In Situ Spatially Resolved Neutron Diffraction of a Sodium Metal Halide Battery

Abstract

In situ spatially resolved neutron diffraction was used to investigate the processes occurring in the cathode of a sodium metal halide cell with mixed Fe/Ni chemistry during cell operation. The in situ diffraction data makes it possible to follow NaCl consumption, Na6FeCl8 formation and consumption and Ni1-xFexCl2 formation during charge as well as the reverse processes during discharge. Data collected at three different positions at different depths within the cathode permit mapping of the reaction progress in time and space. Reactions start near the β″-alumina and proceed towards the cathode interior. Instead of one single reaction front moving through the cell during charge and discharge, separate reaction zones are found for Fe and Ni oxidation as well as for Na6FeCl8 and NaCl usage as Cl− and Na+ source during Ni oxidation. Thus there is one reaction zone per reaction, during charge as well as discharge. The broadness of the reaction zones varies with time, depth and the respective reaction. Our data also yield information about processes like the formation of Ni1-xFexCl2, a possible slight overcharge close to the β″-alumina and finally allow to sketch a simplified mechanism of the processes that occur in the cell during charging.