JP2001093536A - Lithium cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high efficiency lithium cell with improved interface between electrodes and solid electrolyte by improving the solid electrolyte. SOLUTION: The lithium cell is formed by arranging solid electrolyte between a pair of positive and negative electrodes comprising an active substance, the solid electrolyte being plasticized by adding a mixture of titanium oxide and lithium compound to crystalline solid electrolyte powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery, and more particularly, to a lithium battery having an improved solid electrolyte and an improved interface with an electrode.

[0002]

2. Description of the Related Art With the recent remarkable development of portable devices typified by portable telephones and personal computers, demand for batteries as power sources has been rapidly increasing. In particular, lithium ion batteries are batteries that use lithium, which has a small atomic weight and a large ionization energy, and are actively studied as batteries that can obtain high energy densities. Has been reached. These lithium-ion batteries are roughly classified into cylindrical type and square type.In both cases, the positive electrode and the negative electrode are inserted through a separator into a battery case. It has a structure that is injected and sealed.

In the above-mentioned lithium ion battery, lithium cobalt oxide (LiCoO ₂ ) and lithium manganate (LiMn ₂ O ₄ ) are generally used as a positive electrode active material. Further, a carbon material such as coke and carbon fiber is used for the negative electrode active material. LiCoO ₂ or LiM
The charge / discharge voltage of n ₂ O ₄ is about 4V. On the other hand, the charge / discharge potential of the carbon material is around 0V. Therefore, a lithium ion battery achieves a high voltage of about 3.5 V by combining these positive electrode active materials and negative electrode active materials.

[0004] In recent years, with the demand for thin, light and small electronic devices such as video photographing devices, notebook personal computers, and portable information terminals such as portable telephones, the above-mentioned organic electrolytic solution has been required. Instead, a polymer electrolyte battery in which a polymer electrolyte and an organic electrolyte are mixed and disposed between a pair of positive and negative electrodes has been receiving attention. However, since these lithium ion batteries or polymer electrolyte batteries use a liquid as an electrolyte, the problem of liquid leakage cannot be eliminated at all. Further, when an abnormality such as a short circuit occurs in the battery, there is a risk that the organic electrolyte solution reacts and the battery is ignited.
Here, in order to ensure the safety of the battery, development of a lithium battery formed of a nonflammable solid is desired.

In view of such problems, lithium batteries using an inorganic solid electrolyte have been actively developed.
Among them, a lithium battery using a sulfide-based amorphous glass as a solid electrolyte has characteristics in which the lithium ion conductivity in the solid electrolyte is comparable to that of an organic electrolyte, and also has very excellent characteristics as a battery. is there. However, sulfide-based amorphous glass has disadvantages such as being highly hygroscopic and not chemically stable.

On the other hand, oxide-based materials are relatively stable chemically, and oxide-based solid electrolyte materials have been actively studied. In particular, a lithium ion conductive crystalline solid electrolyte having a crystal structure similar to that of a sodium ion conductive solid electrolyte (a NASICON-based material) has recently been used in the range of 1 × 10 ⁻³ to 1 × 10 ⁻⁴ S · cm ⁻¹ lithium ion. It is reported to have conductivity.

For example, in JP-A-5-299101, Li _{1+ (4-n) x} M _x Ti _2-x (PO ₄ ) ₃ (M is a monovalent or divalent cation, and M is a monovalent cation) When n = 1 and M is divalent, n = 2 and x is 0.1 to 0.5) by sintering a granular electrolyte or the like represented by 1 × 10 ⁻³ to 1 × 10
A lithium ion conductivity of ⁻⁴ S · cm ⁻¹ has been obtained.

Japanese Patent Application Laid-Open No. Hei 10-97811 discloses that P ₂ O ₅ , SiO ₂ , TiO ₂ and Al ₂ having a predetermined composition ratio are used.
After melt molding O ₃ , Li ₂ O, etc., L
i _{1 + x + y} Al _x Ti _2-y P _3-y O ₁₂ (0 ≦ x ≦ 0.4, 0 <
(y ≦ 0.6) is disclosed, whereby a solid electrolyte having a lithium ion conductivity of 1.0 × 10 ^{−3 to} 2.0 × 10 ⁻³ S · cm ⁻¹ can be obtained.

