JP2009295470A - Lithium ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composition for a battery which can greatly contribute to capacity increase, productivity enhancement and furthermore cost reduction in a lithium ion secondary battery. <P>SOLUTION: The lithium ion secondary battery provided with at least a positive electrode, nonaqueous electrolyte and a negative electrode uses steel foil which is covered by aluminum or aluminum based alloy as a negative electrode collector and the negative electrode operates at a potential which is 0.5 V or more and 2.5 V or less (v. s. Li/Li+) inside the lithium secondary battery. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, new energy alternatives to fossil fuels such as petroleum and development of efficient energy utilization technologies have been promoted for environmental protection. In April 2003, with the enforcement of the Act on Special Measures Regarding the Use of New Energy by Electric Power Companies, etc., further reduction of carbon dioxide emissions and diversification of energy sources, further increase of new energy such as solar power generation and wind power generation Promotion of dissemination. However, these new energies are easily affected by nature and are unstable power supplies. That is, when a large number of power systems are connected, it is expected that not only the maintenance of the frequency but also the operation of a centralized power source such as thermal power generation will cause a great trouble, and the operation of the power system will be difficult. Therefore, when a large amount of new energy is introduced, it is necessary to smooth the output by the power storage technology, to store the new energy generated power at light loads such as at night.

  Therefore, by developing technologies such as the storage battery main body and the power storage system, it is possible to avoid adverse effects on the power system due to output fluctuations of new energy. As storage battery technologies applied to these, sodium-sulfur batteries (NAS batteries), redox flow batteries, and lead storage batteries are applied to NEDO demonstration test research. On the other hand, due to the rapid development in recent years, the performance of lithium ion secondary batteries and nickel metal hydride secondary batteries has been remarkably improved, and there is great expectation for application due to the increase in size.

  At present, most of the secondary batteries installed in portable electronic devices such as mobile phones and notebook computers in Japan are small lithium-ion secondary batteries for consumer use. In addition, lithium-ion secondary batteries are expected to be put into practical use not only for grid-connected storage batteries as described above, but also for large-scale batteries for in-vehicle use such as hybrid vehicles. Is growing more and more. Therefore, in the development of lithium ion secondary batteries, weight reduction, high capacity, high input / output and high safety are required.

  In such a situation, as one of the techniques for improving the output characteristics of the lithium ion secondary battery, for example, an active material having a specific average particle diameter and a BET specific surface area and a conductive auxiliary agent are contained in a predetermined ratio. It has been proposed to use an electrode in which an active material layer is formed on a current collector (see, for example, Patent Document 1). Moreover, this patent document 1 describes that an aluminum foil, a copper foil, a stainless steel foil or the like may be used as a current collector.

JP 2008-21614 A

The thinner metal foil used for the current collector contributes to higher capacity and lighter weight, and the metal foil has been increasingly thinned in recent years. However, in the case of aluminum foil or copper foil, there is a problem that if the foil is thin and the strength is low in the process of applying the active material, the foil easily breaks in the line when passing through the coating line, which hinders stable continuous production. Furthermore, when a copper foil is used as the current collector for the negative electrode, there is also a problem that the copper surface of the current collector is dissolved due to an increase in potential of the negative electrode when an overdischarge state occurs. In particular, this phenomenon becomes conspicuous under a high temperature environment, and there is a problem that the capacity is deteriorated due to a decrease in current collection efficiency. On the other hand, in the case of stainless steel foil, although foil strength is obtained, there are not only problems in weight reduction, film thickness reduction and flexibility, but also material costs are high.
Therefore, the present invention has been made in view of the above problems, and a battery configuration that can greatly contribute to increase in capacity, productivity, and cost reduction of a lithium ion secondary battery, particularly An object is to provide a negative electrode.

Therefore, the present inventors diligently studied the characteristics necessary for the negative electrode in order to solve the above-mentioned problems, and as a result of repeatedly investigating the suitability as a negative electrode for various materials, a steel foil coated with aluminum or an aluminum-based alloy. (Hereinafter sometimes abbreviated as aluminized steel foil) is used as a negative electrode current collector, and the negative electrode is 0.5 V or more and 2.5 V or less (vs. Li ^/ Li ⁺ ) in a lithium ion secondary battery. The present invention has been achieved by finding that it has sufficient corrosion resistance and strength as a current collector for a negative electrode when it is operated at a potential of 5%.
That is, the present invention is a lithium ion secondary battery having at least a positive electrode, a non-aqueous electrolyte, and a negative electrode, wherein a steel foil coated with aluminum or an aluminum-based alloy is used as a negative electrode current collector, and the negative electrode is a lithium ion secondary battery. The lithium ion secondary battery is characterized in that it operates at a potential of 0.5 V or more and 2.5 V or less (vs Li ^/ Li ⁺ ) in the secondary battery.

  ADVANTAGE OF THE INVENTION According to this invention, the battery structure which can contribute greatly to the increase in capacity | capacitance of lithium ion secondary battery, productivity improvement, and also cost reduction can be provided.

