dissolving LiPF ₆ as a _{1.0mol / L [LiPF 6 -EC /} DEC ( 50:50); manufactured by Kishida Chemical Co., Ltd.].

(4) Separator;
A polypropylene microporous membrane (Celgard (registered trademark) 2400; manufactured by Celgard) was prepared as a separator.

(5) Preparation of PVA aqueous solution;
5% by mass of polyvinyl alcohol (PVA) powder [degree of saponification: 96 mol% or more, average degree of polymerization: 1000, manufactured by Wako Pure Chemical Industries, Ltd.] was dissolved in pure water to prepare a PVA aqueous solution.

(6) Preparation of thermal runaway inhibitor;
A commercially available polyethylene bag [manufactured by J Film Co., Ltd., melting point: 122 ° C.] was cut open, and then cut into a 2 cm × 1.5 cm rectangle to obtain two polyethylene films. Next, after superposing these two polyethylene films, portions corresponding to the three sides of the rectangular film were heat-sealed to prepare a rectangular container (bag).
Subsequently, after pouring PVA aqueous solution into this container (bag), the part corresponding to the one side of the said rectangle was heat-seal | fused, and the thermal runaway inhibitor which enclosed 81.2 mg PVA aqueous solution was produced. The enclosed amount of the PVA aqueous solution was calculated from the mass before and after encapsulation.

(7) Battery production;
A lithium ion secondary battery (laminated type) was produced using the positive electrode, negative electrode, non-aqueous electrolyte, separator, and thermal runaway inhibitor. The thermal runaway inhibitor was disposed at the end in the direction opposite to the terminal side. The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.65 mg per 1 mAh of battery capacity.

(Example 2)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 1 except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 1 was changed. . The amount of the vinyl alcohol unit part of the thermal runaway inhibitor was 0.56 mg per 1 mAh of battery capacity.

(Example 3)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 1 except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 1 was changed. . The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.42 mg per 1 mAh of battery capacity.

(Comparative Example 1)
Instead of the thermal runaway inhibitor of Example 1, a container (bag) was not used, and polyvinyl alcohol powder [degree of saponification: 96%, degree of polymerization: 1000, manufactured by Wako Pure Chemical Industries, Ltd.] was directly placed in the battery. A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 1 except that it was added. The amount of vinyl alcohol unit portion of the polyvinyl alcohol powder was 0.67 mg per 1 mAh of battery capacity.

Example 4
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 1 except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 1 was changed. . The amount of the vinyl alcohol unit part of the thermal runaway inhibitor was 0.34 mg per 1 mAh of battery capacity.

(Example 5)
Polyvinyl alcohol (PVA) powder 10 mass% [saponification degree: 96 mol% or more, average polymerization degree: 1000, manufactured by Wako Pure Chemical Industries, Ltd.] was dissolved in pure water to prepare a PVA aqueous solution. Next, a thermal runaway inhibitor was produced in the same manner as in Example 1 except that 42.6 mg of this PVA aqueous solution was injected.
Then, it carried out similarly to Example 1, and produced the lithium ion secondary battery (laminate type). The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.63 mg per 1 mAh of battery capacity.

(Example 6)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 5 except that the amount of PVA aqueous solution injected into the container (bag) was changed in the production of the thermal runaway inhibitor of Example 5. . The amount of the vinyl alcohol unit part of the thermal runaway inhibitor was 0.57 mg per 1 mAh of battery capacity.

(Example 7)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 5 except that the amount of PVA aqueous solution injected into the container (bag) was changed in the production of the thermal runaway inhibitor of Example 5. . The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.41 mg per 1 mAh of battery capacity.

(Example 8)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 5 except that the amount of PVA aqueous solution injected into the container (bag) was changed in the production of the thermal runaway inhibitor of Example 5. . The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.37 mg per 1 mAh of battery capacity.

Example 9
Polyvinyl alcohol (PVA) powder 15 mass% [saponification degree: 96 mol% or more, average polymerization degree: 1000, manufactured by Wako Pure Chemical Industries, Ltd.] was dissolved in pure water to prepare an aqueous PVA solution. Next, a thermal runaway inhibitor was produced in the same manner as in Example 1 except that 26.3 mg of this PVA aqueous solution was injected.
Then, it carried out similarly to Example 1, and produced the lithium ion secondary battery (laminate type). The amount of the vinyl alcohol unit part of the thermal runaway inhibitor was 0.61 mg per 1 mAh of battery capacity.

