Nucleation Controlled Mechanism of Cathode Discharge in a Ni/NiCl₂Molten 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−.

Elucidating the Rate‐Limiting Processes in High‐Temperature Sodium‐Metal Chloride Batteries

“…[30] Furthermore, model experiments with common NaAlCl 4 reservoir suffer from unintended cross-diffusion of dissolved metal species and their precipitation on the counter electrode, which can affect the assessment of kinetic parameters. [27,28] Thus, the aim of this study is to separate and isolate the different rate-limiting processes which determine the cell resistance as a function of state of charge. In particular, we focus here on deconvoluting the de-/chlorination processes of nickel and iron from microstructural effects of the porous electrodes (porosity, tortuosity, etc.).…”

“…Previously, the de‐/chlorination of various transition metals was studied by electrochemical model experiments in a common reservoir of molten NaAlCl 4 . [ 20 , 24 , 25 , 26 , 27 , 28 ] Diffusional rate limitations were identified to limit the reaction of nonporous iron and nickel foil or wire electrodes. Assuming de‐/chlorination to take place in a continuous, core–shell conversion layer, the transport limitations were assigned to diffusion of Cl − or metal ions through the metal chloride conversion layer, either through pores, [ 20 , 24 , 27 , 28 ] or through the metal chloride itself.…”

“…[ 20 , 24 , 25 , 26 , 27 , 28 ] Diffusional rate limitations were identified to limit the reaction of nonporous iron and nickel foil or wire electrodes. Assuming de‐/chlorination to take place in a continuous, core–shell conversion layer, the transport limitations were assigned to diffusion of Cl − or metal ions through the metal chloride conversion layer, either through pores, [ 20 , 24 , 27 , 28 ] or through the metal chloride itself. [ 25 ] Limited reversibility in the oxidation and reduction of certain metal electrodes, as inferred from asymmetric current–voltage peak shapes in the model experiments, was ascribed to dissolution of cathode species.…”

“…[ 30 ] Furthermore, model experiments with common NaAlCl 4 reservoir suffer from unintended cross‐diffusion of dissolved metal species and their precipitation on the counter electrode, which can affect the assessment of kinetic parameters. [ 27 , 28 ]…”

Fabrication of the Ni-NiCl2 Composite Cathode Material for Fast-Response Thermal Batteries

“…However, the activation time still cannot meet the requirements necessary to ensure a fast response. Therefore, the development of highperformance NiCl 2 -based electrode materials is challenging (Wang et al, 2013;Hosseinifar and Petric, 2016;Rock et al, 2016;Gui et al, 2020).…”