JP2000138073A - Entire solid lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate occurrence of leak of an electrolyte, improve safety, and improve a charge/discharge cycle characteristic by impregnating solid electrolyte and electrodes with a polymeric solid electrolyte containing solute and polymerizing them. SOLUTION: Polymeric solid electrolyte containing solute is impregnated into a pair of electrodes 4 comprising solid electrolyte 5 using lithium oxide containing transition metal such as Ti, V, Cr, Mn, Fe, Co, or Ni, a positive electrode 2, and a negative electrode 3, and they are polymerized. This polymeric solid electrolyte is composed of one or plural kinds of polyethylene-oxide polymeric solid electrolyte and polypropylene-oxide polymeric solid electrolyte, and the solute is composed of one or plural kinds of LiBF4, LiN(CF3SO2)2, LiC(CF3SO2)2. Thus, a contact area of the solid electrolyte 5 and the electrodes 4 can be increased, therefore, movement of lithium ions following a charge/ discharge reaction is smoothened, and internal resistance of this secondary battery can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an all-solid lithium secondary battery using a solid electrolyte as an electrolyte.

[0002]

2. Description of the Related Art Conventionally, various batteries using an aqueous or non-aqueous electrolyte between a pair of positive and negative electrodes have been developed. As portable information terminal devices such as mobile phones have become thinner, lighter and smaller, they have not been satisfactory in terms of rate characteristics, cycle characteristics, storage characteristics, energy density, and the like in charge and discharge.

As a battery satisfying these characteristics,
Research and development of secondary batteries with high voltage and high energy density are being actively conducted, and lithium secondary batteries using metallic lithium for the negative electrode are particularly portable as lightweight and reusable small secondary batteries. It is in the limelight as a power source for devices that do not have information terminals.

[0004] However, the conventional lithium secondary battery uses lithium metal for the negative electrode. Although this lithium metal is highly reactive, it is a flammable material and may be ignited suddenly. Problem.

In order to solve such a problem, there has been proposed a lithium secondary battery in which a negative electrode is replaced by a material having a layered structure such as graphite as a material constituting an electrode (see Japanese Patent Application Laid-Open No. 7-12226). ).

However, since this lithium secondary battery uses graphite or the like having a layered structure as a negative electrode, various terminal groups such as quinone groups and ketone groups bonded to the graphite surface are easily reduced electrochemically. In many cases, gas is generated by the electrochemical reduction reaction involving the terminal groups and the electrolyte in the first initial charging reaction, the internal pressure of the battery rises, the battery ruptures, the battery performance deteriorates, and the Since a flammable organic electrolyte in which a lithium salt is dissolved is used, there is a problem of liquid leakage, and there is also a problem that double and triple safety measures are required to ensure safety.

A lithium secondary battery in which a solid electrolyte comprising an organic polymer and an organic electrolytic solution is interposed between a pair of positive and negative electrodes to suppress the fluidity of the electrolyte has also been proposed (JP-A-8-315855). ).

However, this lithium secondary battery contains an organic electrolyte, and in the charging / discharging process at a high temperature, the organic electrolyte causes a decomposition reaction to generate gas, thereby increasing the internal pressure of the battery. There is a problem of doing.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems in the prior art, and it has been found that the electrolyte generates a gas by an oxidation-reduction reaction and ruptures, or the electrolyte leaks. It is an object of the present invention to provide an all-solid lithium secondary battery which is excellent in safety and has improved charge / discharge cycle characteristics.

[0010]

In order to achieve the above object, according to the all-solid lithium secondary battery of the present invention, the solid electrolyte and the electrode absorb and release ions accompanying the electrochemical redox reaction. In an all-solid lithium secondary battery formed by combining materials exhibiting a phenomenon, the solid electrolyte and the electrode were impregnated with a polymer solid electrolyte containing a solute and polymerized.