The lithium ion secondary battery of the present invention includes at least a positive electrode, a non-aqueous electrolyte, and a negative electrode, uses an aluminum-plated steel foil as a negative electrode current collector, and the negative electrode is 0.5 V to 2.5 V in the lithium ion secondary battery. It operates at a potential of (vs Li ^/ Li ⁺ ) below.

In the present invention, the component composition of the steel sheet to be aluminized is not particularly limited, and, for example, a plain steel cold-rolled steel sheet can be used. Although aluminum plating is not specifically limited, For example, after introduce | transducing a steel plate into a molten aluminum bath, it can perform by spraying wiping gas to the steel plate immediately after pulling up from a plating bath, and adjusting a plating adhesion amount.
There are two types of hot-dip aluminum plating: pure aluminum plating and aluminum-based alloy plating containing Si. In pure aluminum plating, a thick alloy layer is formed at the interface between the plating layer and the base material. Alloy plating is desirable. The Si content is preferably 0.5 to 20% by mass, more preferably 3 to 13% by mass.
The obtained aluminized steel sheet may be passed through a cold rolling mill to form a foil having a total thickness of about 5 to 100 μm. In the present invention, a known surface coating method such as a hot dipping method, a vapor deposition plating method, or an electroplating method can be used as a method for coating the steel foil with aluminum or an aluminum-based alloy.

The negative electrode in this invention is comprised from the aluminum plating steel foil obtained above and the negative electrode active material layer formed on it. The negative electrode active material layer has a negative electrode active material, a conductive agent, a binder, and the like that can insert and desorb lithium ions at an operating potential of 0.5 V or more and 2.5 V or less (vs. Li ^/ Li ⁺ ). Is included.
Examples of the negative electrode active material include titanium oxide particles, and more specifically, Li ₄ Ti ₅ O ₁₂ , TiO _{2 and the} like can be used. As the conductive agent, for example, graphite, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, metal fiber and the like can be used. Examples of the binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene, polypropylene, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and vinylidene fluoride-hexafluoropropylene copolymer. Coalescence etc. can be used.

The positive electrode in the present invention is composed of a positive electrode current collector and a positive electrode active material layer formed thereon. The positive electrode active material layer includes a positive electrode active material capable of inserting and releasing lithium ions, a conductive agent, a binder, and the like.
As the positive electrode current collector and the positive electrode active material layer, known materials that can be used for lithium ion secondary batteries can be used without limitation.

  In the present invention, as the solvent constituting the non-aqueous electrolyte, known solvents that can be used for lithium ion secondary batteries can be used without limitation, and examples thereof include ethylene carbonate (EC), diethyl carbonate (DEC), and propylene. Examples include carbonate, butylene carbonate, dimethyl carbonate, sulfolane, dimethoxyethane, tetrahydrofuran, and dioxolane. These may be used alone or in combination of two or more.

In the present invention, as the solute constituting the nonaqueous electrolyte, a known solute that can be used for a lithium ion secondary battery can be used without limitation. For example, LiClO ₄ , LiPF 6, LiBF ₄ , LiAsF ₆ , LiN ( CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiCF ₃ (CF ₂ ) ₃ SO _{3 and the} like can be mentioned, and these may be used alone or in combination of two or more.

  Hereinafter, although an example and a comparative example explain the present invention still in detail, the present invention is not limited to these.

  C: 0.003% by mass, Al: 0.038% by mass, Si: 0.003% by mass, Mn: 0.12% by mass, P: 0.012% by mass, S: 0.122% by mass, Ni: 0.02% by mass, Cr: 0.02% by mass, Cu: 0.01% by mass, Ti: 0.073% by mass, N: 0.0023% by mass, the thickness having the composition consisting of the balance Fe and inevitable impurities Melting at a bath temperature of 700 ° C. using a molten aluminum base alloy plating bath having a composition of Si: 10% by mass, Fe: 2% by mass and Al: 88% by mass in a 0.5 mm plain steel cold-rolled steel strip Aluminum-based alloy plating was applied. The plating adhesion amount was controlled to 20 μm per side by the gas wiping method. Thereafter, the cold rolling mill was passed through as many times as necessary until the total thickness including the plating layer reached 50 μm. The aluminum-plated steel foil produced by this method was used as the following test material for tensile strength test and current collector for charge / discharge test.

Using a universal precision tensile testing machine, the tensile strength of the aluminum-plated steel foil, copper foil, and aluminum foil as test materials was measured according to the following method.
Measure the maximum load when a specimen with a width of 12.7 mm, a length of 175 mm, and a foil thickness of 50 μm is fixed at a distance between chucks of 125 mm, and pulled at a constant speed of 2 mm per minute. Divided by. At this time, the measured value before the tensile test was used for the cross-sectional area of the specimen. The results are shown in Table 1. The measurement was carried out three times for each test material, and the tensile strength values in Table 1 are average values. As can be seen from Table 1, the aluminized steel foil was found to have excellent tensile strength properties. That is, by using the aluminum-plated steel foil as a current collector, the foil is not cut in the step of applying the active material, and the electrodes can be stably and continuously produced.

