JP2002252033A - Polymer electrolyte and lithium ion polymer secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polymer electrolyte with which high ion conductivity is obtained under room temperature and further under low-temperature environment. SOLUTION: In the polymer electrolyte, which is composed of mixing with a polymer, an electrolytic solution, and a lithium metal salt, a relation of 5.0×10<-2> <=ε×η<=3.0×10<-1> is filled, when making the dielectric constant of the electrolytic solution to ε (non-unit), and viscosity into η (kg/m.s).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer electrolyte having a high ionic conductivity even in a low temperature environment, and a lithium ion polymer secondary battery using the same.

[0002]

2. Description of the Related Art With the spread of portable devices such as video cameras and notebook personal computers in recent years, demand for thin batteries has increased. As this thin battery, a lithium ion polymer secondary battery formed by laminating a positive electrode sheet and a negative electrode sheet is known. This positive electrode sheet is made by forming an active material on the surface of the positive electrode current collector foil,
The negative electrode sheet is made by forming an active material on the surface of the negative electrode current collector foil. A polymer electrolyte layer is interposed between the active material of the positive electrode sheet and the active material of the negative electrode sheet. In this battery, a positive electrode terminal and a negative electrode terminal for taking out a potential difference in each active material as a current are provided on the positive electrode current collector foil and the negative electrode current collector foil, and the stacked body is sealed in a package. Thereby, a lithium ion polymer secondary battery is formed. In this lithium ion polymer secondary battery, desired electricity can be obtained by using the positive electrode terminal and the negative electrode terminal drawn out of the package as terminals of the battery.

[0003] Polymer electrolytes used in lithium ion secondary batteries include polymers, electrolytes and lithium metal salts. As a method for producing this polymer electrolyte,
First, a slurry is prepared by adding and mixing each of the above-mentioned polymer, electrolyte solution and lithium metal salt. Next, the polymer electrolyte slurry is coated and dried by a doctor blade method on a release paper so that the dry thickness of the electrolyte layer is 10 to 150 μm, and the polymer electrolyte is obtained by peeling this layer from the release paper. . Here, the doctor blade method is one of the methods of forming ceramics into a sheet, and the thickness of the slip carried on a carrier such as a carrier film or an endless belt is determined by a knife edge called a doctor blade and a carrier. This is a method of precisely controlling the thickness of the sheet by adjusting the distance between the sheets.

As a general finding of a polymer electrolyte and an electrolyte contained in the polymer electrolyte, a high dielectric constant and a low viscosity of the electrolyte result in a high ionic conductivity of the electrolyte and a high ionic conductivity. It is said that the ionic conductivity of a polymer electrolyte containing an electrolytic solution having a high rate is also good. Therefore,
When the dielectric constant of the electrolytic solution is represented by ε (unitless) and the viscosity is represented by η (kg / m · s), the polymer electrolyte is also used by using the electrolytic solution having a large value of ε / η (m · s / kg). It will exhibit high ionic conductivity.

[0005]

However, in a low-temperature environment such as -20.degree. C., the above-mentioned knowledge of the conventional general electrolyte does not apply to the ionic conductivity of the polymer electrolyte containing the electrolyte. That is, even if the ε / η of the electrolytic solution shows a large numerical value, the ionic conductivity of the polymer electrolyte containing the electrolytic solution does not always show a high numerical value, and the battery using this polymer electrolyte has sufficient performance. There was no problem. An object of the present invention is to provide a polymer electrolyte capable of obtaining a high ionic conductivity even at room temperature or even in a low temperature environment, and a battery using the same.

[0006]

According to the first aspect of the present invention,
In a polymer electrolyte obtained by mixing a polymer, an electrolyte and a lithium metal salt, the dielectric constant of the electrolyte is ε (unitless),
When the viscosity is η (kg / m · s), the polymer electrolyte satisfies the following condition: 5.0 × 10 ⁻² ≦ ε × η ≦ 3.0 × 10 ⁻¹ (1) . In the invention according to claim 1, when the dielectric constant of the electrolytic solution contained in the polymer electrolyte is ε (unitless) and the viscosity is η (kg / m · s),
The numerical value represented by ε × η (kg / m · s) is defined in the above equation (1). When ε × η is less than 5.0 × 10 ⁻² kg / m · s, in a low-temperature environment, the polymer having a high thermal expansion coefficient shrinks, thereby facilitating the discharge of the electrolyte from the polymer, and the discharge of the polymer electrolyte. The ionic conductivity is significantly reduced. If ε × η exceeds 3.0 × 10 ⁻¹ kg / m · s, the transfer resistance of lithium ions increases and the ionic conductivity decreases in a low-temperature environment.

