JP2016081681A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly safe lithium ion secondary battery capable of suppressing generation of thermal runaway by limiting an electric current under an overcharge state.SOLUTION: The lithium ion secondary battery, having a positive electrode collector, a positive electrode mixture layer provided on the positive electrode collector and containing at least a positive electrode active material and a positive electrode lead tab serving as an energization path to the outside, includes a voltage resolution adhesive layer mainly comprised of a material resolved at a predetermined voltage, provided between the positive electrode collector and the positive electrode lead tab.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium ion secondary battery.

  With the widespread use of electronic devices such as notebook computers, mobile phones, and digital cameras, the demand for secondary batteries for driving these electronic devices is increasing. In recent years, these electronic devices have increased power consumption with the progress of higher functionality, and are expected to be smaller. Therefore, higher energy density and higher output density are required for secondary batteries. Is required. As secondary batteries that can achieve high energy density and high output density, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries are considered promising.

  However, since lithium ion secondary batteries use lithium with high chemical activity, highly flammable electrolytes, and lithium transition metal composite oxides with low overcharge stability as battery materials, It is known that if the charging is further continued in the state, the chemical reaction between the battery materials proceeds rapidly, and there is a safety problem that causes heat generation, thermal runaway, ignition, etc. of the battery. For this reason, it is necessary to quickly stop charging before reaching an overcharged state, and a mechanism is employed in which an external circuit performs voltage monitoring, charging stop, and the like. As described above, although various measures are taken for safety, ignition of the on-board battery and the on-board battery continues. For this reason, establishment and introduction of a safety mechanism not only outside the battery but also inside the battery are required.

  Patent Documents 1 to 4 disclose methods for suppressing overcharge of lithium ion secondary batteries.

  Patent Document 1 discloses that the overcharge prevention effect is improved by containing a mixture of cyclohexylbenzene and a tert-alkylbenzene derivative in a nonaqueous electrolytic solution.

  Patent Document 2 discloses that an accident caused by thermal runaway is prevented by including thermally expandable microcapsules in the electrode mixture layer or along the interface between the electrode mixture layer and the current collector. Yes.

  Patent Document 3 discloses that the safety during overcharge is improved by uniformly mixing the positive electrode active material constituting the positive electrode and lithium carbonate.

  In Patent Document 4, the positive electrode has a two-layer structure including a first layer that is a conductive layer formed on a positive electrode current collector and a second layer that is an active material layer, and the first layer is overcharged. It is disclosed that by containing a substance that decomposes at a high potential in a state, the charging current is interrupted when the potential becomes high due to overcharging.

JP 2006-120650 A JP 2001-332245 A JP 2008-181830 A JP 2000-077061 A

  However, when the additive described in Patent Document 1 is mixed with the electrolytic solution, there is a problem that the electrolyte ion conductivity in the electrolytic solution is lowered. Further, there is a problem that the reaction of the additive occurs during high-temperature storage, and the battery cycle life and high-temperature storage characteristics deteriorate.

  In addition, when the thermally expandable microcapsules described in Patent Document 2 are introduced into the positive electrode, the microcapsules gradually expand during high-temperature storage and increase the positive electrode resistance, so that the battery cycle life and high-temperature storage characteristics are degraded. There is a problem.

  In addition, as shown in Patent Document 3, when a compound that generates a gas due to overcharging is introduced into the positive electrode mixture, the amount of the active material in the positive electrode mixture is reduced, so that the positive electrode capacity is reduced. .

  In addition, as shown in Patent Document 4, when a compound that generates a gas due to overcharge is introduced into the first positive electrode layer on the current collector, destruction of the first positive electrode layer proceeds due to gas generation accompanying a voltage increase. At this time, when current interruption due to destruction of the first layer of the positive electrode is insufficient, there is a problem of overheating and thermal runaway.

  An object of the present invention is to provide a highly safe lithium ion secondary battery that can limit current in an overcharged state and suppress thermal runaway.

  The present invention is a lithium ion secondary battery comprising a positive electrode current collector, a positive electrode mixture layer provided on the positive electrode current collector and containing at least a positive electrode active material, and a positive electrode lead tab serving as a current path to the outside. Between the positive electrode current collector and the positive electrode lead tab, a voltage resolving adhesive layer composed mainly of a material that decomposes at a predetermined voltage was provided.

  ADVANTAGE OF THE INVENTION According to this invention, a highly safe lithium ion secondary battery which can restrict | limit an electric current in an overcharge state and can suppress thermal runaway is realizable.