Furthermore, in JP-A 6-111831 discloses a composite oxide with the solid electrolyte formed by the positive electrode integrally formed of a composite oxide of MnO ₂ or an alkali metal and manganese, MnO ₂ or an alkali metal and manganese The lithium compound reacts with the product and L
It has been proposed that by forming an i ₂ MnO ₃ layer, the contact area at the interface between the positive electrode and the solid electrolyte is increased to reduce the internal resistance of the battery, thereby improving the charge / discharge characteristics.

[0010]

Conventionally, in the case of a battery using a solid electrolyte, the electrode and the solid electrolyte are often joined only by pressure welding, the contact area between the electrode and the solid electrolyte is small, and the joining strength is low. However, the resistance at these interfaces was high, the internal resistance of the battery was high, and the charge and discharge characteristics were poor. In particular, there has been a problem that as the charge / discharge current increases, the voltage drop due to the internal resistance of the battery increases, and the current density is limited.

Further, many crystalline solid electrolytes have anisotropy in the ion conduction path, so that the grain boundary resistance in the solid electrolyte becomes a problem. Therefore, a sintered body is often used as a crystalline solid electrolyte.
In Japanese Patent Publication No. 1 (KOKAI), there is a proposal to particularly improve this problem. However, the problem of the ion conduction path also applies to the interface between the electrode and the solid electrolyte, and there is a problem that contact only by pressure welding increases the resistance of the interface.

[0012] Japanese Patent Application Laid-Open No. 6-111831 discloses that
It is a proposal to improve the interface resistance between the electrode and the solid electrolyte,
MnO ₂ is formed by sputtering, Li ₂ Mn
The process of forming O ₃ is complicated, for example, by reacting MnO ₂ with LiOH.

The present invention has been made in view of the above-mentioned conventional problems, and aims to improve an interface between an electrode and a solid electrolyte by improving the solid electrolyte.

[0014]

Means for Solving the Problems As a specific method,
A lithium battery having a solid electrolyte disposed between a pair of positive and negative electrodes mainly composed of an active material, wherein the solid electrolyte is a crystalline solid electrolyte powder containing lithium, titanium, and phosphorus elements, and titanium oxide and a lithium compound. Characterized by the fact that the mixture is calcined by adding a mixture of

The mixing ratio between the titanium oxide and the lithium compound is 1: 9 to 9: 9 in the ratio of the titanium element to the lithium element.
It is desirably 1.

The amount of the mixture of the titanium oxide and the lithium compound is 1 to 3 with respect to the solid electrolyte powder.
Desirably, it is 0 wt%.

Further, the active material of the positive electrode is Li _{1 + x} Mn.
_2-x O ₄ (0 ≦ x ≦ 0.2) or LiMn _2-y Me _y O ₄
(Me = Ni, Cr, Cu, Zn, 0 <y ≦ 0.6), and the active material of the negative electrode is Li ₄ Ti ₅ O ₁₂ or Li
It is preferably made of ₄ Mn ₅ O _12.

[0018]

In the lithium battery of the present invention, a mixture of titanium oxide and a lithium compound is added to the solid electrolyte powder and fired. And the solid junction between the electrode and the solid electrolyte can be formed. Therefore, the internal resistance of the battery can be reduced by increasing the contact area of the interface.

The solid electrolyte powder is desirably a crystalline solid electrolyte having lithium ion conductivity containing lithium, titanium and phosphorus elements. Inorganic solid electrolytes can be roughly classified into crystalline and non-crystalline, but amorphous and oxide-based solid electrolytes have a lithium ion conductivity of about 10 ⁻⁶ S · cm ⁻¹ at room temperature. No solid electrolyte that sufficiently satisfies the characteristics has been found. Furthermore, during the heat treatment, the amorphous solid electrolyte reacts with the active material of the electrode, and often forms a reaction product at the interface between the electrode and the solid electrolyte. Become. Therefore, a crystalline solid electrolyte is desirable.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the lithium battery of the present invention will be described. FIG. 1 is a sectional view showing a configuration example of the lithium battery according to the present invention. In FIG. 1, 1
Denotes a package, 2 denotes a pair of electrodes, 2a denotes a positive electrode, 2b denotes a negative electrode, 3 denotes a solid electrolyte layer, 4 denotes a positive electrode current collector, and 5 denotes a negative electrode current collector.

