JPH11191318A - Ion conductive solid electrolyte and battery using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high ion conductivity even under low temperature environment by impregnating a polymer with a nonaqueous electrolyte consisting of a nonaqueous solvent containing a low viscosity nonaqueous solvent having a specified viscosity and a Li metal salt dissolved thereto. SOLUTION: As the polymer to be impregnated with a nonaqueous electronlyte, vinylidene polyfluoride is preferably used. The content of the polymer is preferably set so that the ratio of a monomer constituting the polymer to the total mole number of the monomer, a nonaqueous solvent and an electrolytic salt is 5-25 mol.%. As the nonaqueous solvent, a low viscosity nonaqueous solvent having a viscosity of 1.5 cP or less at 25 deg.C is used. Examples of such a solvent include dimethyl carbonate and the like. In case of dimethyl carbonate, the content in the solid electrolyte is desirably set to 20-30 mol.% to the total mole number of the monomer constituting the polymer, the nonaqueous solvent and the electrolytic salt.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an ion-conductive solid electrolyte and a battery using the same.

[0002]

2. Description of the Related Art A lithium ion secondary battery using a lithium-containing compound for a positive electrode, a carbon-based material for a negative electrode, and a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent as an electrolyte is a nickel-cadmium secondary battery. It is known that a higher energy density can be obtained as compared with an aqueous secondary battery such as a battery.

[0003] The lithium ion secondary battery generally has a cylindrical or square shape.
2. Description of the Related Art Recently, with the spread of portable electronic devices, particularly mobile phones and notebook computers, thin-shaped devices such as a card type and a flat type have been required.

However, when a power generating element of a conventional lithium ion secondary battery is housed in a thin battery case, liquid leakage is likely to occur, which may damage peripheral electronic components.

Therefore, in order to prevent such liquid leakage,
For example, Japanese Patent Application Laid-Open No. 3-59058 and the like have proposed the use of a solid electrolyte solidified by a polymer, and the development of materials and the like has been advanced.

[0006]

However, in the case of a solid electrolyte, it is more difficult to obtain ionic conductivity than in the case of a liquid electrolyte. For example, the ionic conductivity of a solid electrolyte disclosed in JP-A-3-59058 is 3 mScm ^-1. , Which is about one order of magnitude lower than the ionic conductivity of the non-aqueous electrolyte. Furthermore, in the case of a solid electrolyte, in a low-temperature environment of 0 ° C. or less, the ionic conductivity is reduced to about submicron, and the battery cannot be operated.

Accordingly, the present invention has been proposed in view of such a conventional situation, and provides an ion-conductive solid electrolyte capable of obtaining a high ionic conductivity even at room temperature and also in a low-temperature environment, and uses the same. The purpose is to provide a battery that has been used.

[0008]

In order to achieve the above-mentioned object, an ion-conductive solid electrolyte of the present invention is prepared by impregnating a polymer with a non-aqueous electrolyte obtained by dissolving a lithium metal salt in a non-aqueous solvent. Wherein the non-aqueous solvent contains a low-viscosity non-aqueous solvent having a viscosity at 25 ° C. of 1.5 cP or less.

Further, the battery of the present invention comprises a positive electrode containing a lithium-containing compound, a negative electrode containing any of lithium metal, a lithium alloy or a carbonaceous material capable of occluding lithium, and a non-aqueous solvent. A non-aqueous electrolyte obtained by dissolving a lithium metal salt has an ion-conductive solid electrolyte impregnated with a polymer, and the non-aqueous solvent has a low-viscosity non-aqueous solution having a viscosity at 25 ° C. of 1.5 cP or less. It is characterized by containing a solvent.

A solid electrolyte containing a low-viscosity non-aqueous solvent having a viscosity of 1.5 cP or less has a viscosity of 6 to 8 m at a temperature of 25 ° C.
Shows high ionic conductivity Scm ^-1, 1mScm ^-1 order of ion conductivity can be obtained even at -20 ° C. for a low-temperature environment. Therefore, when used as an electrolyte material for a battery, sufficient performance can be obtained not only at room temperature but also in a low-temperature environment.

When this ion-conductive solid electrolyte is applied to a battery such as a card type having a shape in which liquid leakage easily occurs, since the non-aqueous electrolyte is held in a polymer, the ion-conductive solid electrolyte is not used. A safe battery can be obtained without liquid leakage.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

The ion conductive solid electrolyte of the present invention is obtained by impregnating a polymer with a non-aqueous electrolyte obtained by dissolving a lithium metal salt in a non-aqueous solvent.

