PNNL

Abstract

The sodium-nickel chloride (ZEBRA) battery is typically operated at relatively high temperature (250~350°C) to achieve adequate electrochemical performance. Reducing the operating temperature in the range of 150 to 200°C can lead to enhanced cycle life by suppressing temperature related degradation mechanisms. The reduced temperature range also allows for lower cost materials of construction such as elastomeric sealants and gaskets. To achieve adequate electrochemical performance at lower operating temperatures requires an overall reduction in ohmic losses associated with temperature. This includes reducing the ohmic resistance of ?”-alumina solid electrolyte (BASE) and the incorporation of low melting point molten salt as the secondary electrolyte. In present work, planar-type Na/NiCl2 cells with a thin flat plate BASE (600 ?m) and low melting point secondary electrolyte were evaluated at reduced temperatures. Molten salt formulation for use as secondary electrolytes were fabricated by the partial replace of NaCl in the standard secondary electrolyte (NaAlCl4) with other lower melting point alkali metal salts such as NaBr, LiCl, and LiBr. Electrochemical characterization of the ternary molten salts demonstrated , improved ionic conductivity, and sufficient electrochemical window at reduced temperatures. Furthermore, Na/NiCl2 cells with 50 mol% NaBr-containing secondary electrolyte exhibited reduced polarizations at 175°C compared to the cell with the standard NaAlCl4 catholyte. The cells also exhibited stable cycling performance even at 150oC.