JP5426809B2 - Secondary battery, electronic equipment using secondary battery and transportation equipment - Google Patents

Description

  The present invention relates to a secondary battery, and more particularly to a lithium ion battery. The present invention also relates to an electronic device using a secondary battery and a transport device.

  2. Description of the Related Art Conventionally, secondary batteries that can be used repeatedly through charging and discharging are used in various fields. Representative examples of the secondary battery include a lead storage battery, a nickel metal hydride battery, and a lithium ion battery. Among these, the lithium ion battery has a large electromotive force and can be reduced in size and weight, and thus is useful in various fields.

Conventional lithium ion batteries generally have the following configuration (see, for example, Patent Document 1).
That is, the lithium ion battery includes a positive electrode and a negative electrode that can occlude and release lithium ions, and the positive electrode and the negative electrode are stacked in a state of being separated from each other by a separator formed of a microporous film. This laminated body is accommodated in a case of arbitrary shape, and a non-aqueous electrolyte is injected. This non-aqueous electrolyte contains an effective amount of a lithium salt as an electrical conductivity imparting agent.
The positive electrode is a thin film-coated aluminum foil having a thickness of about 20 μm, and the negative electrode is a thin film-coated copper foil having a thickness of about 10 to 20 μm.

  Such lithium ion batteries have already been put into practical use, for example, for small consumer devices (such as personal computers, mobile phones, and portable music players). However, in the automobile industry, nickel metal hydride batteries and the like have already been used as energy sources for electric cars and hybrid cars, but various problems still remain in the practical use of lithium ion batteries.

  As one of them, from the viewpoint of improving fuel efficiency, improvement in regeneration acceptability is required. In order to improve the regenerative acceptance, it is important to reduce the internal resistance of the battery and to increase the potential difference until full charge by eliminating the flatness of the charge / discharge curve.

  In order to reduce the internal resistance of the battery, it is necessary to reduce the resistance of the non-aqueous electrolyte described above. Examples of conventionally used solvents for non-aqueous electrolytes include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. Among these, when a carbon material is used for the negative electrode, ethylene carbonate is generally used in terms of excellent stability.

  However, since the freezing point of ethylene carbonate is relatively high, the ionic conductivity is remarkably lowered in a low temperature environment, and the resistance of the nonaqueous electrolytic solution is increased. Therefore, it is conceivable to take measures to increase the ionic conductivity by accelerating the movement speed of lithium ions by mixing with a low viscosity solvent such as dimethyl carbonate.

On the other hand, in the above-described lithium ion battery, since the lithium salt is contained in an effective amount in the non-aqueous electrolyte, the charge / discharge curve is flat. Therefore, in order to eliminate the flatness of the charge / discharge curve, it is conceivable to employ non-graphitizable carbon instead of graphite as the negative electrode material.
Japanese Patent Laid-Open No. 11-121022

However, the above-described conventional technology has the following problems.
First, in order to obtain sufficient ionic conductivity, it is necessary to add a considerable amount of a low-viscosity solvent. Since the low-viscosity solvent is highly volatile, the non-aqueous electrolyte is affected by the generated volatile gas. May become flammable and safety may be reduced.

  Secondly, in order to produce non-graphitizable carbon, it is necessary to use an expensive substance such as furfuryl alcohol resin or phenol resin as a starting material and to precisely control the firing atmosphere. For this reason, non-graphitizable carbon is more expensive than graphite. In consideration of the environment, when a water-based binder is used as the binder for the negative electrode, the moisture adsorbed on the non-graphitizable carbon may not be completely removed and may remain. Then, the remaining moisture is decomposed at the time of the first charge and generates gas, which may reduce safety.

  Therefore, the present invention has been made in view of the above problems, and an inexpensive lithium ion battery that can improve regenerative acceptance and safety, an electronic device using the lithium ion battery, and a transport device are provided. The purpose is to provide.

  The present inventors have found that an excellent electrical conductivity can be obtained without adding a lithium salt by containing an effective amount of an ionic liquid in a non-aqueous electrolyte, and the present invention has been completed. . Specifically, the present invention provides the following.

(1) A secondary battery comprising a positive electrode capable of occluding and releasing lithium ions, a negative electrode capable of occluding and releasing lithium ions, and a non-aqueous electrolyte interposed between the positive and negative electrodes,
The nonaqueous electrolytic solution is a secondary battery containing an effective amount of an electrical conductivity imparting agent made of an ionic liquid and not containing 0.5 mol / L or more of a lithium salt.