The material of the package 1 is not limited as long as it can maintain airtightness. For example, an aluminum laminate, a metal such as nickel or aluminum, or a shrink case can be used.

The positive electrode current collector 4 or the negative electrode current collector 5 is provided for current collection of the positive electrode 2a or the negative electrode 2b, and is made of a metal foil such as aluminum (Al), nickel (Ni), copper (Cu). Can be used.

The active material of the positive electrode 2a is Li _{1 + x} Mn _2-x O ₄
(0 ≦ x ≦ 0.2), LiMn _2-y Me _y O ₄ (Me = N
i, Cr, Cu, Zn, 0 <y ≦ 0.6) is selected. The active material of the negative electrode 2b is Li ₄ Ti ₅ O ₁₂ , L
One of i ₄ Mn ₅ O ₁₂ is selected.

The active material of the electrode is preferably a lithium oxide containing a transition metal element. When metal lithium is used for the negative electrode, a battery with a higher voltage can be obtained.However, when the solid electrolyte and metal lithium come into contact, the solid electrolyte may cause a reduction reaction, which may impair the characteristics of the solid electrolyte. is there. Further, in a solid electrolyte battery, since a separator and an organic electrolyte are not used, there is a limit to permit expansion and contraction of an electrode due to a charge / discharge reaction. Therefore, as the active material of the electrode used in the present invention, Li _{1 + x} Mn _2-x O ₄ (0 ≦ x ≦ 0.
2), LiMn _2-y Me _y O ₄ (Me = Ni, Cr, C
u, Zn, 0 <y ≦ 0.6), or Li ₄ Ti ₅ O ₁₂ ,
Li ₄ Mn ₅ O ₁₂ is desirable. Here, there is no clear distinction between the positive electrode active material and the negative electrode active material, and those showing a noble potential by comparing the charge and discharge potentials of the two compounds are shown as the positive electrode, and those showing the noble potential are shown as the negative electrode. 1 to
A battery with a voltage of 3.5 V can be configured.

The electrode 2 is mainly composed of an active material powder, and a conductive agent, a binder, and a solid electrolyte powder are added as necessary.
Further, the electrode 2 is used by applying it on a metal foil using a suitable binder and drying it, or by using a fired active material.

As the conductive agent, there is no problem as long as it is generally used for a lithium ion battery, and examples thereof include acetylene black, graphite, and Ketjen black.
As a binder, polyvinylidene fluoride (PVdF),
Examples include polytetrafluoroethylene (PTFE), polyolefins, and polyimide. Examples of the solid electrolyte powder include a crystalline powder containing at least lithium, titanium, and phosphorus elements. In this case, the solid electrolyte powder does not necessarily have to have the same composition as the solid electrolyte 3.

The electrode 2 is formed by dispersing the active material of the electrode in a solvent or water in which a binder is dissolved to obtain a slurry. The obtained slurry is applied on a metal foil of aluminum, nickel, copper, or the like by a doctor blade method, a casting method, a roll coater method, or the like, and formed by volatilizing a solvent or water.

The method for forming the fired electrode is as follows:
The active material is dispersed in water or a solvent in which a molding aid is dissolved to prepare a slurry. The slurry is applied to a base film by a doctor blade method, a casting method, a roll coater method, etc., dried, and then cut. (2) a method in which the active material is directly or granulated into a powder, charged into a mold, pressed and molded by a press machine, and then fired; (3) a rolled press is used to form the granulated powder. And then forming the sheet into a sheet, cutting the sheet, and firing the sheet. (2),
The granulation of (3) may be wet granulation of granulating from the slurry described in the method of (1) or dry granulation without using a solvent. In the method (2), a molding aid may not be used.

As a molding aid that can be used here, a general organic binder for granulating ceramics can be used.

