JP2016062872A - Positive electrode for lithium ion secondary battery and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for preventing a lithium ion secondary battery from being subjected to thermal runaway and inflammation caused by overcharging of a lithium ion secondary battery and to provide a lithium ion secondary battery using the same.SOLUTION: The positive electrode of a lithium ion secondary battery is so configured that micro-capsules containing an alkaline solution is mixed in a positive electrode. When being overcharged, a shell material of the micro-capsules is decomposed, and the contained alkaline solution is eluded, which causes erosion of an aluminum collector, and electrical contact between the battery and an external circuit is cutoff.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery with improved safety against overcharging, and a lithium ion secondary battery using the same.

  With the widespread use of electronic devices such as notebook computers, smartphones, and tablet terminals, demand for secondary batteries for driving these electronic devices is increasing. In recent years, the power consumption of these electronic devices has increased with the progress of higher functionality, and miniaturization is expected, so high energy density and high output density are required for secondary batteries. It has been. As secondary batteries that can achieve high energy density and high output density, non-aqueous electrolyte secondary batteries such as lithium ion batteries are considered promising.

  However, a lithium ion secondary battery (hereinafter referred to as a lithium ion battery or simply a battery) is composed of lithium having a high chemical activity, a highly flammable electrolyte, and a lithium transition metal composite oxide having a low stability in an overcharged state. Because it is used as a battery material, if it is overcharged after full charge, the chemical reaction between the battery materials will proceed rapidly, causing the battery to cause thermal runaway, ignition, etc. It is known. For this reason, it is necessary to quickly stop charging before reaching a thermal runaway, 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.

  As an example, Patent Document 1 discloses an electrolyte additive that relieves overcharge by increasing the internal resistance of a battery by oxidizing and polymerizing a material added to the electrolyte due to a voltage increase associated with overcharge. Yes. Specifically, this is a technique in which a compound having a benzene ring such as cyclohexylbenzene or diphenyl ether is oxidatively polymerized to increase the internal resistance of the battery.

  In Patent Document 2, as a technique for increasing electrode resistance due to temperature increase associated with overcharge and suppressing thermal runaway during overcharge, an electrode in which an electrode mixture layer made of a positive electrode material or a negative electrode material is laminated on a current collector Discloses an electrode containing thermally expandable microcapsules in the electrode mixture layer or along the interface between the electrode mixture layer and the current collector.

Japanese Patent No. 3938194 Japanese Patent No. 4727021

  However, as shown in Patent Document 1, when an additive that suppresses overcharge is mixed in the electrolyte, overcharge is suppressed for a certain period of time, that is, while the additive is oxidatively polymerized. When the oxidative decomposition of the additive was finished, there was a problem that overcharge proceeded again.

  In the technique disclosed in Patent Document 2, even if the battery temperature rises and the microcapsule thermally expands, if a part of the electrode is in contact with the current collector, the overcharge reaction eventually proceeds and thermal runaway occurs. Therefore, the essential solution has not been reached.

  This invention is made | formed in view of such a subject, The objective is to provide the electrode which prevents the thermal runaway and ignition by the overcharge of a lithium ion battery, and a lithium ion battery using the same.

  One aspect of the present invention for solving the above problem is a positive electrode of a lithium ion battery, in which microcapsules containing an alkaline solution are mixed in the positive electrode, and the microcapsules are in an overcharged state. The shell material is decomposed, the contained alkaline solution is eluted, and the aluminum current collector is corroded, whereby the electrical contact between the battery and the external circuit is interrupted.

  According to the present invention, even when the lithium ion battery is overcharged, the alkaline solution eluted from the inside of the microcapsule corrodes the aluminum current collector, so that the electrical contact between the battery and the external circuit is interrupted. Can prevent overcharge and prevent battery runaway and ignition.

Cross-sectional schematic diagram of positive electrode for laminated battery according to Examples 1 to 4 and Comparative Example 2 Cross-sectional schematic diagram of negative electrode for laminated battery Schematic diagram after punching positive electrode for laminate battery Schematic diagram after punching negative electrode of laminated battery Schematic diagram of positive electrode for laminated battery encapsulated in separator Schematic diagram of negative electrode for laminated battery encapsulated in separator Schematic diagram of electrode body Cross-sectional schematic diagram of the electrode body Schematic diagram of laminated battery Cross-sectional schematic diagram of a positive electrode for a laminated battery according to Comparative Example 1 Schematic diagram of coin battery

  Hereinafter, a positive electrode for a lithium ion battery according to the present invention and a lithium ion battery using the positive electrode will be described.