  Here, the “ionic liquid” refers to a room temperature molten salt that stably exists as a liquid at room temperature. The “effective amount” refers to an amount that can provide a desired electrical conductivity, and may be appropriately set according to the type of ionic liquid, the content of lithium salt, and the like.

  (2) The secondary battery according to claim 1, wherein the non-aqueous electrolyte contains 3.0 mol / L or less of the ionic liquid.

・・・化学式１
（式中、Ｒ_１〜Ｒ_４はアルキル基又はアルコキシル基であり、Ｘはビストリフルオロメチルスルフォニルイミド（ＴＦＳＩ）又はビスペンタフルオロエタンスルフォニルイミド（Ｂｅｔｉ）である。） (3) The secondary battery according to (1) or (2), wherein the ionic liquid is an ionic liquid represented by Chemical Formula 1.

... Chemical formula 1
(In the formula, R _{1 to} R ₄ are an alkyl group or an alkoxyl group, and X is bistrifluoromethylsulfonylimide (TFSI) or bispentafluoroethanesulfonylimide (Beti).)

(4) The secondary battery according to (3), wherein R _{1 to} R ₄ in Chemical Formula 1 are an alkyl group or an alkoxyl group having 3 to 7 carbon atoms.

  (5) The non-aqueous electrolyte further contains an organic solvent for diluting the ionic liquid, and the organic solvent is at least one selected from the group consisting of a chain carbonate, a cyclic ester, and a chain ester, and The secondary battery according to any one of (1) to (4), which is / or a cyclic carbonate.

  (6) An electronic device using the secondary battery according to any one of (1) to (5).

  (7) A transport device using the secondary battery according to any one of (1) to (5).

According to the present invention, the non-aqueous electrolyte contains an effective amount of an electrical conductivity imparting agent made of an ionic liquid and does not contain 0.5 mol / L or more of a lithium salt. As a result, the internal resistance of the battery is reduced, the slope of the potential change with respect to the charge capacity of the negative electrode is increased, and the flatness of the charge / discharge curve is lost.
Thus, since regenerative acceptance is improved, cheap graphite can be adopted as a negative electrode material. Since graphite has the property of being hardly affected by moisture, gas generation or the like during initial charging is suppressed. In addition, since the ionic liquid does not cause gas generation due to volatilization, the non-aqueous electrolyte is maintained in flame retardancy.
Therefore, regeneration acceptability and safety can be improved, and cost can be reduced.

  Hereinafter, although embodiment of this invention is described, it does not specifically limit.

<Overall configuration>
The secondary battery of this invention is a lithium ion battery provided with the positive electrode and negative electrode which can occlude / release lithium ion. Further, a separator is interposed between the positive electrode and the negative electrode, and the positive electrode and the negative electrode are isolated by this separator.

  The positive electrode, the negative electrode, and the separator are stacked or wound while being separated from each other by a separator formed of a microporous film. This laminated body or wound body is accommodated in an outer case, and a nonaqueous electrolyte is injected between the positive electrode and the negative electrode. Note that the outer case may be formed of stainless steel or aluminum and does not have a great influence on the battery characteristics. Therefore, any shape such as a cylindrical shape, a rectangular shape, an elliptical shape, or a laminated shape can be adopted.

[electrode]
The electrode is composed of a positive electrode and a negative electrode, and each of the positive electrode and the negative electrode has a sheet shape in which an active material is coated on a current collector foil.

(Positive electrode active material)
Here, the positive electrode active material is a Li-containing transition metal composite oxide capable of occluding and releasing lithium ions. It is preferable that the chargeable / dischargeable capacity is 90 mAh / g or more and the operating voltage exceeds 3.6 V in that the battery (summary substance) mass and the battery volume can be reduced. This characteristic is particularly important when a plurality of batteries are connected in series, such as a battery pack for transportation equipment. Specifically, a lithium cobalt complex oxide (typically LiCoO ₂ ), a lithium nickel complex oxide (typically LiNiO ₂ ), or a complex oxide including a plurality of transition metals (for example, LiNi _{(1 -x-y) -Mn y Co z} O 2) may be mentioned as preferred examples.

(Negative electrode active material)
On the other hand, the negative electrode active material is a carbonaceous material mainly composed of a carbon material and capable of occluding and releasing lithium ions. Specifically, graphite having high crystallinity is a preferable example in terms of excellent initial Coulomb efficiency and cycle life.

(Binder)
Usually, the electrode contains a binder. As the binder, a polymer that is soluble in an organic solvent is preferable. Specifically, polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polypropylene oxide (PPO), and the like are preferable in that they are not decomposed at the potential of the lithium ion battery and are insoluble in the non-aqueous electrolyte. Take as an example.