A second aspect of the present invention is the polymer electrolyte according to the first aspect, wherein the weight ratio of the polymer to the weight of the polymer and the electrolyte is 0.01 to 0.50. In the invention according to claim 2, when the weight ratio of the polymer to the weight of the polymer and the electrolyte is less than 0.01, the strength of the polymer electrolyte is insufficient, and the weight ratio of the polymer to the weight of the polymer and the electrolyte is caused. Exceeds 0.50, ionic conductivity is lowered.

The invention according to claim 3 is the invention according to claim 1 or 2, wherein the polymer is polyvinylidene fluoride (hereinafter referred to as PVdF), a hexafluoropropylene-vinylidene fluoride copolymer (hereinafter referred to as HFP). -P
VdF. ) Or a polymer electrolyte which is a polymer having a polyvinylidene fluoride-hexafluoropropylene-polyvinylidene fluoride copolymer as a main component. The invention according to claim 4 is the invention according to claim 1 or 2,
The electrolyte is ethylene carbonate (hereinafter, referred to as EC), propylene carbonate (hereinafter, referred to as PC), γ-butyrolactone, dimethyl carbonate,
The polymer electrolyte is a mixed solution containing two or more selected from methyl ethyl carbonate (hereinafter, referred to as MEC) or diethyl carbonate. The invention according to claim 5 is a lithium ion polymer secondary battery produced using the polymer electrolyte according to any one of claims 1 to 4. In the invention according to claim 5, claims 1 to 4
A lithium ion polymer secondary battery produced using any of the polymer electrolytes described above can sufficiently exhibit performance even in a low-temperature environment.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described.
explain. The polymer electrolyte of the present invention is a polymer, an electrolytic solution.
And lithium metal salts, obtained by mixing
You. Excellent ionic conductivity and oxidation-reduction resistance as a polymer
Are selected, PVdF, HFP-PVdF, etc.
No. Lithium metal salt can be dissolved as electrolyte
Functional, EC, PC, γ-butyrolact
, Dimethyl carbonate, MEC or diethyl carbonate
A mixed solution containing two or three or more selected from the group consisting of
I can do it. As the lithium metal salt, LiClO _Four, L
iBF_Four, LiPF₆, LiCF_ThreeSO_Three, LiAsF₆,
Li (CF_ThreeSO_Two)_TwoN, LiC_FourF₉SO_ThreeEtc.
You.

A polymer electrolyte containing a low-viscosity electrolytic solution, which has been conventionally used, has a problem that the polymer having a high coefficient of thermal expansion shrinks when the polymer electrolyte is maintained at a low temperature state. The drainage of liquid is promoted. Therefore, it is considered that the ionic conductivity of the polymer electrolyte is significantly reduced. Therefore, in order for the polymer electrolyte to have excellent ionic conductivity at a low temperature, it is preferable that the contained electrolyte is an electrolyte having a certain viscosity or more than a low-viscosity electrolyte. By using the electrolyte having the above viscosity, when the polymer electrolyte is maintained at a low temperature, the discharge of the electrolyte is not promoted due to the contraction of the polymer.

In particular, when the relationship between the dielectric constant ε (unitless) of the electrolyte and the viscosity η (kg / m · s) is represented by ε × η (kg / m · s),
It was found that a polymer electrolyte containing an electrolytic solution having a value within the range of 5.0 × 10 ⁻² ≦ ε × η ≦ 3.0 × 10 ⁻¹ had high ionic conductivity.

The polymer electrolyte of the present invention has a dielectric constant and viscosity of ε ×
By limiting the numerical range of η, it is possible to suppress the promotion of the discharge of the electrolyte due to the contraction of the polymer in a low-temperature environment. The mixing ratio of the polymer electrolyte is such that the weight ratio of the polymer to the weight of the polymer and the electrolyte is 0.0
1 to 0.50. A preferable polymer and a weight ratio of the polymer to the weight of the electrolytic solution are 0.1 to 0.3.

Next, a lithium ion secondary battery is manufactured using the polymer electrolyte. For example, as a method for manufacturing a sheet-shaped lithium ion polymer secondary battery, first, a sheet-shaped positive electrode collector and a sheet-shaped negative electrode current collector are prepared. Next, the components required for the positive electrode active material layer, the negative electrode active material layer, and the polymer electrolyte, for example, the respective components shown in Table 1 below are mixed for 2 hours by a ball mill, so that the positive electrode active material layer slurry and the negative electrode active material layer slurry are respectively mixed. And a polymer electrolyte slurry are prepared.