  As a result of intensive studies to solve the problem of preventing thermal runaway and ignition of the battery at the time of overcharge, the inventor mixed the microcapsule containing an alkaline solution in the positive electrode of the lithium ion battery, In this case, the shell material of the microcapsule is decomposed, the encapsulated alkaline solution is eluted, and the aluminum current collector is corroded. It has been found that it prevents battery runaway and ignition. According to the present invention, it is possible to improve safety without compromising battery performance while avoiding an increase in cost due to introduction of additive materials without complicating the manufacturing process of the positive electrode.

(Positive electrode)
In the positive electrode of the lithium ion battery according to one embodiment of the present invention, microcapsules containing an alkaline solution are mixed, and the shell material of the microcapsules is decomposed and encapsulated when overcharged. The alkaline solution is eluted and corrodes the aluminum current collector, so that the electrical contact between the battery and the external circuit is interrupted.

  For example, known materials such as acetylene black, ketjen black, carbon black, graphite, and carbon nanotube can be used as the conductive assistant contained in the positive electrode.

  The shell material of the microcapsule included in the positive electrode is preferably a material that has alkali resistance, NMP (N-methylpyrrolidone) resistance, and electrolytic solution resistance, and that decomposes at potential during overcharge. For example, acrylonitrile, vinylidene chloride, methyl methacrylate, or two or three kinds of copolymers of these materials may be mentioned. Further, the decomposition start voltage of the shell material of the microcapsule is preferably, for example, 4.5 V or more and 4.8 V or less.

  The alkali solution encapsulated in the microcapsules is not particularly limited as long as it can corrode the aluminum current collector, and examples thereof include sodium hydroxide solution and potassium hydroxide solution.

  As the positive electrode current collector, a current collector made of aluminum can be used.

The positive electrode active material contained in the positive electrode is not particularly limited, and a conventionally known active material can be used. Examples of the positive electrode active material include a lithium transition metal composite oxide capable of releasing lithium ions, and examples thereof include LiNiO ₂ , LiMnO ₂ , LiCoO ₂ , and LiFePO ₄ . Further, as the positive electrode active material, a plurality of lithium transition metal oxides can be mixed and used.

  The positive electrode can be formed by mixing a positive electrode active material, a binder, a conductive auxiliary agent, and the like in a solvent such as N-methylpyrrolidone, then laminating and drying on an aluminum current collector.

(Negative electrode)
The negative electrode active material contained in the negative electrode is not particularly limited, and occludes lithium ions such as metal materials such as lithium, alloy materials containing silicon and tin, and carbon materials such as graphite and coke. The releasable compounds 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. When using an alloy material or carbon material as the negative electrode active material, the negative electrode active material, a binder, a conductive additive, 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 is preferably a chemically and physically stable material such as polyvinylidene fluoride, polytetrafluoroethylene, EPDM, SBR, NBR, or fluororubber. Examples of the conductive assistant include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, and amorphous carbon.

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

(Nonaqueous electrolyte)
The non-aqueous electrolyte is not particularly limited, and a non-aqueous electrolyte 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, and the like. 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.

(battery)
As a separator for preventing contact between the positive electrode and the negative electrode, it is possible to use a microporous film or nonwoven fabric made of polyolefin such as polyethylene or polypropylene or an aromatic polyamide resin, a porous resin coat containing inorganic ceramic powder, or the like. it can.

  The positive electrode, negative electrode, non-aqueous electrolyte, and separator are stored in a rectangular or cylindrical container made of aluminum or stainless steel, or a container such as an aluminum laminate film for the purpose of preventing leakage of the electrolyte or preventing outside air from entering. Thus, a lithium ion battery can be manufactured.

  The tab connecting the electrode and the terminal of the container is not particularly limited, and a tab made of aluminum, stainless steel, nickel or the like can be used for the positive electrode, and a tab made of copper, nickel or the like can be used for the negative electrode. The tab is accompanied by a resin made of modified polypropylene called sealant, which maintains the insulation between the tab and the aluminum laminate film.

  Hereinafter, the present invention will be described using examples.

Example 1
(Preparation of positive electrode)
In FIG. 1, the cross-sectional schematic diagram of the positive electrode for laminated batteries which concerns on Example 1 is shown. 89 parts by mass of LiNi _0.5 Co _0.2 Mn _0.3 O ₂ (manufactured by Nippon Chemical Industry) as the positive electrode active material 11, 3 parts by mass of acetylene black (HS-100, manufactured by Denki Kagaku Kogyo) as the conductive auxiliary agent 12, 3 parts by mass of polyvinylidene fluoride (# 7200, manufactured by Kureha Battery Materials Japan) as a binder, a microcapsule 14 having an average particle diameter of 9 μm or more and 15 μm or less in which the shell is acrylonitrile and the sodium hydroxide solution is encapsulated 5 parts by mass was added to NMP and dispersed to prepare a homogeneous paste. This paste was applied onto the aluminum foil current collector 13 with a coating machine such as a die coater and dried to obtain the positive electrode layer 10. The film thickness of the positive electrode layer 10 after the drying treatment was 70 μm on one side and 140 μm on both sides. The positive electrode after the drying treatment was subjected to pressure treatment with a roll press so that the density of the positive electrode layer 10 was about 3.3 g / cm ³ .