(Conductive material)
The electrode preferably contains a conductive material in that the resistance can be reduced. As a conductive material, carbon black (acetylene black, etc.) and fine powder such as carbon fiber grown by vapor deposition are used because they have a stable and excellent volume resistivity (less than 100 Ω · cm) with respect to non-aqueous electrolytes. Is preferably used.

  The above electrode is prepared by mixing a polar active material, a conductive material, and a binder with a suitable solvent, and coating the resulting slurry on a suitable current collector foil using a suitable coating device. Can be made.

  Examples of the positive electrode current collector foil include aluminum, titanium, tantalum, and alloys thereof. On the other hand, examples of the negative electrode current collector foil include copper, nickel, and stainless steel. Among these, copper is preferable because it can be easily processed into a thin film and is inexpensive.

[Separator]
The separator is not particularly limited, and may be a separator that has been conventionally used in lithium-ion batteries for small consumer devices. Specifically, a polyolefin-based microporous film such as a nonwoven fabric, polyethylene, or polypropylene can be cited as a preferable example in that the positive electrode and the negative electrode can be separated and the electrolytic solution can be retained.

[Non-aqueous electrolyte]
The non-aqueous electrolyte contains an effective amount of an electrical conductivity imparting agent composed of an ionic liquid.

・・・化学式１ (Ionic liquid)
Examples of the ionic liquid include those having a molecular structure represented by Chemical Formula 1. In Chemical Formula 1, R _{1 to} R ₄ are an alkyl group or an alkoxyl group, and X is any one of bistrifluoromethylsulfonylimide (TFSI), bisperfluoroethanesulfonylimide (Beti), BF ₄ , and PF _6. is there.

... Chemical formula 1

If the carbon number of R _{1 to} R ₄ in Chemical Formula 1 is too small, the ionic liquid may not be formed, and if it is too large, the viscosity becomes high and the movement speed of lithium ions is reduced. Therefore, R _{1 to} R ₄ are preferably 3 or more and 7 or less alkyl groups or alkoxyl groups. More preferably, it is 3-5, More preferably, it is 3-4.

  On the other hand, X in Chemical Formula 1 is more preferably bistrifluoromethylsulfonylimide (TFSI) or bisperfluoroethanesulfonylimide (Beti) in that decomposition and corrosion of the lithium ion battery material can be suppressed.

  By using the ionic liquid as the main component of the supporting salt, the slope of the potential change with respect to the charge capacity of the negative electrode is increased, and the flatness of the charge / discharge curve is lost. Here, FIG. 2 shows the difference in the relationship between the charge state and the input / output density depending on whether the charge / discharge curve is flat.

  As shown in FIG. 2, the input density is large when the flatness is not present, and the slope of the input density with respect to the change in the charging state is large as compared with the case where the flatness is present. For this reason, it turns out that regeneration acceptability is excellent. Further, when the state of charge is low, the output density is moderately suppressed, the input density is much larger than the output density, and there is a sufficient margin. From the above, it can be seen that a secondary battery including a negative electrode having no flatness is also useful as a secondary battery for transportation equipment.

  In order to reduce the resistance of the nonaqueous electrolytic solution, the concentration of the ionic liquid is 3.0 mol / L or less. If the concentration of the ionic liquid is too small, the ionic conductivity of the non-aqueous electrolyte is insufficient. On the other hand, if the concentration is too large, the moving speed of the ionic liquid is insufficient. It is preferably 5 mol / L or less.

(Lithium salt)
The nonaqueous electrolytic solution may contain a lithium salt as a supporting salt. Specifically, one or more of LiPF ₆ , LiBF ₄ , LiClO ₄ , LiTFSI, and LiBeti can be easily ionized in a high dielectric constant solvent, which will be described later, and exhibit excellent charge transfer medium characteristics. Is a suitable example.

  However, when the content of the lithium salt in the nonaqueous electrolytic solution is 0.5 mol / L or more, the flatness of the charge / discharge curve is generated, so that it is necessary to be less than 0.5 mol / L.

(solvent)
The non-aqueous electrolyte may be composed only of an ionic liquid, but may be a liquid obtained by diluting an ionic liquid with an organic solvent. Such an organic solvent preferably has a high dielectric constant and a low viscosity in terms of increasing the ionic conductivity.