As the substrate film, for example, resin films such as polyethylene terephthalate, polypropylene, polyethylene, and tetrafluoroethylene, and metal foils such as aluminum, stainless steel, and copper can be used.

The solid electrolyte 3 is a solid electrolyte obtained by mixing a mixture of titanium oxide and a lithium compound with a crystalline solid electrolyte powder containing elements of lithium, titanium and phosphorus and sintering the mixture. Here, as the solid electrolyte powder, Li
_{1 + x} M _x Ti _2-x (PO ₄ ) ₃ (where M is Al, Sc,
Y, La), Li _{1 + x} Ti _2-x (PO ₄ ) ₃ , Li _0.5-3x R
_{0.5 + x} TiO ₃ (where R is La, Pr, Nd, Sm),
Li _{1 + x + y} Al _x Ti _2-x Si _y P _3-y O ₁₂ , Li _{1+ (4-n)} M
_x Ti _2-x (PO ₄ ) ₃ (M is a monovalent or divalent cation)
And the like. Further, as the lithium compound, L
i ₂ CO ₃ , LiOH, hydrate of LiOH, or Li
₃ PO _{4 and the} like.

Here, the mixing ratio of titanium oxide and lithium compound is 1: 9 to 9: 9 in terms of the ratio of titanium element to lithium element.
It is desirably 1. If the lithium element is more than 1: 9, lithium dissolved in the solid electrolyte powder becomes excessive, and the solid electrolyte may have electronic conductivity. Further, when the lithium element is less than 9: 1, the grain boundary resistance of the solid electrolyte cannot be reduced. In consideration of the above, the most preferable mixing ratio of titanium oxide and lithium compound is 3: 7 by the ratio of titanium element to lithium element.
7: 3.

The amount of the mixture of titanium oxide and lithium compound to be added to the solid electrolyte powder is 1 to 30 wt.
% Is desirable. The amount of this mixture added is 1 wt.
%, The interaction with the solid electrolyte is not sufficiently performed, so that the grain boundary resistance of the solid electrolyte cannot be reduced, and the adhesion to the electrode 2 also decreases. When the amount of the mixture is more than 30 wt%,
The grain boundary component to the solid electrolyte 3 increases, and the lithium ion conductivity of the entire solid electrolyte 3 decreases. In view of the above, the most preferable mixture amount of the titanium oxide and the lithium compound is 5 to 20% by weight.

As a method for forming the solid electrolyte 3, a titanium oxide and a lithium compound are added to a crystalline solid electrolyte powder containing at least lithium, titanium and phosphorus elements such that titanium and lithium have a predetermined element ratio. And mix wet or dry. Hereinafter, the obtained mixed powder is subjected to the same steps as in the method for forming the electrode 2 to be fired, whereby the solid electrolyte 3 is obtained.

As a method of bringing the electrode 2 coated and dried on the metal foil into close contact with the solid electrolyte 3, the solid electrolyte 3 is sandwiched between the positive electrode 2a and the negative electrode 2b, and the positive electrode 2a and the negative electrode 2b
A method in which pressure is applied from the current collector side, or a method in which a sandwich is formed in the same manner as described above using an electrode coated on a metal foil as described above, and the temperature is increased to a temperature near the melting point of the binder. is there. In this case, when pressure is applied from the current collector side of the positive electrode 2a and the negative electrode 2b, the adhesion is further improved, which is effective.

As a method of bringing the fired electrode 2 and the solid electrolyte 3 into close contact with each other, there is a method in which the solid electrolyte 3 is sandwiched between the positive electrode 2a and the negative electrode 2b, and heating under normal pressure or heating under pressure. In particular, heating by pressurization is effective because the adhesion between the electrode 2 and the solid electrolyte 3 is improved and the processing time is short. The electrode 2 and the solid electrolyte 3 used
Can use the positive electrode 2a, the solid electrolyte 3, and the negative electrode 2b which are individually fired, or the fired body of the solid electrolyte 3 and the formed positive electrode 2a, and the negative electrode 2b.

The solid electrolyte battery to which the present invention is applied may be a primary battery or a secondary battery. The battery shape is not limited to a cylindrical type, a square type, a button type, a coin type, a flat type and the like.