In such an ion-conductive solid electrolyte, it is desirable to use polyvinylidene fluoride as the polymer impregnated with the non-aqueous electrolyte because of its excellent ion conductivity and oxidation-reduction resistance.

The content of the polymer is desirably in a range such that the ratio of the monomer constituting the polymer to the total number of moles of the nonaqueous solvent and the electrolyte salt is 5 to 25 mol%. If the content of the polymer is too large, the content of the electrolytic solution becomes relatively small, so that the ionic conductivity decreases. On the other hand, if the content of the polymer is too small, a solid electrolyte cannot be obtained.

As the non-aqueous solvent, the solid electrolyte has a viscosity of 1.5 cP or less, more preferably 1.0 cP or less.
The following low-viscosity non-aqueous solvents are used. By using such a non-aqueous solvent having a low viscosity, a high ionic conductivity can be obtained at room temperature and also in a low-temperature environment of 0 ° C. or lower.

The low-viscosity non-aqueous solvent has a viscosity of 1.5
Those having a cP or less and capable of dissolving the electrolyte salt are selected, and dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (ME
C), 1,3-dioxolane and the like. Among them, dimethyl carbonate is particularly preferred because it can impart high ionic conductivity to the solid electrolyte and can also improve the stability of the solid electrolyte. Table 1 summarizes the viscosities of the low-viscosity non-aqueous solvents mentioned here.

[0018]

[Table 1]

Among them, in the case of dimethyl carbonate, the content in the solid electrolyte is 40 m with respect to the total number of moles of the monomer constituting the polymer, the non-aqueous solvent and the electrolyte salt.
ol%, preferably 20 to 30 mol%. If the content of the low-viscosity nonaqueous solvent is too small, the ionic conductivity of the polymer electrolyte cannot be sufficiently increased. Conversely, if the content of the low-viscosity non-aqueous solvent is too large, the solubility of the electrolyte salt decreases, and the physical stability of the solid electrolyte is impaired.

However, since these low-viscosity non-aqueous solvents have a relatively low dielectric constant, it is desirable to combine them with a non-aqueous solvent having a high dielectric constant.

As such a high dielectric constant solvent, it is desirable to use one having a potential window in the range of -0.3 V to 4.9 V with respect to the lithium potential. For example, ethylene carbonate (EC), propylene carbonate ( PC), γ
-Butyrolactone and the like. These solvents have a viscosity of more than 1.5 cP, but relatively hardly impair the ionic conductivity of the solid electrolyte. For reference, Table 2 summarizes the viscosities of these high dielectric constant solvents.

[0022]

[Table 2]

Further, in order to accelerate the dissolution of the polymer, a solvent such as dimethylacetamide may be used in combination.

Next, as the electrolyte salt to be contained in the solid electrolyte, when the solid electrolyte is used in place of the non-aqueous electrolyte of a lithium ion secondary battery, a lithium salt generally used in a lithium ion secondary battery is used. Salt can be used.

For example, LiClO ₄ , LiBF ₄ , LiP
F ₆ , LiCF ₃ SO ₃ , LiAsF ₆ , Li (CF ₃ S
O ₂ ) ₂ N and lithium metal salts such as LiC ₄ F ₉ SO ₃ .

When the solid electrolyte is used for other purposes, it may be appropriately selected according to the intended use, and may be an alkali metal salt such as sodium or potassium in addition to a lithium salt.

In order to produce the above-mentioned ion-conductive solid electrolyte, a non-aqueous electrolyte is prepared by dissolving an electrolyte salt in a non-aqueous solvent, and this is heated. Then, the polymer is added to the heated non-aqueous electrolyte to completely dissolve it, and the obtained solution is quickly spread on the substrate and slowly cooled.

In dissolving the polymer, a stirrer such as a homogenizer may be used. In this case, the rotation speed of the homogenizer is desirably 50 rpm or more. Further, in order to promote the dissolution of the polymer, the polymer may be dissolved in dimethylacetamide or the like in advance, and the polymer solution may be added to the non-aqueous electrolyte. In addition, the preparation of the solid electrolyte may be performed in the order of adding and dissolving the polymer in the non-aqueous solvent, and then adding and dissolving the electrolyte salt.

The ion conductive solid electrolyte as described above is
For example, it is used as an electrolyte material of a battery. The applied battery may be a primary battery specification or a secondary battery specification.
In the case of the secondary battery specification, for example, the following materials are used as the positive electrode material and the negative electrode material.

First, a lithium-containing compound is used as a positive electrode material, and specifically, a general formula Li _x MO ₂ (however,
M is one or more transition metals, preferably Mn, Co, N
i represents at least one kind, and x is 0.05 ≦ x ≦ 1.1
0) is used.

