Hydrogen-absorbing alloys used for the negative electrode of nickel-metal hydride (Ni-MH) secondary batteries must combine the two roles of a hydrogen storage media and an electrochemical catalyst. Alloys for secondary batteries must also offer endurance during charge/discharge cycles. Hydrogen-absorbing alloys for batteries have been developed from these viewpoints. The hydrogen-absorbing alloys used for Ni-MH can be divided into two main types: the LaNi5-type and the TiNi(x)-type. In the LaNi5-type, we have substituted La for Misch-metal (Mm), which is a mixture or rare earth elements such as La, Ce, Pr and Nd, and we have partially substituted Ni for Co and Al to lengthen the charge/discharge cycle life. To adjust the hydrogen-absorbing equilibrium pressure, we have partially substituted Ni for Mn and Al. Another type of positive electrode, different from that of Ni-Cd secondary batteries, was developed for the use with Ni-MH to study the environment. A Zn-Co additive prevented electrode swelling by suppressing the production of gamma-NiOOH at charging. As a result, cadmium-free, low-pollution, nickel electrodes were developed. As for cell construction materials, new separators were also developed that suggest improved cell reliability. Ni-MH can now be practically charged at 1C using the -DELTAV charging method, and discharge at 4C. Its 1C charge/discharge cycle life is more than 500 cycles. This enables the commercial application of Ni-MH to a wide variety of cordless equipment.