<Example>
Lithium titanate (Li ₄ Ti ₅ O ₁₂ , hereinafter referred to as LTO) as an active material capable of inserting and desorbing lithium ions at an operating potential of 0.5 V to 2.5 V (vs Li / Li ⁺ ) And powdered acetylene black as a conductive agent and polyvinylidene fluoride (PVDF) as a binder in a mass ratio of 88: 6: 6, which is mixed with n-methylpyrrolidone ( A slurry was prepared by dispersing in NMP) solvent.

  The slurry was applied onto the previously prepared aluminum-plated steel foil, dried at 80 ° C. for 3 hours, and then pressed. Then, what was dried under reduced pressure at 150 degreeC for 3 hours was made into the LTO electrode.

<Comparative example>
LiFePO ₄ powder as an active material capable of inserting and desorbing lithium ions at an operating potential of more than 2.5 V and 3.9 V or less (vs Li ^/ Li ⁺ ), acetylene black as a conductive agent, Polyvinylidene fluoride (PVDF) as a binder was blended in a mass ratio of 80:10:10 and dispersed in an n-methylpyrrolidone (NMP) solvent to prepare a slurry.

The slurry was applied onto the previously prepared aluminum-plated steel foil, dried at 80 ° C. for 3 hours, and then pressed. Thereafter, those were allowed to 3 hours drying under reduced pressure at 0.99 ° C. was LiFePO ₄ electrode.

<Production of test cell>
A test cell was produced in a glove box (glove box device with a gas circulation purifier) under an argon atmosphere (dew point −110 ° C. or less). A general three-electrode cell was used as a test cell. An LTO electrode or a LiFePO ₄ electrode was used as a test electrode, and metallic lithium was used as a counter electrode and a reference electrode. The cell configuration was set such that the surface area of the counter electrode was sufficiently large relative to that of the test electrode and was regulated by the potential of the test electrode. As an electrolytic solution, 1.0 mol / dm ³ LiClO ₄ / EC + DEC (50:50 vol%) was used for the LTO electrode, and 1.0 mol / dm ³ LiPF ₆ / EC + DEC (50:50 vol%) was used for the LiFePO ₄ electrode. .

<Charge / discharge test>
The charge / discharge operation test was performed at 20 ° C. in a glove box under an argon atmosphere. A test cell was connected to HJR-1010mSM8 (manufactured by Hokuto Denko) and evaluated under the measurement conditions shown in Table 2.

In addition, the capacity | capacitance of an electrode is the following formula | equation:
{Current value (mA) × Time (h) / Mass of active material (g)} = Capacity (mAh / g)
More calculated. Moreover, although the current density was set to 0.1 C, it calculated from 1C = 175 mA / g in the case of the LTO electrode and 1 C = 170 mA / g in the case of the LiFePO ₄ electrode.

<Results of charge / discharge test>
As a result of examining the cycle characteristics of each electrode, the discharge capacity at the first cycle of the LTO electrode was about 150 mAh / g, and the discharge capacity at the 10th cycle was hardly decreased. In contrast, the discharge capacity at the first cycle of the LiFePO ₄ electrode was about 85 mAh / g, but the discharge capacity at the 10th cycle was about 65 mAh / g, which was about 24% lower. Thus, it can be seen from the comparison between the example and the comparative example that the difference in the type of active material, that is, the difference in operating potential affects the cycle characteristics.
From the above results, it is possible to operate a lithium ion secondary battery using an aluminum-plated steel foil as a current collector in a potential range of 1.0 V or more and 2.5 V or less (vs Li ^/ Li ⁺ ). It was revealed that the electrode characteristics were developed.
The cathode polarization corresponding to the insertion reaction of lithium ions into LTO was defined as charging, and the anodic polarization corresponding to the elimination reaction of lithium ions from LTO was defined as discharging. The charging anode polarization corresponding to elimination reaction of lithium ions from LiFePO _4, was defined as the discharge cathode polarization corresponding to the insertion reaction of lithium ions into LiFePO _4.

Claims (4)

A lithium ion secondary battery comprising at least a positive electrode, a non-aqueous electrolyte, and a negative electrode,
A steel foil coated with aluminum or an aluminum-based alloy is used as a negative electrode current collector, and the negative electrode operates at a potential of 0.5 V or more and 2.5 V or less (vs Li / Li +) in a lithium ion secondary battery. The lithium ion secondary battery characterized by the above-mentioned.   The lithium ion secondary battery according to claim 1, wherein the coating is formed by hot dipping.   The lithium ion secondary battery according to claim 1, wherein the negative electrode active material of the negative electrode contains at least titanium oxide particles. The lithium ion secondary battery according to claim 3, wherein the titanium oxide particles are Li ₄ Ti ₅ O ₁₂ or TiO ₂ .