JP2001236948A - Positive electrode for lithium ion secondary battery - Google Patents

Abstract

(57) Abstract: A lithium ion secondary battery using lithium nickelate as a positive electrode active material, and a positive electrode for a lithium ion secondary battery capable of improving cycle characteristics. SOLUTION: A positive electrode active material 12 formed on a positive electrode current collector foil 10 and a negative electrode active material 16 formed on a negative electrode current collector foil 14 are provided.
In the lithium ion secondary battery in which the lithium ion oxide and the lithium ion battery are opposed to each other with the separator 18 interposed therebetween, lithium nickel oxide is used for the positive electrode active material 12 and nickel fiber is added to this. As a result, nickel elution from lithium nickel oxide can be suppressed, and an increase in internal resistance of the lithium ion secondary battery can be suppressed, so that cycle characteristics can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an improvement in a positive electrode used for a lithium ion secondary battery.

[0002]

2. Description of the Related Art Heretofore, a lithium cobalt compound, a lithium manganese compound, or the like has been used as an active material in a positive electrode for a lithium ion secondary battery. However, cobalt has a problem of high cost, and a lithium manganese compound has a problem of collapse of the crystal structure due to elution of manganese during use.

[0003] Therefore, the use of lithium nickelate (LiNiO ₂ ) as a positive electrode active material has been studied.

However, in the positive electrode using lithium nickelate, the lithium nickelate expands and contracts during charging and discharging of the lithium ion secondary battery, and the space between the lithium nickelate as an active material and the conductive material and the nickel is reduced. There has been a problem that the adhesion between the positive electrode material containing lithium oxide and the current collector foil is reduced, and the internal resistance is increased.

To solve such a problem, it is considered effective to add a fibrous substance to the electrode material as described in, for example, JP-A-3-263693. Due to the addition of the fibrous substance, even if the distance between the active material and the conductive material and the distance between the positive electrode material and the current collector foil are slightly apart, the fibers are entangled with the particles, so that the conductivity can be secured. That's why.

[0006]

However, the above-mentioned prior art is a technique for preventing the collapse of the negative electrode and the like, and cannot be said to solve the problems specific to lithium nickelate. That is, lithium nickelate has a property that nickel elutes with expansion and contraction during charging and discharging of a lithium ion secondary battery. As described above, the eluted nickel becomes a nickel oxide (NiO or the like), which is deposited on the surface of the negative electrode, increases the internal resistance of the battery, and has a problem in that the cycle characteristics of the lithium ion secondary battery deteriorate.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide a lithium ion secondary battery using lithium nickelate as a positive electrode active material.
Lithium ion 2 that can improve cycle characteristics
An object of the present invention is to provide a positive electrode for a secondary battery.

[0008]

In order to achieve the above object, the present invention provides a positive electrode for a lithium ion secondary battery using lithium nickelate as an active material, wherein a nickel fiber is added. Features.

[0009]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 shows a lithium ion 2 according to the present invention.
A sectional view of a configuration example of a lithium ion secondary battery using a positive electrode for a secondary battery is shown. In FIG. 1, a positive electrode current collector foil 10
The positive electrode active material 12 using lithium nickelate is formed on the negative electrode current collector foil 14, and the negative electrode active material 16
Are formed. The positive electrode active material 12 and the negative electrode active material 16 are opposed to each other with a separator 18 interposed therebetween, and constitute a lithium ion secondary battery. In addition,
The positive electrode active material 12, the negative electrode active material 16, and the separator 18 are impregnated with a predetermined electrolytic solution.

The present inventors have found that it is effective to add a nickel fiber to the active material in order to suppress the elution of nickel from lithium nickelate used for the positive electrode active material 12. When the nickel fiber is added to the positive electrode active material 12, the nickel fiber acts as a buffer against the strain caused by the expansion and contraction of the lithium nickelate crystal during charge and discharge of the lithium ion secondary battery, and the crystal strain is relaxed. be able to. For this reason, the elution of nickel based on the distortion of the crystal of lithium nickelate can be suppressed.

