JPH0470744B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, and its purpose is to obtain a lightweight, high-output secondary battery that can be used as a power source for all cordless devices. That is.

BACKGROUND TECHNOLOGY At present, only primary batteries of non-aqueous electrolyte batteries using lithium as a negative electrode have been developed as commercial products, but perfect secondary batteries have not yet been obtained. For secondary batteries that use lithium as the negative electrode, although there are still problems to be solved with the positive electrode, electrolyte, separator, etc., the biggest challenge is to improve the characteristics of lithium, which is the negative electrode. That is, a lithium negative electrode that exhibits satisfactory charge-discharge cycle characteristics has not been obtained. When metallic thium is used for the negative electrode, dendrites are formed on the surface of the lithium electrode during charging, which pierce the separator and cause an internal short circuit in the battery.

Various methods are being considered to solve this problem, and among them, the use of alloys, particularly alloys of lithium and aluminum, is said to be promising. However, lithium-
Even when aluminum alloy is used, there is a problem that electrodeposit falls off from the electrode plate surface during charging and discharging.
It remains unresolved. (M.Hughes et.al., J.
Power Sources 12 83 (1984).

Problems to be Solved by the Invention The present invention improves charge/discharge cycle characteristics by preventing electrodeposit from falling off the electrode surface of a lithium-aluminum alloy during charging and discharging, and provides excellent non-aqueous electrolytic The purpose is to obtain a negative electrode for liquid secondary batteries.

Means for Solving the Problems The present invention is characterized in that a fiber having a lithium-aluminum alloy deposited on the surface of the metal M fiber is used for a negative electrode for a non-aqueous electrolyte secondary battery.
The metal M has a property that lithium does not diffuse into its interior, and is in the form of a fiber with a diameter of 0.1 micron or less. Furthermore, the composition of the lithium-aluminum alloy is in the range of 35 to 45 percent lithium in terms of atomic ratio. When used as an electrode plate for a battery, the fiber must be pressure-molded onto a suitable support to form a plate.

Effect When the negative electrode plate of the present invention is used, the surface area is dramatically increased compared to conventional electrode plates. The amount of electrodeposited material removed from the surface of a lithium-aluminum alloy during lithium electrodeposition has a large relationship with the current density during charging. When the current density during charging is 50 μA/cell or more, the rate of electrodeposition of lithium is higher than the rate of diffusion of electrodeposited lithium into the electrode plate, so the electrodeposited lithium is accumulated on the plate surface. As a result, a portion with a lithium composition of 50% or more is formed on the surface of the electrode plate and falls off, resulting in a decrease in the capacity of the electrode plate and deterioration of cycle characteristics. On the other hand, the current density during charging is 20μA/
cell, the diffusion rate of lithium into the electrode plate is faster than the electrodeposition rate of lithium, and the electrodeposited lithium is not accumulated on the electrode plate surface but diffuses uniformly into the electrode plate. To go.

Compared to plate-shaped lithium-aluminum alloy,
When using the electrode plate of the present invention, the true surface area of the electrode plate is 1000 times or more even if the weight of the lithium-aluminum alloy present is the same.
Therefore, when charging and discharging a battery, the true current density of the lithium-aluminum alloy electrode is less than 1/1000 of the apparent current density. For example, even when the apparent current density is 10 mA/cm ² , the true current density is 10 μA/cm ² or less, so that it is possible to prevent electrodeposit from falling off the surface of the lithium-aluminum electrode during charging.

Furthermore, in the negative electrode according to the present invention, the composition of the lithium-aluminum alloy to be vapor-deposited is 35 to 45 percent lithium in terms of atomic ratio. This state of the electrode plate is a fully charged state, and after the start of discharge, the lithium content decreases. At the end of discharging, the capacity of the battery must be determined in advance so that the composition of the lithium-aluminum alloy is at least 10% lithium in terms of atomic ratio, for the following reasons.

In a lithium-aluminum alloy, when the atomic ratio of lithium is 0 to 7 percent, it forms an α phase, when it is between 47 and 56 percent, it forms a β phase, and when it is between 7 and 47 percent, it forms a mixed phase of α and β phases. There is. Among these, the α phase is a solid solution, and lithium diffuses very slowly in this phase, but lithium diffuses quickly in the α+β phase.

Therefore, during charging and discharging, the composition in the fully charged state and completely discharged state is always α + β phase, and the diffusion of lithium in the electrode plate is maintained at a fast rate, resulting in lithium being accumulated on the surface of the electrode plate. There is no chance of the electrodeposit falling off. By determining the capacity of the battery so that the composition of the lithium-aluminum alloy portion of the negative electrode according to the present invention is always in the α+β phase, electrodeposit can be prevented from falling off during charge/discharge cycles.

Further, in the negative electrode of the present invention, there is a core of 0.1 micron or less made of metal M in which lithium does not diffuse into the center of the lithium-aluminum fiber. This metal M functions as a current collector and at the same time functions to maintain the shape of the lithium-aluminum alloy fiber. That is, when the entire fiber is made only of lithium-aluminum alloy, the shape may change during charging and discharging, and the fiber may be partially cut. However, when metal M exists in the core of the fiber, lithium does not diffuse into the interior of metal M, so there is no change during the charging and discharging reaction, and lithium -
The shape of the aluminum alloy fibers will be maintained, and the original surface area will be maintained as a whole.

