Modeling the Thermodynamic and Kinetic Effects on the Electrochemical Performance of Planar Na-NiCl2 Batteries

Abstract

Na/metal chloride batteries (i.e., Na-NiCl2 or Na-FeCl2 batteries) possess a number of excellent properties including high safety and stability, high energy density, high voltage, no self-discharge, and good cycle life. Relying on these advantageous features, these Na/metal chloride secondary cells have a high potential for applications of transportation and vehicles as well as the advanced stationary energy storage systems (ESS). In these Na/metal chloride batteries, the electrochemical performance is largely determined by the thermodynamics and kinetic reactions in the cathode area comprised of NaCl particles, Ni/NiCl2 and/or Fe/FeCl2 powders, NaAlCl4 secondary electrolyte (i.e., catholyte), and various additives. During the charge-discharge process, complicated electrochemical reactions will occur in this cathode of the cell, including ionic diffusion of Na+ through NaAlCl4 catholyte driven by the chemical potential and the electric potential gradients, dissolution/coarsening of NaCl particles, and precipitation/decomposition of NiCl2 or FeCl2. The presentation introduces a continuum computational model to capture the quantitative effects of reaction thermodynamics and transport kinetics on the electrochemical performance of Na/metal chloride batteries with a Na-NiCl2 cell chemistry. The computational model was developed using a commercial COMSOL multi-physics software package. The model was applied to characterize a novel planar cell, which was designed adopting a disc shaped beta-alumina solid electrolyte (BASE) instead of a conventional cloverleaf shaped BASE. The planar cell geometry was intended to lower the cell operation temperature by optimizing the cell performance. In this multi-physics computational model, relevant physicochemical materials properties of active materials have been incorporated. The cell voltage was calculated using the Butler-Volmer kinetic equation and the temporal volume change of the active materials was computed based on the local current densities. The computation results were used to quantitatively investigate the effects of (1) cell operation temperature, (2) cathode active materials to catholyte ratio, and (3) cell dimension on the charge-discharge characteristics of the planar Na-NiCl2 cell. Especially, the combined effect of operation temperature and cell size on the cell performance was of special interest. It turns out that the electrochemical performance of the Na-NiCl2 cells is highly dependent on the cell operation temperature to show reduced cell voltages with decreased operation temperatures.