If the elution of nickel from lithium nickelate can be suppressed as described above, the collapse of the crystal structure of lithium nickelate can be suppressed, and the increase in internal resistance caused by the finer cathode active material 12 can be suppressed. be able to.

For the above purpose, the nickel fiber added to the positive electrode active material 12 preferably has a diameter of 20 μm or less and a length in the range of 0.5 to 5 mm.
If the nickel fiber has a diameter of 20 μm or more or a length of 5 mm or more, the fiber becomes hard and breaks the separator 18 to cause a short circuit. On the other hand, if the length is too long, the dispersibility of the nickel fiber is deteriorated at the time of forming the electrode, and it is not possible to obtain a sufficient strain buffering effect.

The nickel fiber is used in an amount of 0.1% with respect to a positive electrode material obtained by adding a conductive material and a binder to lithium nickel oxide.
It is added in the range of 10 wt%. If the addition amount is too small, the effect of buffering strain becomes small, and if it is too large, the capacity of the electrode decreases and the weight increases.

As described above, the amount of nickel eluted after performing 100 cycles of charge / discharge at 60 ° C. in a lithium ion secondary battery using the same, with or without nickel fibers added to the positive electrode active material 12. Are shown in Table 1.

[0016]

[Table 1] In Table 1 above, the nickel (Ni) elution ratio indicates the amount of nickel eluted with respect to the weight of nickel contained in the lithium nickelate used for the positive electrode active material 12. As shown in Table 1, it can be seen that the addition of nickel fiber significantly reduces the amount of nickel eluted.

FIG. 2 shows changes in the internal resistance of the lithium ion secondary battery when the charge / discharge cycle of the lithium ion secondary battery is repeated with and without the addition of the nickel fiber. As shown in FIG. 2, when the nickel fiber is added, the internal resistance can be suppressed from increasing as the number of cycles increases, as compared with the case where the nickel fiber is not added. This is because the nickel fiber suppresses the elution of nickel from lithium nickelate, which suppresses the collapse of the crystal structure of lithium nickelate, and between the active material and the conductive material and between the positive electrode active material 12 and the positive electrode collection material. This is because peeling between the electrofoil 12 is prevented.

FIG. 3 shows a change in the capacity retention of the lithium ion secondary battery when the charge / discharge cycle of the lithium ion secondary battery is repeated between the case where the nickel fiber is added and the case where the nickel fiber is not added. FIG. 3 also shows that the case where the nickel fiber was added had a higher capacity retention ratio and improved cycle characteristics. In the example of FIG. 3, it can be seen that the capacity retention rate is improved by about 10% at the value after 100 cycles.

FIGS. 2 and 3 show the measurement results when 1 wt% of a nickel fiber having a diameter of 4 μm and a length of 1 mm was added. This measurement was performed at a temperature of 60 ° C.

[0020]

As described above, according to the present invention,
By adding nickel fibers to the positive electrode active material using lithium nickelate, nickel elution from lithium nickelate can be suppressed, preventing an increase in internal resistance and improving the cycle characteristics of lithium ion secondary batteries. Can be.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a configuration example of a lithium ion secondary battery using a positive electrode for a lithium ion secondary battery according to the present invention.

FIG. 2 is a diagram showing a change in internal resistance with respect to a charge / discharge cycle of a lithium ion secondary battery using a positive electrode active material with and without a nickel fiber as a positive electrode.

FIG. 3 is a diagram showing a change in a capacity retention ratio with respect to a charge / discharge cycle of a lithium ion secondary battery using a positive electrode active material with and without a nickel fiber as a positive electrode.

[Explanation of symbols]

Reference Signs List 10 positive electrode current collector foil, 12 positive electrode active material, 14 negative electrode current collector foil, 16 negative electrode active material, 18 separator.

Claims (1)

[Claims] 1. A positive electrode for a lithium ion secondary battery using lithium nickel oxide as an active material, wherein a nickel fiber is added to the positive electrode.