JPH06101325B2 - Lithium secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a lithium secondary battery, and more particularly to improvement of a negative electrode thereof.

[Conventional technology]

Conventionally, metallic lithium has been used alone for the negative electrode of a lithium secondary battery. However, when lithium is used alone as the negative electrode, the lithium that is electrodeposited in the charging reaction is dendrite-like (dendritic), and this dendrite-like lithium grows through repeated charge and discharge, breaking through the separator and causing an internal short circuit. In addition, there is a problem in that the electrodeposited lithium during charging is very active and reacts with impurities in the electrolytic solution and the solvent of the electrolytic solution to form a passive film on the surface of the negative electrode, which deteriorates the charge / discharge cycle characteristics.

Therefore, by using a lithium-aluminum alloy for the negative electrode, lithium is electrochemically alloyed with aluminum during charging, and lithium is diffused into the aluminum to allow reaction of the deposited lithium with an electrolytic solution solvent and dendrite growth. It has been proposed to suppress and improve charge / discharge cycle characteristics (eg, US Pat. No. 4,002,49
5, U.S. Pat. No. 4,056,885).

According to the above proposal, although dendrite-like precipitation of lithium and suppression of the reaction of the precipitated lithium with the electrolyte solvent and the like are significantly improved, lithium-alloying aluminum has a potential voltage surface and electric capacity. However, there is a problem in that the charge and discharge cycle characteristics can be improved without causing such a decrease in the voltage side and the electric capacity side.

[Problems to be solved by the invention]

The present invention solves the problems that the conventional lithium secondary battery has such as deterioration of charge / discharge cycle characteristics due to passivation of the negative electrode surface and occurrence of internal short circuit due to dendrite growth of electrodeposited lithium, and lithium is used alone. It is an object of the present invention to provide a lithium secondary battery having excellent charge / discharge cycle characteristics even when used as a negative electrode.

[Means for solving problems]

According to the present invention, a film of a lithium compound represented by the general formula (I) LiAlCl ₃ X (I) (wherein X is Br or I) is formed on the surface of the negative electrode which is in contact with the electrolytic solution. Thus, it is possible to provide a lithium secondary battery having excellent charge / discharge cycle characteristics by preventing passivation of the negative electrode and suppressing dendrite growth.

That is, it is considered that the passivation that occurs on the negative electrode surface of the lithium secondary battery progresses because the active electrodeposited lithium is in contact with the electrolytic solution for a long time. Therefore, in order to prevent this, it is necessary to reduce the contact between the electrodeposited lithium and the electrolytic solution without interfering with the charge / discharge reaction. For that purpose, the interface between the negative electrode and the electrolytic solution should have an electron conductivity. It suffices if there is no ion conductive membrane.

Therefore, as described above, when a film of a lithium compound represented by the general formula (I), that is, LiAlCl ₃ X, is formed on the surface of the negative electrode that is in contact with the electrolytic solution, these films function as an ion-selective permeable film. Therefore, the charge / discharge reaction is not hindered, but the chance of direct contact between the active electrodeposited lithium and the electrolytic solution is significantly reduced. As a result, the reaction between the electrodeposited lithium and impurities in the electrolytic solution and the solvent of the electrolytic solution is reduced, passivation of the negative electrode surface is suppressed, reversibility of lithium is improved, and charge / discharge lithium characteristics are improved. In addition, the formation of passivation film on the surface of the negative electrode (which is formed so as to be scattered on the surface of the negative electrode) is reduced, so that the electrodeposited lithium during charging is less likely to be deposited as dendrites. The dendrite growth is suppressed, and an internal short circuit due to the dendrite growth does not occur.

A specific example of the lithium compound represented by the general formula (I) is LiAl.
Cl ₃ Br and LiAlCl ₃ I, but these LiAlCl ₃ Br and LiAlCl ₃ I
The formation of a film on the negative electrode surface of, for example, LiAlCl ₃ B in the electrolytic solution
r or LiAlCl ₃ I, or the negative electrode of LiAlCl ₃ Br or Li.
It is carried out by immersion in a molten bath of AlCl ₃ I. Li
When the negative electrode is immersed in a molten bath of AlCl ₃ Br or LiAlCl ₃ I,
Needless to say, a film of LiAlCl ₃ Br or LiAlCl ₃ I is formed on the negative electrode surface when pulled up from the bath and cooled.
LiAlCl ₃ Br and LiAlCl ₃ the case of adding I in the electrolyte solution, it dissociates as shown in the following ^{_{formula, LiAlCl 3 Br → Li + +}} AlCl 3 Br - LiAlCl 3 I → Li + + AlCl 3 I - dissociation The formed AlCl ₃ Br ⁻ and AlCl ₃ I ⁻ combine with lithium (Li) in the negative electrode to form a film on the surface of the negative electrode in the form of LiAlCl ₃ Br or LiAlCl ₃ I.