(Anode production)
FIG. 2 is a schematic cross-sectional view of the negative electrode for a laminated battery according to Example 1. Materials used for the negative electrode layer 21 formed on the copper foil current collector 22 include 96 parts by mass of spherical natural graphite (SMG, manufactured by Hitachi Chemical Co., Ltd.) as an active material, and vapor grown carbon fiber (VGCF-H) as a conductive additive. 2 parts by weight of Showa Denko) and 2 parts by weight of styrene butadiene rubber (BM-400B, manufactured by Nippon Zeon) as a binder are added to an aqueous carboxymethyl cellulose solution (DN800-H, manufactured by Daicel), and subjected to dispersion treatment. A uniform paste was prepared. In addition, carboxymethylcellulose was prepared so that it might be 1 weight part with respect to 100 weight part of the said composition. This paste was applied onto a copper foil current collector 22 (12 μm thick) with a coating machine such as a die coater and dried to obtain a negative electrode layer 21. The negative electrode layer 21 after the drying treatment was 60 μm on one side and 120 μm on both sides. The negative electrode layer 21 was subjected to pressure treatment with a negative electrode roll press after the drying treatment so that the density of the negative electrode layer 21 was about 1.7 g / cm ³ .

(Laminated battery production)
FIG. 3 shows a schematic diagram after punching of the positive electrode for a laminated battery, and FIG. 4 shows a schematic diagram after punching of the negative electrode for a laminated battery. The obtained positive electrode 44 was punched as shown in FIG. 3, and the negative electrode 45 was punched as shown in FIG. The positive electrode dimension was 60 mm × 80 mm, and the end 10 mm × 80 mm was an aluminum current collector. The negative electrode dimension was 62 mm × 82 mm, and the end 12 mm × 82 mm was a copper foil current collector. FIG. 5 is a schematic view of a positive electrode for a laminated battery contained in the separator 31, and FIG. 6 is a schematic view of a negative electrode for a laminated battery contained in the separator 31. 7 shows a schematic diagram of the electrode body 52, and FIG. 8 shows a schematic sectional view thereof. Each of the prepared electrodes (positive electrode 44, negative electrode 45) is wrapped with a polyethylene separator 31 (Hypore, manufactured by Asahi Kasei E-Materials) (FIGS. 5 and 6), and each electrode is laminated as shown in FIG. Thus, one electrode body 52 was obtained (FIG. 7). An aluminum tab 42 (4 mm × 92 mm, thickness 150 μm) was used for the positive electrode 44, and a nickel tab 43 (4 mm × 92 mm, thickness 150 μm) was used for the negative electrode 45. Each tab 42, 43 was provided with a sealant 41.

In FIG. 9, the cross-sectional schematic diagram of a laminated battery is shown.
The obtained electrode body 52 was enclosed in an aluminum laminate film 51. Then, LiPF ₆ was added to a mixed organic solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 so as to have a concentration of 1 mol / L, and vinylene carbonate was further added by 2% by weight. The non-aqueous electrolyte prepared above was injected, and the aluminum laminate film 51 was thermally welded to produce a laminate battery.

(Example 2)
A laminated battery was produced in the same manner as in Example 1 except that vinylidene chloride was used as the shell material of the microcapsule 14 added to the positive electrode 44.

(Example 3)
A laminated battery was produced in the same manner as in Example 1 except that methyl methacrylate was used as the shell material of the microcapsule 14 added to the positive electrode 44.

Example 4
A laminated battery was produced in the same manner as in Example 1 except that a copolymer of acrylonitrile, vinylidene chloride, and methyl methacrylate was used as the shell material of the microcapsule 14 added to the positive electrode 44.

(Comparative Example 1)
In FIG. 10, the cross-sectional schematic diagram of the positive electrode for laminated batteries which concerns on the comparative example 1 is shown.
A laminated battery was produced in the same manner as in Example 1 except that the microcapsule 14 was not added in Example 1 and the mass part of the microcapsule 14 was replaced with the positive electrode active material 11.

(Comparative Example 2)
A laminated battery was produced in the same manner as in Example 1 except that ethanol was used as the alkaline solution in the microcapsule 14 in Example 4.