[0014]

[Table 1]

Next, the obtained positive electrode active material layer slurry is applied on a positive electrode current collector by a doctor blade method and dried,
A positive electrode is formed by rolling. Similarly, the negative electrode is formed by applying the obtained negative electrode active material layer slurry on a negative electrode current collector by a doctor blade method, drying and rolling the negative electrode active material layer slurry. The positive electrode or negative electrode active material layer is formed so that the thickness after drying is 20 to 250 μm.
The polymer electrolyte is formed by applying and drying the obtained polymer electrolyte slurry on a release paper by a doctor blade method so that the dry thickness of the electrolyte layer is 10 to 150 μm, and peeling off the release paper. The formed positive electrode, polymer electrolyte and negative electrode are sequentially laminated, and the laminate is thermocompression-bonded to form a sheet-like electrode body.

Finally, the positive electrode lead and the negative electrode lead made of Ni are welded to the positive electrode current collector and the negative electrode current collector, respectively, and are housed in a bag-shaped laminated package material having an opening. The opening is sealed by thermocompression bonding under the conditions, whereby a sheet-shaped lithium ion polymer secondary battery can be manufactured.

[0017]

Next, embodiments of the present invention will be described. <Example 1> First, HFP-PVdF was used as a polymer.
40 g of (Kynar2810; product containing 12% by weight of hexafluoropropylene) manufactured by Elf Atochem. Also, E
A solution obtained by mixing C and PC so that the weight ratio of EC: PC is 50:50 is used as an electrolyte.
0 g was prepared. Dielectric constant ε (no unit), viscosity η of this electrolyte
(kg / m · s) and ε × η (kg / m · s) and ε / η
(m · s / kg) was obtained, and ε × η = 1.43 × 1
0 ⁻¹ kg / m · s, ε / η = 3.5 × 10 ⁴ m · s / kg
Met. The ionic conductivity of the electrolyte solution measured at -20 ° C was 1.17 mS / cm. Next, 200 g of dimethylformamide was added and dissolved in 40 g of the HFP-PVdF as a solvent, and then 80 g of the electrolyte solution having the above composition was added and mixed for 2 hours by a ball mill to prepare a polymer electrolyte slurry.

<Example 2> A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid in which EC and PC were mixed at a weight ratio of EC: PC of 60:40 was used as an electrolytic solution. The dielectric constant ε (no unit) and the viscosity η (kg / m · s) of the used electrolyte solution were measured, and ε × η and ε / η were determined. Ε × η = 1.41 × 10 ⁻¹ kg / m
S and ε / η were 3.7 × 10 ⁴ m · s / kg. The ionic conductivity of this electrolytic solution at -20 ° C was 1.
23 mS / cm. <Embodiment 3> EC, PC and MEC are converted to EC: PC: ME.
A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid mixed so that C became 40:40:20 by weight was used as the electrolyte. The dielectric constant ε (no unit) of the used electrolyte solution and the viscosity η (kg / m
× η and ε / η were determined, and ε × η = 1.0
1 × 10 ⁻¹ kg / ms, ε / η = 3.2 × 10 ⁴ ms
/ Kg. The ionic conductivity of this electrolyte at -20 ° C was 1.71 mS / cm. <Embodiment 4> EC, PC and MEC are converted to EC: PC: ME.
A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid mixed so that C became 48:32:20 by weight was used as the electrolyte. Measure the dielectric constant ε (no unit) and viscosity η (kg / m
× η and ε / η were determined, and ε × η = 1.0
0 × 10 ⁻¹ kg / ms, ε / η = 3.4 × 10 ⁴ ms
/ Kg. The ionic conductivity of the electrolyte at -20 ° C was 1.78 mS / cm. <Example 5> A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid in which EC and MEC were mixed so that the weight ratio of EC: MEC was 50:50 was used as the electrolytic solution. The dielectric constant ε (unitless) and viscosity η (kg / m · s) of the used electrolyte solution were measured, and ε × η and ε / η were determined, respectively, where ε × η = 5.0 × 10 ^−2. kg / m
s, ε / η = 3.4 × 10 ⁴ m · s / kg. The ionic conductivity of this electrolyte at -20 ° C was 2.
It was 36 mS / cm. <Example 6> A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid in which PC and MEC were mixed so that the weight ratio of PC: MEC was 50:50 was used as the electrolytic solution. The dielectric constant ε (no unit) and viscosity η (kg / m · s) of the used electrolyte solution were measured, and ε × η and ε / η were determined, respectively, where ε × η = 5.1 × 10 ^−2. kg / m
s, ε / η = 2.1 × 10 ⁴ m · s / kg. The ionic conductivity of this electrolyte at -20 ° C was 2.
It was 31 mS / cm.