The film of LiAlCl ₃ Br or LiAlCl ₃ I achieves the purpose as long as it is formed on the surface of the negative electrode that is in contact with the electrolytic solution.
As in the case of immersing a negative electrode in a molten bath of LiAlCl ₃ Br and LiAlCl ₃ I, LiAlCl ₃ Br film or LiAlCl ₃ I coating be formed on the entire surface of the negative electrode, and crimping the negative electrode to the negative electrode current collector As long as the electron conductivity between the negative electrode lithium and the negative electrode current collector is secured, it does not hinder the electric reaction.

When adding LiAlCl ₃ Br or LiAlCl ₃ I to the electrolyte, LiAlCl ₃
Even if LiBr and AlCl ₃ are added to the electrolyte solution instead of Br, a film of LiAlCl ₃ Br can be formed on the surface of the negative electrode, and even if Lil and AlCl ₃ are added instead of LiAlCl ₃ I, it is formed on the surface of the negative electrode. This may be done because a film of LiAlCl ₃ I can be formed.

When the amount of LiAlCl ₃ Br or LiAlCl ₃ I added to the electrolytic solution increases, they do not dissociate and remain as a solid, so the amount added varies depending on the type of electrolytic solution. It is preferably about 5% by weight.

In the present invention, lithium or a lithium alloy is used for the negative electrode. Examples of the lithium alloy include lithium-
Aluminum alloys, lithium-magnesium alloys, lithium-gallium alloys, lithium-indium alloys, lithium-gallium-indium alloys, and lithium alloys obtained by adding a small amount of one or more metals as a third component to these lithium alloys. Is used. In the present invention, even when a lithium alloy is used for the negative electrode, in addition to the effect of alloying lithium, the effect of forming a film of the lithium compound represented by the general formula (I) is added to improve charge / discharge cycle characteristics. However, in the present invention, it is possible to achieve an improvement in charge-discharge cycle characteristics even when lithium is used alone in the negative electrode, and in this case, there is a decrease in voltage and electric capacity as in the case of using a lithium alloy. Since it does not occur, the effect of the present invention is exerted in the most practical form. That is, the present invention is particularly significant when lithium is used alone as a negative electrode.

In the battery of the present invention, as the lithium ion conductive organic non-aqueous electrolyte, for example, 1,2-dimethoxyethane, 1,2-
Diethoxyethane, ethylene carbonate, propylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc. alone or in a mixed solvent of two or more kinds, for example, LiCl
One or more kinds of electrolytes such as O ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ dissolved therein are used. Further, in order to stabilize the electrolyte such as LiPF ₆ in the electrolytic solution, it is also preferable to add a stabilizer such as hexamethylphosphoric triamide to the electrolyte.

Then, as the positive electrode active material forming the positive electrode, for example, titanium disulfide (TiS ₂ ), molybdenum disulfide (MoS ₂ ), molybdenum trisulfide (MoS ₃ ), iron disulfide (FeS ₂ ), zirconium sulfide (ZrS ₂ ). ), Niobium disulfide (NbS ₂ ), phosphorus nickel trisulfide (NiPS ₃ ), vanadium selenide (VSe ₂ ) and other transition metal chalcogen compounds are used. In particular, titanium disulfide is preferred because it has a layered crystal structure, a large diffusion constant of lithium ions inside the titanium disulfide, a smooth charge-discharge reaction on the positive electrode side, and good reversibility of lithium.

〔Example〕

 Next, the present invention will be described in more detail with reference to examples.

Example 1 LiI and AlCl ₃ were mixed in equimolar amounts and reacted at 90 ° C. for 1 hour to synthesize LiAlCl ₃ I, which was mixed with 60% by volume of 4-methyl-1,3-dioxolane and 1,2-dimethoxy. Ethane 34.8% by volume,
1 wt% was added to an organic non-aqueous electrolytic solution in which 1.0 mol / l of LiPF ₆ was dissolved in a mixed solvent consisting of 5.2% by volume of hexamethylphosphoric triamide. The LiAlCl ₃ ion conductivity of the electrolyte solution by the addition of I is 0.017S / cm, also electrolyte ion conductivity in the state without the LiAlCl ₃ I is 0.011S / cm, the LiAlCl ₃ I The ionic conductivity was improved by the addition.