(Example 10)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 9, except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 9 was changed. . The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.52 mg per 1 mAh of battery capacity.

(Example 11)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 9, except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 9 was changed. . The amount of vinyl alcohol unit portion of the thermal runaway inhibitor was 0.43 mg per 1 mAh of battery capacity.

(Example 12)
A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 9, except that the amount of PVA aqueous solution injected into the container (bag) in the production of the thermal runaway inhibitor of Example 9 was changed. . The amount of the vinyl alcohol unit portion of the thermal runaway inhibitor was 0.32 mg per 1 mAh of battery capacity.

(Comparative Example 2)
As a fiber, a core-sheath type composite fiber having a core component of polypropylene and a sheath part of polyethylene having a fineness of 0.8 dtex and a fiber length of 5 mm is used. After forming a fiber web by a wet papermaking method, a hot roll press at 130 ° C. The core-sheath type composite fiber was fused to prepare a nonwoven fabric having a basis weight of 10 g / m ² .
Next, on the nonwoven fabric, a polypropylene fiber having a fineness of 0.02 dtex and a fiber length of 2 mm is made (5 g / m ² basis weight), and the core-sheath type composite fiber is fused again by a hot roll press at a temperature of 130 ° C. The thickness was adjusted with a roll calender to prepare a composite nonwoven fabric having a thickness of 35 μm and a basis weight of 15 g / m ² .
Separately, an aqueous PVA solution in which 10% by mass of polyvinyl alcohol (PVA) powder [degree of saponification: 96%, degree of polymerization: 1000, manufactured by Wako Pure Chemical Industries, Ltd.] was dissolved in pure water was prepared.
Next, impregnating the composite non-woven fabric with the PVA aqueous solution, removing excess solution with a roller, drying, and adjusting the thickness with a roll calender to coat the surface of the composite non-woven fabric with polyvinyl alcohol, containing PVA A composite nonwoven fabric (thickness: 35 μm, additional weight of PVA: 3.1 g / m ² ) was produced.
And it replaced with the thermal runaway inhibitor of Example 1, and cut | disconnected the said PVA containing composite nonwoven fabric so that a vinyl alcohol unit partial amount might be 0.64 mg per battery capacity 1mAh, and it piled up with a separator, and arrange | positioned between electrodes. A lithium ion secondary battery (laminate type) was produced in the same manner as in Example 1 except that.

(High temperature safety test)
The lithium ion secondary batteries of each Example and Comparative Example were subjected to 5 cycles of 0.2 C constant current charge / discharge in a voltage range of 3-4.2 V, and after confirming that the batteries were operating normally, The following high temperature safety test was conducted.
In the high-temperature safety test, the battery was charged at a constant current of 0.2 C to 4.2 V, further charged for 5 hours by the constant voltage method until fully charged, and then the battery was heated in a hot air oven set at a temperature of 140 ° C. The open circuit voltage (OCV) change was measured and the time until the battery voltage became 2 V or less was measured. The results are shown in Table 1.

  As is clear from Table 1, if the amount of vinyl alcohol unit part per 1 mAh of battery capacity is about the same, the time until the battery voltage becomes 2 V or less is shorter and safer when the PVA is in an aqueous solution state. It was found to be excellent in performance. Further, it was found that the amount of PVA can be reduced when the PVA is in the state of an aqueous solution if the time is comparable.

  The lithium ion secondary battery of the present invention can be used as a power source for electronic devices such as camera-integrated VTRs, mobile phones, and laptop computers.

Claims (3)

  A thermal runaway inhibitor for a lithium ion secondary battery, characterized in that an aqueous solution of polyvinyl alcohol is enclosed in a container.   A lithium ion secondary battery comprising the thermal runaway inhibitor according to claim 1.   The lithium ion secondary battery according to claim 2, wherein the thermal runaway inhibitor is incorporated in an amount of 0.4 mg or more per 1 mAh of battery capacity as the amount of vinyl alcohol unit portion.