Effect of Pretreatment on Electrochemical Properties of Positive Electrodes for Use in Ni-MH Batteries

Abstract

Effect of discharging before initial activation (pretreatment) of Co(OH)2-coated Ni(OH)2 positive electrodes for use in Ni-metalhydride (Ni-MH) batteries on their electrochemical properties was investigated. The high-rate dischargeability (HRD) of the positive electrode was improved by the pretreatment. The effect of the pretreatment was more conspicuous as the amount of the Co(OH)2-coated Ni(OH)2 as an active material increased. The charge transfer resistance (Rct) of the positive electrode evaluated by AC impedance measurements significantly increased at the depths of charge (DOCs) less than 20%, while the increase in Rct was effectively suppressed by the pretreatment. Moreover, the cyclic voltammogram of the pretreated positive electrode showed that metallic Co was produced during the pretreatment and oxidized to CoOOH. The formation of the conductive oxidation products from not only the pristine Co(OH)2 but also metallic Co can lead to the formation of more dense conductive network which may be responsible for the improvement of HRD and Rct.