A New Activation Process for a Zr‐Based Alloy as a Negative Electrode for Ni/MH Electric Vehicle Batteries

Abstract

The electric vehicle (EV) and the hybrid-electric vehicle (HEV) have attracted significant interest in recent years, and the Ni/MH battery has been investigated as a highly promising power source for these advanced vehicles. However, the energy density of a Ni/MH battery must be improved in order to reach the target EV driving distance. The development of hydrogen storage electrodes with a high discharge capacity is one of the key requirements for improving the energy density of the Ni/MH battery.Recently, efforts have concentrated on Zr-based Laves phase alloys because they exhibit a larger hydrogen storage capacity than the AB 5 -type alloy as well as good cycle life durability. 1-3 However, Zr-based Laves phase alloys require many charge-discharge cycles to achieve activation because dense oxide layers on the surface inhibit the diffusion of hydrogen atoms into the bulk alloy. This slow activation can lead to overcharge of the Ni(OH) 2 electrode and the consequent formation of ␥-NiOOH, which degrades the electrode. In order to avoid overcharge of the Ni(OH) 2 electrode, it is necessary to fully activate the Zr-based alloy electrode within about five cycles.In order to improve the Zr-based alloy activation behavior, several pretreatments have been proposed. Lee et al. reported that Zrbased alloy activation was greatly improved by adding light rareearth elements, attributed to the formation of a porous La oxide film on the surface. 4-6 Beside the standard metallurgical methods, KBH 4 treatment, a pulse-charging and a hot-charging treatment have been suggested. 7-10 However, these proposed treatments are not satisfactory because they do not avoid overcharge of the Ni(OH) 2 electrode during the formation process.It was reported that immersion in a boiling 6 M KOH solution is very effective for improving the activation characteristics of Zrbased alloys. 11 After immersion, the Zr-oxide layer is eliminated and a Ni-rich region is formed at the alloy surface. However, it is difficult to apply this formation process, because the high immersion temperature could damage the separator or the Ni(OH) 2 electrode. To use an immersion treatment to form a Ni/MH cell, the temperature should be lowered.We recently found that the activation characteristics of Zr-based alloys depend on the applied current density. Charging at a low current density leads to more rapid activation, however, the reason for this improved activation is not yet clear.In this work, we introduce a new activation process by combining the hot-immersion treatment with a slow-charging method. It was reported in our previous work that the Zr 0.7 Ti 0.3 Cr 0.3 Mn 0.3 V 0.4 Ni 1.0 alloy (termed "alloy 1" throughout this paper) is one of the most promising materials for the MH electrode. 10 An alloy 1 electrode was treated through two steps as follows: the alloy electrodes were immersed at 80ЊC for 12 h in a KOH solution and then charged at a very low current density. The effects of this treatment on the alloy electrochemical and physicochemical properties are di...

A Study on the Electrode Characteristics of V-Ti alloy Surface-Modified by Ballmilling Process

“…Since the electric vehicle ͑EV͒ and the hybrid electric vehicle ͑HEV͒ have attracted a great deal of interest in recent years, there have been extensive studies on the Ni/MH battery as a promising power source for these vehicles. 1 However, the energy density of a Ni/MH battery must be improved to meet the recent goal for advanced electric vehicles. The development of metal hydrides with a high discharge capacity is a key parameter for increasing the energy density of the Ni/MH battery.…”