(Microcapsule potential decomposition investigation)
94 parts by mass of each microcapsule without inclusion material, 3 parts by mass of acetylene black (HS-100, manufactured by Denki Kagaku Kogyo), 3 parts by mass of polyvinylidene fluoride (# 7200, manufactured by Kureha Battery Materials Japan) are used as NMP. The mixture was added and dispersed to prepare a homogeneous paste. The paste was applied on an aluminum foil current collector with a coating machine such as a die coater and dried to obtain an electrode for 70 μm microcapsule evaluation. The electrochemical behavior of the electrode was investigated. Specifically, a bipolar cell having the above electrode as a working electrode and a lithium metal as a counter electrode was prepared, and a potentio / galvanostat device (1287 type, manufactured by Solartron) and a frequency response analyzer (1260 type, manufactured by Solartron) Was used to perform cyclic voltammetry (CV) measurement by sweeping at a sweep rate of 5 mV / s and a potential range of 3.0 V to 5.0 V.

In CV measurement, the voltage at which the oxidation current was observed at 0.05 mA / cm ² was defined as the oxidation start potential of the microcapsule shell material.

  Table 1 shows the results. When comparing Examples 1 to 4, Comparative Example 1 and Comparative Example 2 in Table 1, oxidative decomposition does not show up to about 5.5 V without the microcapsules, whereas with the microcapsules, it is 4.5 V or more. It is suggested that oxidative degradation starts at the potential. From this, it is considered that the microcapsule is decomposed immediately after the start of overcharge, and even if the overcharge starts accidentally, it is considered that the decomposition of the shell starts immediately after the start of overcharge and the elution of the encapsulated material begins. It can be said that the shell material used is a very good material for overcharge countermeasures.

(Overcharge test)
Ten laminated batteries were manufactured, and the battery voltage was charged to 4.2 V by constant current charging first, and was fully charged. Thereafter, an overcharge test was performed for a predetermined time (for example, 1.5 hours) with 1C charging, and the number of smoke and ignition of the battery was investigated. When the battery voltage reached the upper limit voltage of the charging / discharging device or 0 V within a predetermined time, the test was terminated at that time.

  Table 2 shows the results of the overcharge test. The average test end voltage in Table 2 indicates the test end voltage of 10 batteries. When the battery voltage reaches 0 V, the average test end voltage indicates the battery voltage just before that.

  From the results, it can be seen that all the batteries of the examples were cut off from electrical contact in the vicinity of 5 V, while all of the batteries of the comparative examples were overcharged and ignited. From this, it is suggested that the battery of the example breaks down the shell material of the microcapsule during overcharge, and the inclusion material corrodes the aluminum current collector, thereby blocking the electrical contact between the battery and the external circuit. The

  The lithium ion secondary battery using the microcapsule shown in the example suppresses the thermal runaway by blocking the electrical contact between the battery and the external circuit at the time of overcharging, that is, the thermal runaway suppressing ability is extremely high. It was confirmed that the battery was excellent.

  As described above, according to the present invention, it is possible to produce an excellent battery having a thermal runaway suppression capability only by adding microcapsules to the positive electrode, and it is possible to suppress complication of the manufacturing process. . Therefore, the manufacturing process of a normal lithium ion secondary battery can be used as it is, and the increase in electrode manufacturing cost can also be suppressed.

  The present invention is not limited to a laminate type, but includes a conductive additive 12, an aluminum foil current collector 13, a separator 31, a metal lithium 62, a gasket 63, a positive electrode can 64, and a negative electrode can as shown in FIG. It can be used for electrodes of various lithium ion secondary batteries such as coin type, square type, and cylindrical type.

  The present invention is useful for lithium ion secondary batteries and the like.

DESCRIPTION OF SYMBOLS 10 Positive electrode layer 11 Positive electrode active material 12 Conductive support agent 13 Aluminum foil collector 14 Microcapsule 21 Negative electrode layer 22 Copper foil collector 31 Separator 41 Sealant 42 Positive electrode tab 43 Negative electrode tab 44 Positive electrode 45 Negative electrode 51 Aluminum laminate film 52 Electrode body 62 Lithium metal 63 Gasket 64 Positive electrode can 65 Negative electrode can

Claims (4)

  A positive electrode of a lithium ion battery, in which microcapsules containing an alkaline solution are mixed in the positive electrode, and when the battery is overcharged, the shell material of the microcapsules is decomposed and the contained alkaline solution is eluted. And the positive electrode by which an electrical contact with a battery and an external circuit is interrupted | blocked by corroding an aluminum electrical power collector.   The positive electrode according to claim 1, wherein a decomposition starting voltage of the shell material of the microcapsule is 4.5 V or more and 4.8 V or less.   The positive electrode according to claim 1, wherein the shell material of the microcapsule is acrylonitrile, vinylidene chloride, methyl methacrylate, or a copolymer of two or three of these materials.   The lithium ion battery which comprised the positive electrode in any one of Claims 1-3.