In the above all-solid lithium secondary battery, the polymer solid electrolyte comprises one or more of a polyethylene oxide-based polymer solid electrolyte and a polypropylene oxide-based polymer solid electrolyte, and the solute is LiBF ₄ , LiN (CF ₃ S
Desirably, it is composed of one or more of O ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₂ .

[0012]

In the all-solid-state lithium secondary battery of the present invention, a liquid, particularly an organic electrolyte, is not used for the electrolyte.
Eliminating electrolyte leakage, the electrolyte does not react with moisture or air, and the solid electrolyte and the electrode are impregnated with a polymer solid electrolyte containing a solute and polymerized. The polymer solid electrolyte is present in the gap between the solid electrolyte particles in the state and the electrode active material particles, so that the contact area between the solid electrolyte and the electrode can be increased, and the movement of lithium ions accompanying the charge / discharge reaction is smooth. Thus, an all-solid lithium secondary battery having excellent secondary battery characteristics can be obtained.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the all-solid lithium secondary battery of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view showing an embodiment of the all-solid lithium secondary battery of the present invention. In FIG. 1, reference numeral 1 denotes an all-solid lithium secondary battery including a pair of electrodes 4 including a positive electrode 2 and a negative electrode 3 and a solid electrolyte 5. The positive electrode 2 and the negative electrode 3 forming the pair of electrodes 4 include a current collector. 6 is formed by applying
A solid electrolyte 5 is interposed between the pair of electrodes 4 and is packaged with a package 7 capable of maintaining airtightness.

The all-solid-state lithium secondary battery of the present invention exhibits the occlusion and release phenomena of ions accompanying the electrochemical oxidation-reduction reaction, and uses a combination of a material having a charge / discharge potential to be described later and an electrode Is not particularly limited as long as it shows such a phenomenon and has a predetermined charge / discharge potential. For example, LiCoO ₂ , L
iNiO ₂ , LiCrO ₂ , LiVO ₂ , LiNi _1/2
Co _1/2 O ₄ , LiMn ₂ O ₄ , LiMnO ₂ , Li _{4
Mn 5 O 12, LiTiO 2,} LiFeO 2, LiRuO
₂ , LiWO ₂ , λ-MnO ₂ , V ₂ O ₅ , Li ₄ Ti
Known materials such as ₅ O ₁₂ , anatase TiO ₂ and Nb ₂ O ₅ can be used.

In consideration of the electromotive force of the secondary battery, it is desirable that the difference between the charge and discharge potentials of the material forming the electrode 4 is large, and the charge and discharge potential of the material forming the solid electrolyte 5 is further higher than those. Lower is more preferred. For example, the active material forming the electrode 4 is made of a material in which the charge and discharge potential of the positive electrode 2 is 2.5 to 4.0 V with reference to metallic lithium, and the charge and discharge potential of the negative electrode 3 is 2.0 to 3.0 V. More preferably, the solid electrolyte 5 is made of a material having a charge / discharge potential of 1.5 to 2.9 V with respect to lithium metal as a reference potential.

Metallic lithium is used as a reference potential, a material having the lowest charge / discharge potential is used as the material of the solid electrolyte 5, and a material exhibiting an intermediate charge / discharge potential is applied to the active material constituting the negative electrode 3, and the highest charge / discharge potential is applied. A material exhibiting a discharge potential is combined as an active material constituting the positive electrode 2, and a large difference between the charge and discharge potentials of the positive electrode 2 and the negative electrode 3 with metal lithium as a reference potential enables a higher charge and discharge potential to be obtained. It is a desirable combination. That is, the charge / discharge potential of the all-solid lithium secondary battery of the present invention is determined by the difference between the charge / discharge potentials of the positive electrode 2 and the negative electrode 3 as in other secondary batteries.