Schematic which shows an example of the lithium ion secondary battery which concerns on embodiment of this invention.

  Hereinafter, the electrode for lithium ion secondary batteries and the lithium ion secondary battery which concern on embodiment of this invention are demonstrated.

  FIG. 1 is a diagram showing an example of a lithium ion secondary battery according to an embodiment of the present invention. The battery 1 includes a positive electrode current collector 2, a positive electrode mixture layer 3, a negative electrode current collector 4, a negative electrode mixture layer 5, a separator 6, a positive electrode lead tab 7, a negative electrode lead tab 8, a voltage decomposition adhesive layer 9, a tab sealant 10, and The exterior material 11 is comprised. Although not shown in FIG. 1, the inside of the battery 1 is filled with an electrolytic solution.

  The positive electrode current collector 2 is not particularly limited, and a conventionally known current collector made of a material such as aluminum, stainless steel, or nickel plated steel can be used.

The positive electrode mixture layer 3 is formed by mixing a positive electrode active material, a conductive additive, and a binder in a single solvent or a mixed solvent such as N-methylpyrrolidone, methyl ethyl ketone, and toluene, and then mixing the mixed solution with the positive electrode current collector. It can be formed by applying on the body 2 and drying. Moreover, the positive electrode active material contained in the positive mix layer 3 is not specifically limited, A conventionally well-known active material can be used. Examples of the positive electrode active material include lithium transition metal composite oxides that release lithium ions, and examples thereof include LiNiO ₂ , LiMn ₂ O ₄ , LiCoO ₂ , and LiFePO ₄ . In addition, a plurality of lithium transition metal oxides can be mixed and used as the positive electrode active material. The binder is preferably a chemically and physically stable material such as polyvinylidene fluoride, polytetrafluoroethylene, EPDM, SBR, NBR, or fluororubber. Examples of the conductive aid include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, and amorphous carbon.

  The negative electrode current collector 4 is not particularly limited, and a current collector made of copper foil or the like can be used.

  The negative electrode mixture layer 5 includes a negative electrode active material and a binder. Moreover, you may add a conductive support agent as needed. The negative electrode active material is not particularly limited, and a compound capable of occluding and releasing lithium ions, such as a metal material such as lithium, an alloy material containing silicon, tin or the like, or a carbon material such as graphite or coke is used. They can be used alone or in combination. When a lithium metal foil is used as the negative electrode active material, the lithium foil can be formed by pressure bonding on a metal current collector such as copper. Also, when using an alloy material or carbon material as the negative electrode active material, the negative electrode active material, a binder, a conductive aid, etc. are mixed in a solvent such as water or NMP, and then applied onto a metal current collector such as copper. It can be formed by drying.

  The binder in the negative electrode mixture layer 5 is preferably a chemically and physically stable material such as polyvinylidene fluoride, polytetrafluoroethylene, EPDM, SBR, NBR, or fluororubber. Examples of the conductive aid include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, and amorphous carbon.

  The separator 6 is used to prevent a short circuit between the positive electrode mixture layer 3 and the negative electrode mixture layer 5. It is made of a material having resistance to the electrolytic solution to be used, and has micropores for allowing lithium ions to pass therethrough. Examples of the material include a microporous membrane or nonwoven fabric made of polyolefin such as polyethylene or polypropylene, or an aromatic polyamide resin, a porous resin coat containing inorganic ceramic powder, and the like. Moreover, although what has what is called a shutdown function that a micropore closes by temperature rise is more preferable, it does not restrict to this.

  The positive electrode lead tab 7 and the negative electrode lead tab 8 are metals that are conductors. In order to improve the sealing performance of the outer packaging material 11, a configuration is generally adopted in which a tab sealant 10 is added to the positive electrode lead tab 7 and the negative electrode lead tab 8 so that the electrolyte does not leak after heat sealing. The positive electrode lead tab 7 and the negative electrode lead tab 8 are added to the positive electrode current collector 2 and the negative electrode current collector 4, respectively, and current is taken out from the inside of the battery 1.

  The voltage resolving adhesive layer 9 is composed of a resin and a conductive material that are altered or decomposed at a high voltage when the lithium ion secondary battery is overcharged. Examples of the resin include polyacrylic resin, polymethacrylic resin, polyester resin, polyurethane resin and the like. It has been shown by the inventors' cyclic voltammetry that these resins generate an oxidative decomposition current in the voltage range around 4.5V. Therefore, these resins are oxidatively decomposed at a voltage of 4.5 V or more, and the strength of the resin is significantly reduced. A material obtained by adding a known material such as acetylene black, ketjen black, carbon black, graphite, or carbon nanotube to these resins as a conductive material is bonded between the positive electrode current collector 2 and the positive electrode lead tab 7 ( If it is provided as a voltage-resolving adhesive layer), this portion is decomposed at a high voltage and the strength is lowered, so that the electrical resistance between the two increases and the current can be suppressed.

