Nucleation Controlled Mechanism of Cathode Discharge in a Ni/NiCl2 Molten Salt Half-Cell Battery

Abstract

The cathode of the Na-NiCl2 battery discharges through a complex process of NiCl2 reduction coupled with microstructural growth of Ni particulates. The electrochemical mechanism of this process is a leading determinant of the battery's power rating and cyclability. The present work investigates this cathode discharge mechanism by employing a half-cell test system, and combining scanning electron microscopy (SEM) with cyclic voltammetry (CV) and chronoamperometry. The charge/discharge characteristics of a Ni cathode are probed in a chloro-basic melt of NaCl-saturated NaAlCl4 at 280°C. The CV and chronoamperometric data are fitted to established models of electrodeposition kinetics. From these analyses, the rate determining step of cathode discharge is identified as instantaneous nucleation and growth of electrodeposited Ni. SEM results indicate that, NaCl-decorated sites of the cycled cathode surface facilitate this nucleation mechanism. Scan rate dependent CV indicates that, the charge transfer step of the nucleation reaction involves electro-reduction of NiCl42−. The CV data also reveal the equilibrium concentration and the diffusion coefficient of this dissolved species of NiCl2. The oxidation of Ni to NiCl2 (cathode-charge step) under potentiostatic conditions follows a modified Cottrell equation for interfacial diffusion of Cl−.