Specifically, LiCoO ₂ , whose charge / discharge potential is about 4 V with metallic lithium as a reference potential, is
Is selected as the active material constituting the negative electrode 3, the active material constituting the negative electrode 3 is LiMn having a charge / discharge potential of about 3 V.
O ₂ can be selected, and the material of the solid electrolyte 5 needs to have a charge / discharge potential lower than the charge / discharge potential of the active material constituting the negative electrode 3. Li ₄ Ti exhibiting a charge / discharge potential of 0.55 V
It is desirable to use ₅ O ₁₂ . With such a combination, a laminate is formed as each of the positive electrode 2, the negative electrode 3, and the solid electrolyte 5, and when the charge / discharge potential is measured, it can be confirmed that the laminate exhibits a charge / discharge potential of about 1V.

Further, as a specific example of using a metal oxide for the negative electrode 3, an anatase type TiO ₂ having a charge / discharge potential of about 1.8 V with metallic lithium as a reference potential is used as an active material constituting the negative electrode 3. LiM which has a charge / discharge potential of about 4 V for the active material constituting the positive electrode 2 can be selected.
n ₂ O ₄ was selected, and Li ₄ Ti ₅ O ₁₂ having a charge / discharge potential of about 1.55 V was selected as the material of the solid electrolyte 5.
When a laminate is formed as the positive electrode 2, the negative electrode 3, and the solid electrolyte 5 by combining these, and the charge / discharge potential is measured, it can be confirmed that the laminate exhibits a charge / discharge potential of about 2.2V.

The solid electrolyte 5 includes Ti, V, Cr, Mn,
Since a lithium oxide containing a transition metal such as Fe, Co, and Ni is used, and the selection of such a combination is wide, the lithium oxide containing a transition metal is 1.5 to 1. Most preferably, it is lithium titanate (Li ₄ Ti ₅ O ₁₂ ) having a charge / discharge potential of 6 _V.

As described above, when the solid electrolyte 5 and the electrode 4 are formed by combining a material exhibiting the occlusion and release phenomena of ions accompanying the electrochemical oxidation-reduction reaction, the negative electrode does not use metallic lithium. The deposition reaction of lithium accompanying the discharge reaction can be suppressed, and since no organic electrolyte is used, no liquid leakage occurs and safety can be improved.

The all-solid lithium secondary battery as described above is
The positive electrode 2 and the negative electrode 3 are prepared by adding a binder made of a Teflon-based or styrene-based organic material to a solid electrolyte material, and the electron-conductive material such as acetylene black, Ketjen black or graphite is added to the active material. And a binder made of a Teflon-based or styrene-based organic substance is added, molded and molded, and heat-treated at a temperature of about 80 to 200 ° C.

In the all-solid lithium secondary battery of the present invention, the solid electrolyte 5 and the electrode 4 are polymerized by impregnating a solid polymer electrolyte containing a solute. This polymer solid electrolyte is composed of one or more of a polyethylene oxide-based polymer solid electrolyte and a polypropylene oxide-based polymer solid electrolyte, and the solutes are LiBF ₄ , LiN (C
It is composed of one or more of F ₃ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₂ .

As described above, when the solid electrolyte 5 and the electrode 4 are impregnated with the polymer solid electrolyte containing a solute and polymerized, the presence of the polymer solid electrolyte in the gaps between the solid electrolyte particles and the electrode active material particles is reduced. The contact area between the solid electrolyte and the electrode can be increased, the movement of lithium ions accompanying the charge / discharge reaction becomes smooth, and the internal resistance of the secondary battery can be reduced. An all-solid lithium secondary battery having excellent cycle characteristics can be obtained.

As the polymer solid electrolyte, it is desirable to use one or more of a polyethylene oxide-based polymer solid electrolyte and a polypropylene oxide-based polymer solid electrolyte. In addition to polyethylene oxide-based polymer solid electrolyte or polypropylene oxide-based polymer solid electrolyte, there are polymer solid electrolytes such as polyethyleneimine-based, polyalkylene sulfide-based, and polyvinylpyrrolidone-based polymers. The solid electrolyte is characterized by being chemically stable at home in charge and discharge, and hardly causing lithium ion deposition.