  As the high dielectric constant solvent, a cyclic carbonate is preferable because it is stable in the potential window of the lithium ion battery. Furthermore, when a non-aqueous electrolyte contains a lithium salt, it is necessary to promote ionization of the lithium salt in order to maintain high ionic conductivity. Preferable examples include ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), and vinyl ethylene carbonate (VEC) because they can strongly promote ionization of the lithium salt.

  The low-viscosity solvent has an action of increasing the moving speed of lithium ions or ionic liquids. Since ionic liquids and high dielectric constant solvents have high viscosity and reduce the moving speed of ions, the addition of a low viscosity solvent is desired. As such a low-viscosity solvent, a chain carbonate, a cyclic ether, or a chain ether is preferable because it is stable in the potential window of the lithium ion battery. Specific examples include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), dimethyl ester (DME), diethyl carbonate (DEC), and γ-butyrolactone (GBL).

[Production method]
Such a lithium ion battery can be suitably used in electronic equipment and transportation equipment, and can be produced, for example, by the following method. First, in a dry atmosphere, the positive electrode is accommodated in the positive electrode can and the negative electrode is accommodated in the sealing plate, and the positive electrode and the negative electrode are laminated or wound with the separator sandwiched therebetween. Subsequently, the positive electrode can and the sealing plate are caulked and sealed through a gasket so as to enclose the laminate or the wound body, thereby manufacturing a battery.

<Example 1>
For the negative electrode active material, trimethylpropylammonium TFSI “MCMB25-28” (manufactured by Osaka Gas Chemical Co., Ltd.) was used, and 90 parts by mass of the negative electrode active material and 10 parts by mass of the binder (# 1120 manufactured by Kureha Corporation) Were kneaded in N-methyl-2-pyrrolidone (NMP) as a solvent to obtain a paste-like negative electrode mixture. Next, this negative electrode mixture was applied to a copper foil current collector with a thickness of 20 μm using an applicator and dried at 120 ° C. for 5 minutes. After drying, the electrode compressed by a roll press was punched out to Φ14 to obtain a negative electrode.

LiCoCO ₂ (Honjo Chemical Co., Ltd.) was used as the positive electrode active material, 90 parts by mass of the positive electrode active material, 5 parts by mass of the binder (manufactured by Kureha Co., Ltd. # 1120), and 5 parts by mass of acetylene black, It knead | mixed in NMP which is a solvent, and was set as the paste-form positive electrode compound material. Next, this positive electrode mixture was applied to a 14 μm thick aluminum foil current collector using an applicator and dried at 120 ° C. for 5 minutes. After drying, the electrode compressed by a roll press was punched out to Φ16 to obtain a positive electrode.

  Further, a polyolefin microporous membrane (manufactured by Celgard) was used as the separator.

・・・化学式２ An ionic liquid represented by Chemical Formula 2 is diluted with a solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed at a volume ratio of 1: 3 to prepare an electrolytic solution of ionic liquid 0.5 mol / L. did.

... Chemical formula 2

  In a dry atmosphere, the positive electrode was accommodated in the positive electrode can and the negative electrode was accommodated in the sealing plate, and the positive electrode and the negative electrode were laminated with the separator sandwiched therebetween. Subsequently, after pouring an electrolyte between the positive electrode and the negative electrode, the positive electrode can and the sealing plate were caulked and sealed via a gasket so as to enclose the laminate, thereby producing a coin cell type lithium ion battery. .

・・・化学式３ <Example 2>
A lithium ion battery was produced in the same procedure as in Example 1 except that the ionic liquid (butyltrimethylammonium TFSI) represented by Chemical Formula 3 was used.

... Chemical formula 3

・・・化学式４ <Example 3>
A lithium ion battery was produced in the same procedure as in Example 1 except that the ionic liquid represented by Chemical Formula 4 (trimethylpentylammonium TFSI) was used.

... Chemical formula 4

・・・化学式５ <Example 4>
A lithium ion battery was produced in the same procedure as in Example 1 except that the ionic liquid represented by Chemical Formula 5 (hexyltrimethylammonium TFSI) was used.

... Chemical formula 5

・・・化学式６ <Example 5>
A lithium ion battery was produced in the same procedure as in Example 1 except that the ionic liquid represented by Chemical Formula 6 (methylpropylpiperazinium TFSI) was used.

... Chemical formula 6

<Comparative example>
A lithium ion battery was produced in the same procedure as in Example 1 except that a LiPF ₆ 1 mol / L solution was used as the electrolytic solution.

