JPH07192752A - Lithium secondary battery - Google Patents

Abstract

PURPOSE:To provide a lithium secondary battery in which the growth of dendrites is retarded, charge/discharge efficiency is stable, and cycle life is long in addition to the advantage of high energy density brought by use of an organic electrolyte. CONSTITUTION:An organic solution prepared by blending a lithium base electrolyte and a lithium complex ion forming agent is used as an electrolyte 4, and a lithium alloy is used in a negative electrode 3. A smooth lithium deposition surface is formed on the lithium alloy negative electrode, and deterioration in surface condition is prevented. The high retainability of initial discharge capacity, long cycle life, high reliability, and large discharge capacity can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery in which dendrites are less likely to grow, the charge / discharge efficiency is stable, and the life is long.

[0002]

2. Description of the Related Art Heretofore, a lithium secondary battery has been known in which an organic solution containing a lithium-based electrolyte is used as an electrolytic solution and metallic lithium is used as a negative electrode. This has an advantage that a battery output with high energy density can be obtained by using an organic electrolyte solution. However, there is a problem that dendrites easily grow on the surface of the lithium negative electrode during charging and penetrate the separator (electrolyte layer) between the positive and negative electrodes to cause a short circuit problem.

In view of the above, the present inventors have tried to use a lithium alloy for the negative electrode. As a result, it has been found that this method can overcome the above-mentioned dendrite problem without impairing the advantage of using an organic electrolyte. However, in further intensive research, the effect of suppressing the dendrite growth was recognized in the case of the conventional normal current density, but when the current density was increased and charging / discharging was repeated, the electrodeposition state of lithium on the negative electrode surface changed. It was found that the charging and discharging efficiency deteriorated and the charging and discharging efficiency decreased.

[0004]

SUMMARY OF THE INVENTION Therefore, the present invention is intended to overcome the problem of deterioration of charge / discharge efficiency when a lithium alloy is used for the negative electrode, and further, a battery having a high energy density by using an organic electrolyte solution. It is an object to develop a lithium secondary battery that takes advantage of the output, has a dendrite that does not grow easily, shows stable charge and discharge efficiency during repeated charge and discharge, and has a long life.

[0005]

Means for Solving the Problems The present invention is a lithium secondary battery characterized in that an organic liquid containing a lithium-based electrolyte and a lithium complex ion forming agent is used as an electrolyte and a lithium alloy is used as a negative electrode. Is provided.

[0006]

In the above-mentioned prior art, the present inventors have found that the deterioration of the charge / discharge efficiency under the high current density is caused by the deterioration of the electrodeposition state of lithium in the lithium alloy negative electrode, which causes the deterioration of the state of the negative electrode surface. The present invention has solved the problem by using a lithium complex ion forming agent. That is, in the organic electrolytic solution containing the lithium complex ion-forming agent, a smooth or flat and dense lithium electrodeposition surface is formed on the lithium alloy negative electrode even at a higher current density than in the case of electrodeposition from ordinary lithium single ions. The deterioration of the surface condition of the negative electrode is prevented, the growth of dendrites is prevented, the deterioration of discharge capacity and charge / discharge efficiency is prevented, high efficiency is maintained, and repeated charge / discharge can be stably performed for a long life. A battery having excellent properties can be obtained.

[0007]

EXAMPLES The lithium secondary battery of the present invention uses an organic liquid in which a lithium-based electrolyte and a lithium complex ion forming agent are mixed in the electrolytic solution, and uses a lithium alloy in the negative electrode.
An example thereof is shown in FIG. This is an example of a coin type, 3 is a negative electrode made of a lithium alloy, and 4 is an electrolyte layer (separator). In addition, 1, 7 is a battery can, 2,
6 is a nickel plate for collecting current, 5 is a positive electrode, and 8 is an insulating sealing material.

An appropriate lithium salt can be used as the lithium-based electrolyte in the electrolytic solution, and examples thereof include LiClO ₄ , LiBF ₄ , LiPF ₄ , LiAsF ₃ and Li.
AlCl ₄ , Li (CF ₂ SO ₂ ) _{2 and the} like can be mentioned.