LiAlCl ₃ I electrolyte solution, and positive electrode using titanium disulfide as an active material and polytetrafluoroethylene as a binder. Pressure molded using 0.3 mm thick, 11 mm diameter molded body (15 mAh equivalent amount), negative electrode A button-type battery was fabricated by using a lithium metal plate (equivalent to 20.8 mAh) having a thickness of 0.2 mm and a diameter of 8 mm. The above battery is schematically shown in FIG.

In FIG. 1, reference numeral 1 is a negative electrode can made of stainless steel and having its surface plated with nickel, and 2 is a negative electrode current collector made of a stainless steel net spot-welded to the inner surface of the negative electrode can 1. 3 is a negative electrode, and this negative electrode has a thickness of 0.2 mm as described above,
It consists of a lithium plate with a diameter of 8 mm, and its theoretical electricity is 20.8 m.
It is equivalent to Ah. Reference numeral 4 is a film made of LiAlCl ₃ I on the surface of the negative electrode 3, and this film 4 is formed by LiAlCl ₃ I added to the electrolytic solution. Reference numeral 5 is a separator made of a microporous polypropylene film, and 6 is an electrolyte solution absorber made of a polypropylene nonwoven fabric. Reference numeral 7 is a positive electrode, which is made of titanium disulfide as an active material as described above, and has a disk shape with a thickness of 0.3 mm and a diameter of 11.0 mm, and its theoretical electricity amount is equivalent to 15 mAh. A positive electrode current collector 8 made of stainless steel mesh is arranged on one surface. 9 is a positive electrode can made of stainless steel with nickel plating on the surface,
10 is a polypropylene gasket.

Example 2 LiBr and AlCl ₃ was heated to 100 ° C. in equimolar amounts to synthesize LiAlCl ₃ Br, thickness 0.2mm in a molten bath of this LiAlCl ₃ Br, diameter 8m
Immerse m lithium plate, and immediately after immersion, pull up
It was cooled to solidify LiAlCl ₃ Br. In this way, a lithium plate having a film of LiAlCl ₃ Br formed on its surface was used as a negative electrode, and an electrolyte solution having the same composition as in Example 1 but containing no LiAlCl ₃ I was used. A battery was produced in the same manner as in Example 1.

Comparative Example 1 A battery having the same configuration as in Example 1 was prepared except that LiAlCl ₃ I was not added to the electrolytic solution.

The batteries of Examples 1 and 2 and Comparative Example 1 were subjected to a charge / discharge cycle test at a charging current and a discharging current of 1 mA, a charging end voltage of 2.7V and a discharge end voltage of 1.5V. The results are shown in FIG.

As shown in FIG. 2, both the batteries of Examples 1 and 2 have a larger discharge capacity in each cycle than the battery of Comparative Example 1 showing the conventional product, and the cycle when viewed at the end of 1.5V. There were many. This is because the formation of a film of LiAlCl ₃ I or LiAlCl ₃ Br on the surface of the negative electrode reduces the contact between active electrodeposited lithium and the electrolytic solution during charging, suppressing passivation of the negative electrode and dendrite-like deposition. It is due to the result. The slight difference in the data between the battery of Example 1 and the battery of Example 2 is mainly due to the difference in ionic conductivity between LiAlCl ₃ I and LiAlCl ₃ Br, which means that the means for forming a film on the surface of the negative electrode is different. It is not due to the difference.

Further, as described in Example 1, by adding LiAlCl ₃ I to the electrolytic solution, the ionic conductivity of the electrolytic solution is improved, so that improvement of the internal resistance of the battery can be expected.

〔The invention's effect〕

As described above, in the present invention, the surface of the side of the negative electrode in contact with the electrolytic solution, by forming a film of a lithium compound such as LiAlCl ₃ I and LiAlCl ₃ Br, charge-discharge cycle characteristics of the lithium secondary battery Was able to improve.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the lithium secondary battery of the present invention. FIG. 2 is a diagram showing charge / discharge cycle characteristics of the batteries of Examples 1 and 2 and the battery of Comparative Example 1. 3 ... Negative electrode, 4 ... Coating, 7 ... Positive electrode

Claims (1)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a lithium ion conductive organic non-aqueous electrolyte and a negative electrode,
A film of a lithium compound represented by the general formula (I) LiAlCl ₃ X (I) (wherein X is Br or I) is formed on the surface of the negative electrode which is in contact with the electrolytic solution. Lithium secondary battery.