$AB_2$ type Zr-based Laves phase alloys have been studied for potential application as a negative electrode in a Ni-MH battery because of their large discharge capacity and relatively good durability for charge-discharge cycling. However, they have some disadvantages of poor activation behavior in KOH solution, bad rate-capability and poor low-temperature dischargeability. Therefore, in order to solve the above disadvantages of these alloy systems, several attempts have been tried.
First, in order to investigate the effect of the La addition on activation behavior of ZrCrNi alloy, the surface morphologies of the as-cast and the activatied sample are analysed by SEM. From the results of surface observation it is found that spherical La rich phase is evenly distributed in the as-cast sample and the crack is initiated from La rich phase and then propagated into the alloy surface during the activation process. It is also observed that La rich phase is swelled due to the hydride formation which is followed by inducing cracks on the alloy surface around the La rich phase. Therefore it is suggested that the easy activation behavior is attributed to the new surface which is generated by the volume expansion of the segregated La rich phase on hydriding. It is considered that this method is effective for easy activation of Zr based alloy systems, in which added La elements are segregated as a second phase in the surface.
Second, to improve the activation behavior of the AB2 type multi-component $Zr_{0.7}Ti_{0.3}Cr_{0.3}Mn_{0.3}V_{0.4}Ni$ alloy developed in this study, the hot-charging treatment with simultaneous immersing and charging in hot KOH solution has been performed. We have found that the activation behavior and the rate capability of the $AB_2$ type alloy electrode are improved after pretreatment. During this treatment the charging of the alloy with atomic hydrogen occurred rapidly, resulting in the formation of cracks and a new clean surface. Furthermore, Ni with high catalytic activity was enriched near the surface by dissolving the surface oxide layer and the new surface with Ni layer existed as the metallic state because of reductive atmosphere by the potential shift to the negative direction. Owing to the aforementioned effects, $Zr_{0.7}Ti_{0.3}Cr_{0.3}Mn_{0.3}V_{0.4}Ni$ alloy electrode treated at 80℃$ and 50 mA $g^{-1}$ for 8 hours were fully activated after the first cycle and had a high rate capability of 85% at 500 mA$g^{-1}$.
Third, electrode performances such as the rate capability and the low-temperature dischargeability depend on the surface property and the bulk property of MH as well as the electrode preparation method. In order to investigate the effect of Cu, Ni powder as a compacting material on the electrode performance, the negative electrodes were prepared by mixing $Zr_{0.7}Ti_{0.3}Cr_{0.3}Mn_{0.3}V_{0.4}Ni$ alloy powder with copper powder of various sizes and contents, and were analyzed with electrochemical techniques and scanning electron microscopy (SEM) after cycling. The electrode performances such as the rate capability and the low- temperature dischargeability of the Cu-compacted electrode are improved as the size of Cu powder decreased or its content increased. These performances are better than those of the Ni-compacted electrode. The Cu-compacted electrode shows a high exchange current density for the hydrogen evolution reaction and a low contact and reaction resistance. From SEM analysis of the cycled electrode compacted with copper powder, it is also found that the surface of the MH(metal hydride) particles of the cycled Cu-compacted electrode is covered with copper grains (2-3μ) and whiskers. Therefore, it is suggested that the improved electrode characteristics are attributed to the copper layer deposited on the MH particles by the dissolution and precipitation (DP) process.