[Evaluation]
For lithium-ion batteries prepared in Examples 1 to 5 and Comparative Examples, the charge and discharge current density of 0.65 mA / cm ^2, charge termination voltage 4.2 V, under the conditions of the discharge end voltage 2.7V, discharge capacity and closed circuit The relationship with voltage (CCV) was measured. The result is shown in FIG.

  Subsequently, the presence or absence of flatness of the discharge curve was analyzed based on FIG.

  Here, “no flatness” means that the discharge curve passes through the range of the midpoint ± 0.2 V of the upper and lower limit voltage at 50% SOC (charged state), and is 0. It is defined as a state that satisfies both conditions of no voltage drop of 4 V or more.

  According to this definition, a battery including a negative electrode having a flat charge / discharge curve has a high OCV (open circuit voltage) at 50% SOC in the middle of the control, and therefore, normally, the upper limit voltage from the OCV. It has only margin of about 0.4V up to (for example, 4.2V). On the other hand, a battery including a negative electrode that exhibits a non-flat charge / discharge curve usually has an allowance of 0.6 V from the OCV to the upper limit voltage because the OCV is about 3.6 V. This indicates that if the internal resistance is equal, a battery including a negative electrode showing a charge / discharge curve with no flatness can exhibit excellent regeneration acceptability.

  On the other hand, a characteristic of a battery including a negative electrode showing a flat charge / discharge curve is that the voltage drops greatly (for example, 0.4 V or more) in a range of less than 20% SOC during discharge. That is, a voltage difference close to 1 V is always ensured up to the lower limit voltage in the SOC range of 100 to 20%, which is expected to be frequently used as a battery for transportation equipment. Such a battery is not suitable as a power source for transportation equipment because the output density is too large with respect to the input density and the balance between input and output is not balanced.

  On the other hand, in a battery including a negative electrode showing a charge / discharge curve with no flatness, the input density is increased and the output density is moderately suppressed, so that the input and the output are balanced. Therefore, such a battery is useful as a power source for transportation equipment.

  As FIG. 1 shows, the discharge curve of the battery produced in Examples 1-5 did not have flatness. On the other hand, the discharge curve of the battery produced in the comparative example had flatness. That is, it was confirmed that the batteries produced in Examples 1 to 5 are useful as a power source for transportation equipment because they have excellent regenerative acceptance and a balance between input and output.

It is a figure which shows the discharge curve of the battery which concerns on the Example of this invention. It is a figure which shows the relationship between the charge condition and input / output density in a secondary battery.

Claims (5)

A secondary battery comprising: a positive electrode made of a lithium-containing transition metal oxide capable of occluding and releasing lithium ions; a negative electrode capable of occluding and releasing lithium ions; and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode. There,
The non-aqueous electrolyte is prepared from only an electrical conductivity imparting agent comprising an ionic liquid and an organic solvent for diluting the ionic liquid ,
The non-aqueous electrolyte contains the ionic liquid in an amount of 0.5 mol / L to 1.5 mol / L,
The organic solvent is at least one selected from the group consisting of a chain carbonate, a cyclic ester, and a chain ester, and / or a cyclic carbonate.
The secondary battery is an ionic liquid represented by Formula 1.

... Chemical formula 1
(In the formula, R _{1 to} R ₄ are an alkyl group or an alkoxyl group, and X is bistrifluoromethylsulfonylimide (TFSI) or bispentafluoroethanesulfonylimide (Beti).) 2. The secondary battery according to claim 1, wherein R _{1 to} R ₄ in Chemical Formula 1 are an alkyl group or an alkoxyl group having 3 to 7 carbon atoms.   An electronic device using the secondary battery according to claim 1.   A transportation device using the secondary battery according to claim 1. A secondary battery comprising: a positive electrode made of a lithium-containing transition metal oxide capable of occluding and releasing lithium ions; a negative electrode capable of occluding and releasing lithium ions; and a nonaqueous electrolytic solution interposed between the positive electrode and the negative electrode. A manufacturing method comprising:
An electrical conductivity imparting agent comprising an ionic liquid represented by Chemical Formula 1, at least one selected from the group consisting of a chain carbonate, a cyclic ester and a chain ester, and / or a cyclic carbonate comprising the ionic liquid. and an organic solvent for dilution, only either et al., supra method of manufacturing a secondary battery of preparing a non-aqueous electrolyte solution as an ionic liquid containing less 0.5 mol / L or more 1.5 mol / L.

... Chemical formula 1
(In the formula, R _{1 to} R ₄ are an alkyl group or an alkoxyl group, and X is bistrifluoromethylsulfonylimide (TFSI) or bispentafluoroethanesulfonylimide (Beti).)

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