  Moreover, it is also possible to add a voltage foaming agent that decomposes at a high voltage into the above-mentioned voltage resolving adhesive layer. In this case, the resin constituting the voltage decomposing adhesive layer does not necessarily need to be decomposed at a high voltage, and any of those that are decomposed at a high voltage and those that are not decomposed at a high voltage can be used. As the voltage foaming agent, a substance capable of decomposing at a high voltage such as carbonates such as lithium, zinc and lead, azo compounds, nitroso compounds, hydrazine derivatives and bicarbonates can be used. For example, it was confirmed that 4,4'-oxybis (benzenesulfonylhydrazide), which is a hydrazine derivative, is decomposed at 4.4 to 4.8V. Since the current positive electrode potential of the lithium ion secondary battery is around 4.2 V, 4,4′-oxybis (benzenesulfonylhydrazide) is decomposed and generates gas in the overcharged voltage range. As a result, the voltage decomposing adhesive layer 9 is broken, the strength as the adhesive layer is lowered, and the electric resistance is increased as in the case of using the voltage decomposable resin.

  A non-aqueous electrolyte is used as the electrolyte (not shown). The type of the non-aqueous electrolyte is not particularly limited, such as a solution in which a supporting salt is dissolved in a solvent such as an organic solvent, an ionic liquid that is an electrolyte and solvent, a solution in which a supporting salt is further dissolved in the ionic liquid, etc. Can be mentioned.

  As the organic solvent, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones, oxolane compounds and the like can be used. A mixed solvent such as propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate can also be used.

The supporting salt used for the non-aqueous electrolyte is not particularly limited. For example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF _{3 SO 2) 3, LiN (FSO} 2) 2, LiN (CF 3 SO 2) (C 4 F 9 SO 2), mention may be made of _LiN (CF 3 _{SO 2) 2} and the like.

  The ionic liquid used for the non-aqueous electrolyte is not particularly limited as long as it is a salt that is liquid at room temperature. For example, alkyl ammonium salt, pyrrolidinium salt, pyrazolium salt, piperidinium salt, imidazolium salt, pyridinium salt, sulfonium Examples thereof include salts and phosphonium salts. Further, it is more preferable that it is electrochemically stable in a wide potential region.

  As described above, according to the lithium ion secondary battery of the present invention, when the lithium ion secondary battery is overcharged and the voltage rises, the positive electrode current collector 2 and the positive electrode lead tab 7 are set at a predetermined voltage. The voltage-resolving adhesive layer 9 provided between the electrodes is decomposed and / or deteriorated, and the strength of the voltage-resolving adhesive layer 9 is reduced to increase the resistance between the positive electrode current collector 2 and the positive electrode lead tab 7. As a result, it limits the current and increases the battery external voltage while suppressing the voltage rise and temperature rise inside the battery, so it can be raised to the stop voltage of the system at an early stage, suppressing thermal runaway caused by overcharged state It becomes possible to do.

  In addition, since the lithium ion secondary battery according to the present invention does not mix an additive with the non-aqueous electrolyte, the electrolyte ion conductivity in the non-aqueous electrolyte does not decrease, and in the overcharged state In the absence, the battery cycle life and the high temperature storage characteristics are not deteriorated.

  In addition, since the lithium ion secondary battery according to the present invention does not introduce thermally expandable microcapsules into the positive electrode, the battery cycle life and the high-temperature storage characteristics are not deteriorated in a state that is not an overcharged state.

  Further, in the lithium ion secondary battery according to the present invention, a voltage decomposition adhesive layer 9 is provided between the positive electrode current collector 2 and the positive electrode lead tab 7 instead of introducing a compound that generates gas into the positive electrode mixture. Therefore, the positive electrode capacity does not decrease due to a decrease in the amount of the active material in the positive electrode mixture.

  Further, the lithium ion secondary battery according to the present invention does not structurally destroy the conductive layer on the positive electrode current collector by the gas generated in the overcharged state, but the voltage decomposing adhesive layer 9 is decomposed and / or decomposed in the overcharged state. Or in order to change and reduce the strength of the voltage resolving adhesive layer 9, it is not necessary to provide any structure for suppressing overcharge to the positive electrode mixture layer 3, and the reliability of current limitation and thermal runaway suppression is ensured. high.