Example As an example of a battery according to the present invention, a battery using titanium sulfide (TiS ₂ ) as a positive electrode and an electrode according to the present invention as a negative electrode will be described.

The positive electrode plate is made by pressure molding 500 mg of titanium sulfide (TiS ₂ ) powder onto a stainless steel mesh, and the size is
The size was 10 mm x 10 mm and the thickness was 2 mm. The electrode was incubated in a 1 mol/lithium perchlorate (LiClO ₄ )-propylene carbonate electrolyte before assembly into the battery.
A lithium electrode is used as the counter electrode, and lithium is electrodeposited on the titanium sulfide electrode, so that the composition becomes Li _0.5 TiS ₂ (fully charged state) and electricity is applied.

Next, about 130 mg of a lithium-aluminum alloy having a composition of 40 percent lithium and 60 percent aluminum in atomic ratio is deposited on about 120 mg of nickel fibers having a diameter of 0.1 microns or less. The fibers after deposition were approximately 0.22 microns in diameter. The fibers thus obtained were pressure molded onto a stainless steel mesh to form a porous electrode plate with a size of 10 mm x 10 mm and a thickness of 1 mm.

FIG. 1 is a cross-sectional view showing the structure of the battery according to the present invention. In the figure, 1 is a positive electrode plate containing Li ₀₅ TiS ₂ , and 2 is a porous electrode plate made by depositing a lithium-aluminum alloy on nickel fiber. In the negative electrode plate according to the present invention, 3 is a separator made of polypropylene nonwoven fabric. 4 is the electrolyte, here 1 mol/
A solution of lithium perchlorate (LiClO ₄ ) in propylene carbonate was used. 5 is a stainless steel mesh that is a positive electrode current collector, 6 is a positive electrode terminal, 7 is a stainless steel mesh that is a negative electrode current collector, 8 is a negative electrode terminal, and 9 is a battery case.

FIG. 2 shows the characteristics when the battery according to the present invention is charged and discharged at a current of 1 mA/cell. In the figure, the curve A→B indicates charging, and the curve C→D indicates discharging. In A and D, the battery is in a fully discharged state, with the positive electrode having a composition of Li _0.5 TiS ₂ and the negative electrode having an atomic ratio of 25 percent lithium and 75 percent aluminum. In addition, in B and C, the battery is in a fully charged state, in which case the composition of the positive electrode is Li _0.5 TiS ₂ and the composition of the negative electrode is 40% lithium and 60% aluminum by atomic ratio.
It is a percentage. In addition, in FIG. 2, the coulombic efficiency of charging and discharging was about 84%, and the charging time was 3.6 hours, while the discharging time was about 3.0 hours. This charge-discharge characteristic remained almost unchanged even after 300 cycles.

In the above example, nickel was used as the metal that prevents lithium from diffusing into the core of the lithium-aluminum alloy fiber, but other metals such as iron, copper, titanium, stainless steel, etc. It is known that lithium does not diffuse into nickel, so these metals can also be used in place of nickel.

Further, in the above examples, titanium sulfide was used as the positive electrode and a propylene carbonate solution of lithium perchlorate was used as the electrolyte, but it goes without saying that the negative electrode of the present invention can be used in combination with other positive electrodes or non-aqueous electrolytes. do not have. For example, the positive electrode active material
Chalcogen intercalation compounds such as VSe ₂ and NbSe ₃ ,
Oxides such as V ₂ O ₅ , MnO ₂ , TiO ₂ , M0O ₃ , WO ₂ can be used, and as the electrolyte, aprotons such as γ-butyrolactone and dimethoxyethane can be used, except for aqueous solutions that react with lithium. Any nonaqueous electrolyte in which a lithium salt such as LiClO ₄ , LiBF ₄ , or LiAsF ₆ is dissolved in a neutral solvent can be used.

Effects of the Invention As shown in the Examples, the battery using the lithium-aluminum alloy negative electrode of the present invention exhibits excellent charge-discharge cycle characteristics. The reason for this is that the diameter of the fibers formed by vapor-depositing lithium-aluminum alloy is less than 0.25 microns, so the surface area of the reaction surface is extremely large, and the true current density of the negative electrode during charging and discharging is about 10 μA/ ^cm2 . Because it is small and the composition range of the negative electrode during charging and discharging is entirely in the α+β phase, lithium in the negative electrode plate diffuses quickly, preventing lithium from accumulating on the plate surface and falling off from the surface. It is something that can be done.
In addition, at the center of the lithium-aluminum alloy fiber of the negative electrode, there is a core made of metal that prevents lithium from diffusing into the interior, which not only serves as a current collector, but also prevents the fiber from changing shape or breaking during charging and discharging. It is something.

As described above, by using the negative electrode of the present invention, a lightweight, high-output nonaqueous electrolyte secondary battery with stable charge/discharge cycle characteristics can be obtained.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a battery according to the present invention, and FIG. 2 is a diagram showing the charging and discharging characteristics of the battery according to the present invention. 1...Positive electrode plate, 2...Negative electrode plate, 3...Separator, 4...Electrolyte solution.

Claims (1)

[Claims] 1. A lithium-aluminum alloy having a composition of 35 to 45 percent lithium in terms of atomic ratio is vapor-deposited on the surface of a fiber with a diameter of 0.1 micron or less made of a metal in which lithium does not diffuse into the interior, to form a fiber with a diameter of 0.25 micron or less, and A negative electrode for a non-aqueous electrolyte secondary battery, which is made by pressure-molding fibers into a plate shape.