As the lithium complex ion forming agent, an appropriate agent that forms a lithium complex ion can be used. A macrocyclic compound or the like is preferably used from the viewpoint of forming a flat lithium electrodeposition surface on the negative electrode even at a high current density. Specific examples of the macrocyclic compound include 12 crown 4 derivatives, 14 crown 4 derivatives, 15 crown 5 derivatives, 1
Examples include 8-crown-6 derivatives and macrocyclic diazapolyether derivatives.

As the organic liquid as a solvent in the electrolytic solution, a suitable one which dissolves a lithium salt may be used. Generally, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, sulfolane, γ-butyrolactone, 1,2-dimethoxyethane, diethyl ether, 1,3-dioxolane, methyl formate. , Methyl acetate,
N, N-dimethylformamide, a mixture thereof or the like is used.

The concentration of the lithium-based electrolyte in the electrolytic solution may be appropriately determined, but is generally 0.1 to 3 mol / liter. The concentration of the lithium complex ion forming agent is
From the viewpoint of stable formation of the electrodeposition surface, the stability constant of the formed lithium complex ion is preferably 10 ² or more, and from this point, 0.1 to 2 times the molar number of lithium ion in the electrolytic solution, In particular, the molar number is preferably 0.5 to 1.5 times. If the number of moles is less than 0.1 times, the addition effect may be poor, and if it exceeds 2 times, the resistance of the electrolytic solution increases and the internal resistance of the battery tends to increase.

As the lithium alloy forming the negative electrode, for example, a suitable lithium alloy composed of an intermetallic compound of Al, Bi, Sn, In or the like and Li, or Ag, A
Li-X-Te-based alloys containing an X component consisting of at least one of 1, 1, Mg, Zn, and Ca, among others, such as charge / discharge cycle life, high electromotive force, high discharge capacity, and high energy density. From the viewpoint, the Li: Ag: Te atomic ratio of the Li-Ag-Te alloy is preferably 80 to 150: 1-2.
Examples thereof include 0: 2 to 30. Particularly, those containing 80 atomic% or more of Li are preferable.

The lithium secondary battery of the present invention is not particularly limited except that the above-mentioned electrolytic solution and negative electrode are used, and can be formed in a conventional manner. Therefore, for the positive electrode, for example, MnO ₂ , LiCoO ₂ , Li _w Co _1-xy M _x P _y O
_{2 + z} (where M is one or more transition metals, w is 0 <w ≦ 2, x is 0 ≦ x <1, y is 0 <y <1, and z is −
1 ≦ z ≦ 4. ) Or Li to Li · Co phosphate and / or Co to Li · Co oxide as a component, and 0.1 mol or more of Co and 0.2 per mol of Li.
A positive electrode material or the like having an active material such as one containing P or more of P may be used.

The form of the lithium secondary battery can be appropriately determined according to the purpose of use. In the coin-type secondary battery illustrated in FIG. 1 and the like, a sheet-shaped negative electrode, a positive electrode, or the like is usually used, but a lithium-metal alloy, in particular, a sheet-shaped negative electrode made of the Li—Ag—Te-based alloy described above is, for example, It can be formed by an appropriate method such as a vapor deposition method, a rolling method, or a hot extrusion method, and the thickness thereof is arbitrary, but it may be about several to several hundreds μm. Further, the sheet-shaped positive electrode may be formed, for example, by a casting method, a compression molding method, a roll molding method, etc. together with a conductive material such as acetylene black or Ketjen black and a binder such as polytetrafluoroethylene or polyethylene, if necessary. It can be formed by a method of molding by an appropriate method.

In the above coin type secondary battery, etc., the electrolytic solution is usually used in the form of being contained in a separator interposed between the positive electrode and the negative electrode. The formation of such a separator is, for example, a porous polypropylene film or a porous material. polymer,
It can be carried out by an appropriate method according to a conventional method such as a method of impregnating or filling the glass filter with the electrolytic solution.

Charging of the lithium secondary battery of the present invention can be performed by an appropriate method such as a method of continuously supplying a constant current or a method of supplying a pulse current using a pulse power source.

Example 1 Li, Ag and Te in atomic ratio Li: Ag: Te = 9
It was blended at 0: 10: 2, melted at 500 ° C in an argon atmosphere to be alloyed, and punched by a pressing method using a sliced piece to obtain a disk-shaped negative electrode having a diameter of 20 mm and a thickness of 1 mm. Obtained.