Comparative Example 1 PC and MEC were replaced with PC: ME
A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that the liquids mixed so that C became 25:75 by weight were used as the electrolyte. Dielectric constant ε of electrolyte used
(No unit), viscosity η (kg / m · s) was measured, and ε × η and ε / η were determined, respectively. Ε × η = 2.1 × 10
^-2 kg / m · s and ε / η = 1.7 × 10 ⁴ m · s / kg. The ionic conductivity of the electrolyte at -20 ° C was 2.59 mS / cm. <Comparative Example 2> EC: PC: ME: EC: PC: ME
A polymer electrolyte slurry was prepared in the same manner as in Example 1 except that a liquid mixed so that C became 10:10:80 by weight was used as an electrolyte. Measure the dielectric constant ε (no unit) and viscosity η (kg / m
× η and ε / η were determined respectively, and ε × η = 1.7.
× 10 ⁻² kg / m · s, ε / η = 1.7 × 10 ⁴ m · s /
kg. The ionic conductivity of the electrolyte at -20 ° C was 2.59 mS / cm. <Comparative evaluation> The polymer electrolyte slurries obtained in Examples 1 to 6 and Comparative Examples 1 and 2 were coated and dried by a doctor blade method on an aluminum foil so that the dry thickness of the electrolyte layer was 50 μm. By peeling off from the foil, a sheet-like polymer electrolyte was produced. The produced polymer electrolyte was punched out into a size of 10 mm in diameter by a metal mold, placed in a coin cell, and measured for ionic conductivity at -20 ° C. Table 2 shows the ionic conductivity of the polymer electrolytes of Examples 1 to 6 and Comparative Examples 1 and 2, respectively.

[0020]

[Table 2]

As is clear from Table 2, the ionic conductivity of the electrolytic solution and the ionic conductivity of the polymer electrolyte measured in Examples 1 to 6 and Comparative Examples 1 and 2 did not have a proportional relationship. In the polymer electrolytes of Comparative Examples 1 and 2, ε × η of the used electrolyte solution was lower than the lower limit of 5.0 × 10 ⁻² kg / m · s of the present invention. -20 ° C
The value of the ionic conductivity of the electrolyte at
The value of the ionic conductivity of the polymer electrolyte at 0 ° C. is small. This is presumably because, as described above, since the electrolyte has a low viscosity, the polymer shrinks at a low temperature, thereby promoting the discharge of the electrolyte. On the other hand, in the polymer electrolytes of Examples 1 to 6, the ε × η of the used electrolyte was 5.0 × 10 ⁻² kg / m · s to 3.0 × 10.
^-1 kg / m · s. The ionic conductivity of the polymer electrolyte shows a large numerical value, respectively.
Lithium ion secondary batteries produced using this polymer electrolyte have reached a level where practically sufficient performance can be obtained in a low temperature environment. It is considered that the reason for this is that, because the viscosity of the electrolytic solution was high, the discharge was not promoted even by polymer shrinkage at low temperature.

[0022]

As described above, according to the present invention, when the polymer and the dielectric constant are ε (unitless) and the viscosity is η (kg / m · s), 5.0 × 10 ⁻² ≦ A polymer electrolyte obtained by mixing an electrolyte solution satisfying ε × η ≦ 3.0 × 10 ⁻¹ (1) has a high ionic conductivity even at room temperature or even at low temperature. By using an electrolyte, a lithium ion secondary battery that can sufficiently exhibit performance even in a low-temperature environment can be manufactured.

Continuation of the front page (72) Inventor Akihiro Higami 1-297 Kitabukuro-cho, Omiya-shi, Saitama F-term in Mitsubishi Materials Research Laboratory (reference) 5G301 CA16 CA30 CD01 5H029 AJ02 AJ06 AK03 AL07 AM03 AM05 AM07 AM16 BJ04 BJ12 EJ13 EJ14 HJ01 HJ10 HJ20

Claims (5)

[Claims] 1. A polymer electrolyte obtained by mixing a polymer, an electrolytic solution and a lithium metal salt, wherein, when the dielectric constant of the electrolytic solution is ε (unitless) and the viscosity is η (kg / m · s), 0.0 × 10 ⁻² ≦ ε × η ≦ 3.0 × 10 ⁻¹ (1) 2. The polymer electrolyte according to claim 1, wherein the weight ratio of the polymer to the weight of the polymer and the electrolyte is 0.01 to 0.50. 3. The polymer according to claim 1, wherein the polymer is a polymer containing polyvinylidene fluoride, hexafluoropropylene-polyvinylidene fluoride copolymer or polyvinylidene fluoride-hexafluoropropylene-polyvinylidene fluoride copolymer as a main component. The polymer electrolyte according to any one of the preceding claims. 4. The electrolytic solution according to claim 1, wherein the electrolytic solution is a mixed solution containing two or more selected from ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. Polymer electrolyte. 5. A lithium ion polymer secondary battery produced by using the polymer electrolyte according to claim 1.