[0038]

EXAMPLE [Electrode] Lithium hydroxide and manganese dioxide were mixed at a molar ratio of Li: Mn of 1.1: 1.9, and the mixture was heated and fired at 650 ° C. in the atmosphere for 15 hours. Lithium manganese composite oxide (Li _1.1 Mn
_1.9 O ₄ ) was synthesized and used as an active material for a positive electrode. Next, lithium hydroxide and manganese dioxide are mixed such that the molar ratio of Li and Mn is 4: 5, and the mixture is mixed with 60
Heating and baking at 0 ° C. for 15 hours to synthesize a lithium manganese composite oxide (Li ₄ Mn ₅ O ₁₂ ), which was used as an active material of the negative electrode.

A slurry was prepared by dispersing the above-mentioned active materials of the positive electrode and the negative electrode and acetylene black in N-methyl-2-pyrrolidone in which polyvinylidene fluoride was dissolved. The obtained slurry was applied on an aluminum foil by a doctor blade method to evaporate N-methyl-2-pyrrolidone. Further, a roll pressure was applied for the purpose of improving the filling rate of the powder of the dried sheet electrode. The thickness of each electrode finally obtained was 30 μm. This is 25mm x 25
The positive electrode 2a and the negative electrode 2b were obtained by cutting into a size of mm.

[Solid Electrolyte] A crystalline solid electrolyte powder containing elements of lithium, titanium and phosphorus is LiTi ₂ (P
O ₄ ) ₃ was used. To this powder, 10 wt% of titanium oxide and lithium hydroxide in the proportions shown in Table 1 were added to the solid electrolyte powder.
A mixture of titanium oxide and a lithium compound in a ratio of titanium element to lithium element of 5: 5 was added in the amounts shown in Table 2.

These mixed powders were wet-mixed using isopropyl alcohol (IPA) as a medium. After sufficient mixing, isopropyl alcohol was volatilized to obtain a mixed powder of solid electrolyte powder, titanium oxide and lithium hydroxide. A slurry for sheet formation was prepared using the obtained mixed powder, isobutyl methacrylate as a binder, a plasticizer, a dispersant, and toluene. The obtained slurry was applied on a polyethylene film by a doctor blade method, and toluene was volatilized to obtain a green sheet for a solid electrolyte. At this time, the sheet thickness was 70 μm.

Next, the above-mentioned green sheet was 35 mm ×
Cut to a size of 35 mm, and 800 ° C to 1000
The solid electrolyte 3 was obtained by firing at 2 ° C. for 2 hours. At this time, the solid electrolyte 3 has a thickness of 50 μm and a size of 28 mm × 28 mm.
It was the size of.

[Evaluation Cell] The sintered body of the solid electrolyte is sandwiched between the positive electrode 2a and the negative electrode 2b obtained above, and the melting point of the polyvinylidene fluoride is near the melting point for the purpose of further improving the adhesion between the electrode and the solid electrolyte. Heated at a temperature of about 170 ° C.

The positive electrode current collector 4 was joined to the positive electrode 2a of the laminate, and the negative electrode current collector 5 was joined to the negative electrode 2b. Aluminum laminate was cut into 35mm x 35mm size.
One sheet was prepared, the laminated body to which the current collector was bonded was sandwiched, and the outer periphery of the aluminum laminate was thermocompression-bonded to assemble the 35 mm × 35 mm prismatic lithium battery shown in FIG.

[Evaluation] Using the thus obtained prismatic lithium battery, a charging / discharging device was used to set the charging conditions to 100
μA / cm ² , 200 μA / cm ² , 500 μA / cm ²
The above-mentioned rectangular solid electrolyte battery is charged to 1.5 V with the current of, and after the voltage reaches 1.5 V, charging is stopped and held for 5 minutes. Then, the battery was charged to 1.5 V again, and after reaching this voltage, the charging was stopped and the charge / discharge cycle was evaluated for 5 minutes.

The results are shown in Tables 1 and 2. The numbers in the table indicate the discharge capacity for each discharge current, and the unit is mA.
h.