As the solute, LiBF ₄ , LiN (CF
It is desirable to use one or more of ₃ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₂ . In addition to these, there are LiClO ₄ , LiPF ₆ , LiAsF _{6 and the} like, but LiBF ₄ , LiN (CF ₃ SO ₂ ) ₂ ,
LiC (CF ₃ SO ₂ ) _{2 and the} like are thermally stable in a polyethylene oxide-based polymer solid electrolyte or a polypropylene oxide-based polymer solid electrolyte, and have high withstand voltage.

The polyethylene oxide-based polymer solid electrolyte or the polypropylene oxide-based polymer solid electrolyte is prepared by adding a polymerization initiator such as a peroxide or an azo to a polymer solid electrolyte in which a solute has been dissolved, and It can be polymerized quickly by irradiation or heating.

As the current collector 6, a metal foil such as aluminum (Al), copper (Cu) or nickel (Ni) can be used.

The material of the package 7 is not limited as long as it can maintain airtightness. For example, a laminate made of aluminum, a metal such as nickel (Ni), aluminum (Al), or a shrink case can be used.

[0029]

Next, the all solid lithium secondary battery of the present invention was evaluated as described in detail below.

EXAMPLE First, 80% by weight of LiCoO ₂ as an active material constituting the positive electrode, 11% by weight of acetylene black as an additive for imparting electronic conductivity, and 9% by weight of a Teflon-based binder were used. After mixing, a known organic solvent was added to the mixture at the same weight ratio and mixed to prepare a positive electrode forming paste.

On the other hand, L as an active material constituting the negative electrode
After mixing 80% by weight of i ₄ Mn ₅ O ₁₂ , 11% by weight of acetylene black as an additive for imparting electronic conductivity, and 9% by weight of a Teflon-based binder, the mixture is mixed with a known organic solvent. A paste for forming a negative electrode was prepared by adding and mixing at a weight ratio.

Next, a 20 μm-thick aluminum foil was used as a current collecting plate, and a paste for forming a positive electrode and a paste for forming a negative electrode were applied on the aluminum foil, dried sufficiently, and the solvent was removed. Thus, the thickness of the positive electrode was adjusted to 80 μm, and the thickness of the negative electrode was adjusted to 75 μm.

On the other hand, Li ₄ Ti ₅ O ₁₂ is used as a solid electrolyte.
Was mixed with 10% by weight of a Teflon-based binder, and a known organic solvent was added to the mixture at the same weight ratio and mixed to prepare a paste for forming a solid electrolyte.

Next, the obtained paste for forming a solid electrolyte is applied to a positive electrode or a negative electrode, and is sufficiently dried to remove the solvent.
It was adjusted to μm.

Therefore, in this embodiment, the highest charge / discharge potential of about 4 V with LiC
oO ₂ is used as a positive electrode active material, Li ₄ Mn ₅ O ₁₂ having a charge / discharge potential of about 3 V is used as a negative electrode active material, and Li ₄ Ti ₅ O ₁₂ having a charge / discharge potential of about 1.55 V, which is the lowest, is a solid electrolyte. As an electrochemical device.

In the solid polymer electrolyte to be impregnated and cured in the laminate, 92 wt% of polyethylene oxide in a monomer state and 8 wt% of LiBF ₄ as a solute were dissolved. Since LiBF ₄ is hard to dissolve directly in polyethylene oxide, it was previously dissolved in N-methyl-2-pyrrolidone and then mixed with monomeric polyethylene oxide. This mixture was impregnated into the laminate, NMP was evaporated and dried at a predetermined temperature, and then impregnated with a polymer solid electrolyte to polymerize. The obtained laminate was assembled into an airtight cell for measurement.

After the electrode thus obtained and the electrode having the solid electrolyte layer applied thereto were cut into a 30 mm square size,
After vacuum drying at a temperature of 120 ° C. for 2 hours, both electrodes were bonded to each other to form a laminate, and the roll was pressed to improve the adhesion.