As the negative electrode material, lithium metal,
A lithium alloy and a carbonaceous material capable of storing lithium are used. Examples of carbonaceous materials include pyrolytic carbon, coke (pitch coke, needle coke, petroleum coke, etc.), graphites, non-graphitizable carbons, glassy carbons, organic polymer compound fired bodies, carbon fibers, activated carbon And the like.

[0032]

Embodiments of the present invention will be described below based on experimental results. However, the present invention is not limited to these.

Examples 1 to 5 These examples are examples of various solid electrolytes in which dimethyl carbonate is used as a low-viscosity non-aqueous solvent and the content thereof is changed within a range of 40 mol% or less.

These solid electrolytes were prepared as follows.

First, polyvinylidene fluoride, dimethylacetamide, dimethyl carbonate, ethylene carbonate, propylene carbonate, and LiPF ₆ were weighed at mixing ratios shown in Table 3. The mixing ratio of polyvinylidene fluoride in the table is the molar ratio of the monomer which is a repeating unit.

[0036]

[Table 3]

Then, polyvinylidene fluoride was added to dimethylacetamide as a solvent, and dissolved by mixing and stirring. Next, the obtained polymer solution, ethylene carbonate, propylene carbonate, and dimethyl carbonate were mixed, stirred and uniformly mixed, and then LiPF ₆ was added and stirred. These stirrings were performed at a rotation speed of 50 rpm or more using a homogenizer. Here, the mixture is in a liquid state because the temperature rises to about 100 ° C. due to frictional heat during stirring, but when the stirring is completed, the temperature drops, and a polymer solid impregnated with a liquid component is obtained. However, in Example 5 in which the mixture ratio of dimethyl carbonate was 40 mol%, the electrolyte was not completely impregnated with the polymer, and only a part thereof was solidified.

Comparative Example 1 For comparison, this is an example of a solid electrolyte containing no low-viscosity non-aqueous solvent.

Without using dimethyl carbonate, polyvinylidene fluoride, dimethylacetamide, ethylene carbonate, propylene carbonate and LiPF ₆ were used in Table 3.
A solid electrolyte was prepared in the same manner as in Example 1 except that the mixing ratio was set as shown in Table 1.

Evaluation of Ionic Conductivity of Solid Electrolyte The ionic conductivity of the solid electrolyte prepared as described above was measured at room temperature (25 ° C.) and in a low temperature environment of −20 ° C. by an AC impedance method. Table 4 shows the results.

[0041]

[Table 4]

As shown in Table 4, the solid electrolyte containing dimethyl carbonate, particularly, the content was 40 mol%.
The solid electrolyte of less than 5 mScm ^-1 has an ionic conductivity of 5 mScm ^-1 or more at room temperature, and 1 mS even under a low temperature environment.
An ionic conductivity of at least cm ^-1 is obtained. Furthermore, in the solid electrolytes of Examples 3 and 4 in which the content of dimethyl carbonate was 20 mol% to 30 mol%, a high ionic conductivity of about 8 mScm ^-1 was obtained at room temperature. However, the solid electrolyte of Example 5 in which the content of dimethyl carbonate was 40 mol% had a higher ionic conductivity than the solid electrolyte of Comparative Example 1, but was insufficiently gelled as described above.

Thus, dimethyl carbonate, which is a low-viscosity solvent, has the effect of increasing the ionic conductivity of the solid electrolyte, and its proper content is less than 40 mol%, more preferably 20 mol% to 30 mol%. all right.

Evaluation of Charge / Discharge Characteristics of Battery The battery used in this evaluation was prepared by using the ion-conductive solid electrolyte of Example 3, a positive electrode containing lithium nickelate, and a negative electrode containing graphitized mesophase carbon microbeads as a power generating element. It is a flat type battery.

As shown in FIG. 1, the flat battery has a negative electrode 3 and a positive electrode 4 on which ion-conductive solid electrolytes 1 and 2 are formed.
Are laminated with a separator 5 interposed therebetween, and flat battery casings 6 and 7 electrically connected to these are disposed on the negative electrode 3 side and the positive electrode 4 side of the laminate. An insulating material 7 is arranged around the end face of the laminate, and the battery is hermetically sealed by bonding the edges of the battery exterior materials 6 and 7 to the insulating material 8.

This battery was manufactured as follows.

First, a positive electrode mixture was prepared by mixing lithium nickelate, graphite as a conductive agent, polyvinylidene fluoride as a binder, and N-methylpyrrolidone as a dispersant. Then, the positive electrode mixture was applied to a positive electrode current collector, dried, and cut into a predetermined size (32 cm ² ) to produce a positive electrode 4.