[0047]

[Table 1]

[0048]

[Table 2]

From the above, it can be seen that lithium batteries within the scope of the present invention have excellent charge / discharge characteristics. In particular, it is remarkable that the decrease in the discharge capacity is small even when the discharge current increases.

Further, the lithium battery after the evaluation is disassembled, and
When the electrode and the solid electrolyte were separated, the sample within the scope of the present invention was separated between the current collector and the electrode because the electrode and the solid electrolyte were firmly adhered. On the other hand, in the samples outside the scope of the present invention, the adhesion between the electrode and the solid electrolyte was weak, and the sample was separated between the electrode and the solid electrolyte.

In the lithium battery of the present invention, titanium oxide and a lithium compound are mixed with the solid electrolyte powder and fired, whereby the solid electrolyte powder and the titanium oxide and the lithium compound interact with each other, and the grain boundary resistance of the solid electrolyte is reduced. And a solid bond between the electrode and the solid electrolyte can be formed even at the interface between the electrode and the solid electrolyte. Therefore, it is assumed that the resistance at the interface between the electrode and the solid electrolyte was also reduced, and as a result, the internal resistance was reduced.

[0052]

As described above, according to the lithium battery of the present invention, a solid electrolyte obtained by adding a mixture of lithium oxide and a lithium compound to a crystalline solid electrolyte powder containing lithium, titanium, and phosphorus is fired. By using
The grain boundary resistance of the solid electrolyte can be reduced, and the interfacial resistance between the electrode and the solid electrolyte can be reduced, whereby a lithium battery having excellent charge / discharge characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of a lithium battery according to the present invention.

[Explanation of symbols]

1 ··· package, 2 ··· pair of electrodes, 2a ·
... Positive electrode, 2b ... Negative electrode, 3 ... Solid electrolyte layer, 4 ... Positive electrode current collector, 5 ... Negative electrode current collector

Continuing from the front page (72) Inventor Shinji Magome 3-5 Koukodai, Seika-cho, Soraku-gun, Kyoto Kyoto The inside of Central Research Laboratory, Sera Corporation (72) Inventor Tohru Hara 3-5 Koukodai, Seika-cho, Soraku-gun, Kyoto Kyoto Inside the Central Research Laboratory of Sera Corporation (72) Nobuyuki Kitahara 3-5-5 Kodai, Seika-cho, Soraku-gun, Kyoto Prefecture Kyoto Central Research Laboratory (72) Inventor Eiji Higuchi 3-5-2 Kodai, Seika-cho, Soraku-gun, Kyoto Prefecture Address Kyocera Corporation Central Research Laboratory F-term (reference) 5H003 AA01 BA01 BA03 BB05 BC01 BD00 BD03 BD04 5H024 AA02 BB01 BB07 CC04 EE06 FF23 HH00 HH01 5H029 AJ01 AK03 AL03 AM12 BJ04 CJ02 CJ08 DJ16 HJ01 HJ02

Claims (4)

[Claims] 1. A lithium battery comprising a solid electrolyte disposed between a pair of positive and negative electrodes mainly composed of an active material,
A lithium battery, wherein the solid electrolyte is obtained by adding a mixture of titanium oxide and a lithium compound to a crystalline solid electrolyte powder containing lithium, titanium, and phosphorus elements and firing the mixture. 2. The lithium battery according to claim 1, wherein a mixing ratio of the titanium oxide and the lithium compound is 1: 9 to 9: 1 in a ratio of the titanium element to the lithium element. 3. An amount of the mixture of the titanium oxide and the lithium compound is 1 to 30 watts with respect to the solid electrolyte powder.
The lithium battery according to claim 1, wherein the lithium battery is t%. 4. The method according to claim 1, wherein the active material of the positive electrode is Li _{1 + x} Mn _2-x O _4.
(0 ≦ x ≦ 0.2) or LiMn _2-y Me _y O ₄ (Me
= Ni, Cr, Cu, Zn, 0 <y ≦ 0.6), and the active material of the negative electrode is Li ₄ Ti ₅ O ₁₂ or Li ₄ M
Lithium battery according to claim 1, claim 2 or claim 3, characterized in that it consists of n ₅ O _12.