(Comparative Example) LiCoO ₂ was used as an active material constituting the positive electrode, and LCoO ₂ was used as an active material constituting the negative electrode.
Except that i ₄ Mn ₅ O ₁₂ was employed, the positive electrode was adjusted to a thickness of 80 μm and the negative electrode was adjusted to a thickness of 75 μm in the same manner as in Example 1, and Li ₄ Mn ₅ O was used as a solid electrolyte.
_The thickness was adjusted to 20 μm in the same manner as in Example 1 except that ₁₂ was used.

In this comparative example, the laminate was not impregnated and cured with a solid polymer electrolyte, and the electrode obtained in the same manner as in Example 1 and the electrode having the solid electrolyte layer applied thereto were not cured.
After being cut into a size of mm square, vacuum drying was performed at a temperature of 120 ° C. for 2 hours, and then both electrodes were bonded to each other to produce a laminate. It was assembled into an airtight cell for measurement.

(Evaluation) Using the cell for evaluation thus obtained, a charging / discharging device was used to set the charging condition to 500 μA.
The cell for evaluation is charged to 2.5 V with a current of
After the voltage reaches 2.5 V, the charging is stopped and held for 5 minutes, then discharged with a discharge current of 500 μA to a voltage of 0.5 V, and then charged again to 2.0 V, and after reaching this voltage, A charge / discharge cycle test in which charging was stopped and held for 5 minutes was performed, and the amount of discharged electricity was determined for each fixed cycle to evaluate battery performance as a secondary battery.

As a result, in the sample of the comparative example, the amount of discharge electricity was 80% or less of the initial value in 20 charge / discharge cycles, whereas in the sample of the above example, the discharge electricity was equal in the same charge / discharge cycle. The amount retained at 80% or more of the initial value, indicating that the cycle characteristics were excellent. That is,
The polymer electrolyte is impregnated into the solid electrolyte and the voids in the electrode and polymerized, so that the contact area between the solid electrolyte and the electrode can be increased, and the movement of lithium ions during the charge / discharge reaction becomes smoother. You can see that

It should be noted that the present invention is not limited to the above-described embodiment, but can be implemented with appropriate modifications without departing from the scope of the invention.

[0043]

As described above, the all-solid lithium secondary battery of the present invention impregnates the solid electrolyte and the electrode with the polymer solid electrolyte containing a solute and polymerizes the same. Liquid leakage does not occur, safety is improved, and the solid electrolyte and the electrode active material particles have a polymer solid electrolyte in the gap, so that the contact area between the solid electrolyte and the electrode can be increased. In addition, the movement of lithium ions accompanying the charge / discharge reaction becomes smooth, the internal resistance of the secondary battery can be reduced, and an all-solid lithium secondary battery having excellent secondary battery characteristics, particularly excellent cycle characteristics, can be obtained. .

[Brief description of the drawings]

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

[Explanation of symbols]

1 ‥‥‥ All-solid lithium secondary battery, 2 ‥‥‥ positive electrode, 3 ‥
{Negative electrode, 4} A pair of electrodes, 5} Solid electrolyte

Claims (2)

[Claims] 1. An all-solid lithium secondary battery in which a solid electrolyte and an electrode are formed by combining a material exhibiting an occlusion and release phenomenon of ions associated with an electrochemical oxidation-reduction reaction, wherein the solid electrolyte and the electrode contain a solute. An all-solid lithium secondary battery characterized by being impregnated with a polymer solid electrolyte and polymerized. 2. The polymer solid electrolyte comprises one or more of a polyethylene oxide-based polymer solid electrolyte and a polypropylene oxide-based polymer solid electrolyte, and the solute is LiBF ₄ , LiN (CF _{3).
SO 2) 2, LiC (CF} 3 SO 2) according to claim 1, characterized in that it consists of any one or more of the ₂
2. The all-solid lithium secondary battery according to 1.