Next, a negative electrode mixture was prepared by mixing the graphitized mesophase carbon microbeads, polyvinylidene fluoride as a binder, and N-methylpyrrolidone as a dispersant. Then, the negative electrode mixture was applied to a negative electrode current collector, dried, and cut into a predetermined size (32 cm ² ) to prepare a negative electrode 3.

Then, a solid electrolyte prepared with the same composition as in Example 3 was applied to the positive electrode 4 and the negative electrode 3 in a solution state, and solidified by cooling. Next, the positive electrode 4 on which the solid electrolytes 1 and 2 are formed and the negative electrode 3 are overlapped via a separator 5 made of polypropylene, and battery exterior materials 5 and 6 are arranged on both sides of the laminate, and an insulating material 8 and the battery exterior materials 6 and 7 were adhered to the insulating material 8 to produce a flat battery.

The flat battery thus manufactured was charged and discharged for 5 cycles, and the discharge capacity and charge and discharge efficiency were examined for each cycle.

In the charge / discharge test, constant current charging was performed at a current density of 250 μAcm ⁻² until the closed circuit voltage reached 4.2 V, and then switched to constant voltage charging, and further charged until the total charging time reached 10 hours. And then 250 μAc
This was performed by repeating a charge / discharge cycle in which constant current discharge was performed until the closed circuit voltage reached 2.5 V at a current density of m ⁻² .

FIG. 2 shows the relationship between the number of cycles and the discharge capacity and charge / discharge efficiency.

As can be seen from FIG. 2, this battery has a substantially constant discharge capacity from 2 to 5 cycles and a high charge / discharge efficiency of 95% or more.

From these results, it was found that a solid electrolyte containing a low-viscosity solvent can provide sufficient performance as an electrolyte material for a battery.

[0055]

As is clear from the above description, the ion-conductive solid electrolyte of the present invention has a viscosity at 25 ° C. of 1.
Since it contains a low-viscosity non-aqueous solvent of 5 cP or less,
It exhibits a high ionic conductivity of 6 to 8 mScm ^{-1 in} a 5 ° C environment and an ionic conductivity of about ¹ mScm ^-1 in a low temperature environment of -20 ° C. Therefore, when used as an electrolyte material for a battery, sufficient performance can be obtained not only at room temperature but also in a low-temperature environment. In addition, even when the ion-conductive solid electrolyte is applied to a battery such as a card type having a shape in which liquid leakage easily occurs, the electrolyte does not leak out, and a safe battery can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a battery to which the present invention is applied.

FIG. 2 is a characteristic diagram showing charge / discharge cycle characteristics of a battery.

[Explanation of symbols]

 1,2 ion conductive solid electrolyte, 3 negative electrode, 4 positive electrode

Claims (7)

[Claims] A polymer is impregnated with a non-aqueous electrolyte obtained by dissolving a lithium metal salt in a non-aqueous solvent, wherein the non-aqueous solvent has a low viscosity of 1.5 cP or less at 25 ° C. An ion-conductive solid electrolyte comprising an aqueous solvent. 2. The ion conductive solid electrolyte according to claim 1, wherein the polymer is polyvinylidene fluoride. 3. The method according to claim 2, wherein the ratio P of the monomer constituting the polyvinylidene fluoride to the total number of moles of the nonaqueous solvent and the electrolyte salt is 5 mol% ≦ P ≦ 25 mol%. Ion conductive solid electrolyte. 4. The ion-conductive solid electrolyte according to claim 1, wherein the low-viscosity non-aqueous solvent is dimethyl carbonate. 5. The ratio S of dimethyl carbonate to the total number of moles of the monomer constituting the polyvinylidene fluoride, the nonaqueous solvent and the electrolyte salt is 0 mol% <S <40 mol.
%. The ion-conductive solid electrolyte according to claim 4, wherein 6. A positive electrode containing a lithium-containing compound,
A negative electrode containing any of lithium metal, a lithium alloy or a carbonaceous material capable of occluding lithium,
A non-aqueous electrolyte having a lithium metal salt dissolved in a non-aqueous solvent has an ion-conductive solid electrolyte impregnated in a polymer, and the non-aqueous solvent has a viscosity at 25 ° C. of 1.5 cP or less. A battery comprising a low-viscosity non-aqueous solvent. 7. The lithium-containing compound constituting the positive electrode,
The battery according to claim 6, wherein the battery is a composite oxide of lithium and a transition metal.