  Hereinafter, specific examples of the present invention will be described.

Example 1
First, as the positive electrode mixture layer 3, 92 parts by mass of LiMn ₂ O ₄ (manufactured by Nippon Chemical Industry), 5 parts by mass of acetylene black (HS-100, manufactured by Denki Kagaku Kogyo), polyvinylidene fluoride (# 7200, Kureha Battery) 3 parts by mass (made by Materials Japan) was added to N-methylpyrrolidone (NMP) and subjected to a dispersion treatment to prepare a homogeneous paste. This paste was applied onto a 20 μm-thick aluminum foil as the positive electrode current collector 2 and subjected to a drying treatment, whereby a positive electrode mixture layer 3 was obtained. The film thickness of the positive electrode mixture layer 3 after the drying treatment was about 100 μm.

After compressing the positive electrode mixture layer 3 using a press so that the density of the positive electrode mixture layer 3 is 2.8 g / cm ³ , the positive electrode mixture layer 3 is 3 cm × 3 cm, and the positive electrode current collector 2 is A positive electrode plate was cut out so as to be included in a size of 2 cm × 3 cm.

  Next, 30 parts by mass of acetylene black (HS-100, manufactured by Denki Kagaku Kogyo) and 70 parts by mass of polyester A (molecular weight: 22,000, Tg: 72 ° C.) are added to a mixed solvent of cyclohexanone and isophorone, followed by dispersion treatment. To prepare a homogeneous paste. Hexamethylene diisocyanate-based curing agent is added in an equivalent amount to the OH group of polyester A, and printed on the portion where the positive electrode lead tab 7 on the positive electrode current collector 2 of the positive electrode cut out previously is bonded by screen printing, A voltage decomposition adhesive layer 9 was provided by performing a drying process. The thickness of the voltage-resolved adhesive layer 9 after drying was 10 μm. After the positive electrode lead tab 7 was temporarily fixed thereon, aging was performed at 80 ° C. for 5 days, and the positive electrode current collector 2 and the positive electrode lead tab 7 were bonded simultaneously with the thermosetting of the voltage resolving adhesive layer 9.

  Next, 92 parts by mass of high-rate SMG (manufactured by Hitachi Chemical Co., Ltd.), 5 parts by mass of graphite (SFG6, manufactured by TIMCAL), and polyvinylidene fluoride (# 7200, manufactured by Kureha Battery Materials Japan) are used as the negative electrode mixture layer 5. 3 parts by mass was added to N-methylpyrrolidone (NMP) and dispersed to prepare a homogeneous paste. This paste was applied onto a 12 μm-thick copper foil as the negative electrode current collector 4 and subjected to a drying treatment, whereby a negative electrode mixture layer 5 was obtained. The film thickness of the negative electrode mixture layer 5 after the drying treatment was about 70 μm.

After the negative electrode mixture layer 5 is compressed using a press so that the density of the negative electrode mixture layer 5 is 1.8 g / cm ³ , the negative electrode mixture layer 5 is 3.2 cm × 3.2 cm, and the negative electrode collector It cut out so that the electric body 2 might be contained by the magnitude | size of 2 cm x 3.2 cm, and was set as the negative electrode plate.

  The negative electrode lead tab 8 was bonded to the negative electrode current collector portion of the negative electrode plate with an ultrasonic welding machine.

  A separator made of polypropylene (Hypore, manufactured by Asahi Kasei E-Materials) as a separator 6 is cut out in a size of 4 cm × 8 cm, and the positive electrode plate and the positive electrode mixture layer 3 and the negative electrode mixture layer 5 face each other on the separator surface. A negative electrode plate was laminated.

  As the exterior material 11 of the battery 1, aluminum pouch sheets were cut out and arranged above and below the laminated electrode plates and separators. Next, two sides including the sealant 10 of the electrode plate positive electrode lead tab 7 and the negative electrode lead tab 8 and the other side were heat sealed with a seal width of about 2 mm.

  After the whole was vacuum-dried, an electrolyte solution was injected from the remaining one side that was not heat-sealed. The remaining one side was heat-sealed after deaeration. As an initial charge, after performing constant current charge at 0.1 C to 4.25 V, in order to remove the generated gas, one side that was sealed last was opened, and after deaeration, heat sealing was performed again. Such a battery was obtained.