On the other hand, lithium carbonate, basic cobalt carbonate and an aqueous phosphoric acid solution having a phosphoric acid content of 85% are used as Li: Co: P =
It was mixed in an atomic ratio of 2: 1.5: 0.5, put in an alumina crucible, and heat-treated in an electric furnace at 900 ° C. for 24 hours to obtain lithium phosphate and lithium cobalt phosphate. A mixture (active material) of salt and cobalt oxide was formed and pulverized by a ball mill to obtain a powder having a particle size of 20 μm or less. Next, 46 parts by weight of the powder, acetylene black 4
Parts by weight, 2 parts by weight of polyvinylidene fluoride, and 50 parts by weight of n-methylpyrrolidone are mixed, and the resulting mixture has a diameter of 20 mm,
It was applied on a nickel plate having a thickness of 0.5 mm and dried in a vacuum to form a coating layer (positive electrode layer) having a thickness of 150 μm to obtain a positive electrode.

On the other hand, 0.8 mol / liter of a macrocyclic diazapolyether ligand represented by the following formula was added to a solution prepared by dissolving 1 mol of LiClO ₄ in 1 liter of propylene carbonate to prepare an electrolytic solution. Then, it is impregnated with a porous polypropylene film (separator) having a thickness of 25 μm, the negative electrode and the positive electrode are arranged on the upper and lower sides of the obtained separator, which is sealed with a positive electrode can and a negative electrode can, and shown in FIG. A lithium secondary battery having a different form was obtained. The stability constant of the lithium complex ion in the solution was measured by potentiometric titration and found to be 10 9.5.

[0020] In the formula, l, m, and n mean the number of bonds in a (CH ₃ —O—CH ₃ ) unit, and l = 2, m = 2, and n = 1, respectively.

For the above secondary battery, 2.5 mA / cm
^When the charge / discharge cycle was repeated 50 times between 2.5 and 4.1 V at the charge and discharge current densities of ² , the discharge capacity retention ratio was examined and found to be 90%.

Example 2 As the lithium complex ion-forming agent, 15-crown 5 was used at a concentration of 1.0 mol / liter, and an electrolyte solution having a lithium complex ion stability constant of 10 4.3 was used. A lithium secondary battery was obtained in the same manner as in Example 1. The above-mentioned lithium secondary battery was tested for discharge capacity retention after repeating 50 charge / discharge cycles according to Example 1 at a current density of 1 mA / cm ² of charge and discharge, and found to be 90%.
Met.

Comparative Example 1 A lithium secondary battery was obtained in the same manner as in Example 1 except that an electrolyte solution containing no crown ether derivative was used.
The discharge capacity retention rate after repeating the 50 charge / discharge cycles was 10%.

Comparative Example 2 A lithium secondary battery was obtained in the same manner as in Example 2 except that an electrolyte solution containing no crown ether derivative was used.
When the discharge capacity retention ratio was examined after repeating the 50 charge / discharge cycles, it was 20%.

[0025]

According to the present invention, a lithium electrodeposition surface having a smooth and dense structure can be formed on a lithium alloy negative electrode, the deterioration of the negative electrode surface state due to the growth of dendrites can be prevented, and the initial discharge capacity can be reduced. It is possible to obtain a lithium secondary battery that has excellent sustainability, excellent charge / discharge cycle life, reliability, and discharge capacity, and can be used for a long time.

[Brief description of drawings]

FIG. 1 is a sectional view of an example.

[Explanation of symbols]

 1,7: Battery can 2,6: Nickel plate for current collection 3: Lithium alloy negative electrode 4: Electrolyte layer (separator) 5: Positive electrode 8: Insulation sealing material

Claims (3)

[Claims] 1. A lithium secondary battery, wherein an organic liquid containing a lithium-based electrolyte and a lithium complex ion forming agent is used as an electrolytic solution, and a lithium alloy is used as a negative electrode. 2. The lithium secondary battery according to claim 1, wherein the lithium complex ion forming agent is a macrocyclic compound, and the stability constant of the formed lithium complex ion in the electrolytic solution is 10 ² or more. 3. The lithium secondary battery according to claim 1, wherein the lithium alloy of the negative electrode contains lithium in an amount of 80 atomic% or more.