Hydrogen Absorbing Electrode and Nickel Metal-Hydridge Battery

Abstract

The object of the present invention is to provide a nickel metal-hydride battery excellent in cycle performance, high-rate discharging ability, and output power performance, by utilizing a hydrogen absorbing electrode comprising a hydrogen absorbing alloy powder as an active material, which is excellent in resistance to corrosion and high-rate discharging performance. Provided are a hydrogen absorbing electrode comprising 100 parts by weight of a hydrogen absorbing alloy powder which contains, as a main component, a rare earth element and a transition metal element, and has a saturation mass susceptibility of 1.0 to 6.5 emu / g, and 0.3 to 1.5 part by weight of an oxide or hydroxide of a rare earth element, the oxide or hydroxide has as a main component one or two or more rare earth elements selected from a group consisting of Dy, Ho, Er, Tm, Yb, and Lu and is in the form of powder whose average diameter is equal to or less than 5 μm, and a nickel metal-hydride battery comprising a nickel electrode as a positive electrode and a hydrogen absorbing electrode as a negative electrode.

Description

TECHNICAL FIELD [0001] The present invention relates to a hydrogen absorbing electrode in which a hydrogen absorbing alloy powder is used as an active material, and a nickel metal-hydride battery using the electrode, more specifically to a hydrogen absorbing electrode excellent in cycle performance and high-rate discharge ability at low temperature, and a nickel metal-hydride battery using the electrode which is improved in output power performance and cycle life. BACKGROUND ART [0002] In recent years, electric motor-driven equipment that include mobile electronic equipment such as mobile computers, digital cameras, etc. for which compaction of size and weight is demanded tend to rapidly increase. As a power supply of such electric motor-driven equipment, a sealed nickel metal-hydride storage battery is more widely used than a nickel cadmium storage battery, lead storage battery, etc., because the former provides a higher energy density per unit volume or unit weight, has a higher r...

AI Technical Summary

Benefits of technology

[0033] According to an aspect of the invention as described in Claims 1 and 2, it is possible to provide a hydrogen absorbing electrode for a nickel metal-hydride battery which has a discharge capacity as good as that of a conventional one and is excellent in high-rate discharging ability and charge / discharge cycle performance. (Claim 15)

[0034] According a second aspect of the invention as described in Claim 3, it is possible to provide a hydrogen absorbing electrode for a nickel metal-hydride battery which is particularly excellent in high-rate discharging ability.

[0035] According to a third aspect of the invention as described in Claim 4, it is possible to provide a hydrogen absorbing electrode for a nickel metal-hydride battery which is particularly excellent in charge / discharge cycle performance.

[0036] According to a fourth aspect of the invention as described in Claims 5 to 7, it is possible to provide a hydrogen absorbing electrode for a nickel metal-hydride battery which is excellent in high-rate discharging ability.

[0037] According to a fifth aspect of the invention as described in Claims 8 and 9, it is possible to provide a hydrogen absorbing electrode for a nickel metal-hydride battery which is particularly excellent in charge / discharge cycle performance.

[0038] According to a sixth aspect of the invention as described in Claims 10 and 11, it is possible to provide a nickel metal-hydride battery which exhibits an excellent charge / discharge cycle performance, while maintaining its high output power performance.

Problems solved by technology

However, the aforementioned hydrogen absorbing alloy has a number of drawbacks: it is vulnerable to corrosion due to electrolyte; the hydrogen absorbing electrode (negative electrode) has a poorer high-rate discharging ability and charge receptivity than a nickel electrode (positive electrode), and thus to maintain a balance against the positive electrode, the negative electrode must comprise a hydrogen absorbing alloy having a larger volume (about 1.5 time) than that of the positive electrode, which makes it difficult to raise the energy density of the negative electrode.

The hydrogen absorbing alloy, though being highly resistant to corrosion and having a long life, is slow in activation and, if it is used neat as an electrode, requires a considerable time for initial activation before it exhibits a sufficiently high discharging activity.

However, this treatment is not as effective for improving the cycle life of the battery.

Furthermore, the treatment impedes the fresh attachment of oxides to the exposed surface of the alloy powder, thereby reducing the contact resistance of the powder, and improving its reactivity.

Thus, according to this patent document, the time spent for procedures necessary for activating a hydrogen absorbing alloy is reduced, and discharging ability that is excellent from the initial phase of charge / discharge cycles is obtained, but the cycle performance is still inadequate.

The hydrogen absorbing electrode produced by the method has a high-rate discharging ability better than a conventional comparable hydrogen absorbing electrode, but its high-rate discharging ability does not necessarily reach a level sufficiently high to meet the stern standard sought by hybrid electric vehicles (HEVs), electric motor-driven tools, etc.

However, when the storage battery undergoes charge / discharge cycles, the alloy experiences a series of cycles consisting of hydrogen absorption and hydrogen desorption, and in conjunction with these cycles, the alloy repeats expansion / shrinkage which causes the alloy powder to have strain, and breaks the powder into finer particles.

Therefore, as a result of the repeated charge / discharge cycles, the alloy exposes its fresh surfaces which are then exposed to the electrolyte to be corroded by the latter, which will lead to the reduction of charge reserve capacity.

Because of this, the method described in the second patent document is not likely to bring about a significant improvement in the cycle performance.

Furthermore, the repetition of charge / discharge cycles leads to the rapid decline of high-rate discharging ability.

However, a sufficient life-improving effect could not be obtained particularly at high temperature probably because the enhanced corrosion resistance conferred by the yttrium compound to the hydrogen absorbing alloy powder may not be sufficiently high, or probably because the corrosion resistance enhancing effect of the yttrium compound may be impaired by a lighter rare earth element such as La used in combination.

In any case, the aforementioned means did not bring about a battery possessed of an excellent high-rate discharging ability and a long cycle life.

However, when the method is actually practiced, it is found in some cases that the internal resistance of hydrogen absorbing electrode increases which may lead to the decline of output power performance.

This probably explains the reason why addition of a particulate compound of a rare earth element did not produce a satisfactory result as expected.

Images