(Example 2)
As resin of the voltage decomposition adhesive layer 9, acrylic polyol resin A (molecular weight: 10,000, Tg: 88 ° C.), lithium carbonate as a voltage decomposition foaming agent is added in an amount of 10 parts by mass with respect to 100 parts by mass of the acrylic polyol resin A, A battery according to Example 2 was produced by the same material and method as in Example 1 except that an equivalent amount of an isocyanate curing agent was added.

(Comparative example)
A battery according to a comparative example was produced by the same material and method as in Example 1 except that the positive electrode lead tab 7 was directly ultrasonically welded to the positive electrode current collector 2 without using the voltage resolving adhesive layer 9.

(Voltage decomposition evaluation)
The voltage-resolved adhesive used in Examples 1 and 2 was applied to a 20 μm-thick aluminum foil and dried, and punched into a 13.5 mm circle. A coin cell was prepared using a counter electrode as a lithium foil, and voltage was measured by cyclic voltammetry. The degradation behavior was measured. As a result, an oxidative decomposition current started to be observed in the vicinity of 4.5 to 4.7 V in all cases. Therefore, it was confirmed that the voltage decomposing adhesive layer 9 was decomposed in this voltage range.

(Battery evaluation)
The batteries obtained in Examples 1 and 2 and the comparative example were charged with a constant current and a constant voltage up to 4.3 V, and then discharged with a constant current up to 3.0 V. More specifically, after performing charge / discharge at 0.1 C twice, charge / discharge at 0.2 C was performed once. Thereafter, constant current and constant voltage charging was performed at 0.2 C up to the power supply voltage of the apparatus of 15 V, and the time required for the battery voltage to reach 15 V and the final temperature reached were measured. The temperature was measured by attaching a thermocouple to the battery case 11. Table 1 shows the results of the elapsed time until reaching 15V and the battery temperature when reaching 15V in the batteries obtained in Examples 1 and 2 and the comparative example.

[Test results]
From the overcharge characteristics of the batteries shown in Table 1, in the batteries of Examples 1 and 2 using the voltage resolving adhesive layer 9, the time required for the voltage to rise to 15 V is short, so it can be said that the voltage rise rate is fast. . This is presumably because the resistance of the bonded portion increased due to the decomposition of the voltage-resolved adhesive layer 9. On the other hand, since the battery of the comparative example does not include the voltage resolving adhesive layer 9, no resistance increase occurs between the positive electrode current collector 2 and the positive electrode lead tab 7, and the voltage increase rate is very slow.

  Further, the temperature when reaching 15 V is as low as 85 to 90 ° C. in Examples 1 and 2, indicating that the actual voltage of the electrode portion does not increase and heat generation is small. At this temperature, there is a margin up to around 130 ° C., which is said to be the temperature at which thermal runaway starts, and it is considered that the risk of thermal runaway is low. On the other hand, in the comparative example, since the voltage of the electrode portion was high, the battery temperature rose to 120 ° C., which is near the melting point of the separator 6. It is clear that the temperature of the battery further rises when measured with a device with a high power supply voltage, and there is a risk of reaching a temperature at which thermal runaway starts from an exothermic reaction such as a short circuit due to melting of the separator 6 or decomposition of the electrolyte. It is considered expensive.

  The present invention can be used for a lithium ion secondary battery with high safety.

1: Battery 2: Positive electrode current collector 3: Positive electrode mixture layer 4: Negative electrode current collector 5: Negative electrode mixture layer 6: Separator 7: Positive electrode lead tab 8: Negative electrode lead tab 9: Voltage decomposition adhesive layer 10: Tab sealant 11: Exterior material

Claims (4)

  In the lithium ion secondary battery, comprising: a positive electrode current collector; a positive electrode mixture layer provided on the positive electrode current collector and including at least a positive electrode active material; and a positive electrode lead tab serving as an energization path to the outside. A lithium ion secondary battery comprising a voltage resolving adhesive layer mainly composed of a material that decomposes at a predetermined voltage between an electric body and the positive electrode lead tab.   2. The lithium ion secondary battery according to claim 1, wherein the material constituting the voltage decomposing adhesive layer includes a resin material that is oxidatively decomposed at the predetermined voltage. 3.   The lithium ion secondary battery according to claim 1, wherein the material constituting the voltage decomposing adhesive layer includes a voltage decomposing additive material that is oxidatively decomposed at the predetermined voltage.   2. The lithium ion secondary battery according to claim 1, wherein the material constituting the voltage decomposing adhesive layer includes both a resin material that is oxidatively decomposed at the predetermined voltage and a voltage decomposing additive material. 3. .

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