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, Na 6 FeCl 8 formation and consumption and Ni 1-x Fe x Cl 2 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 Na 6 FeCl 8 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 Ni 1-x Fe x Cl 2 , 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.Sodium metal halide batteries, first invented in the 80's 1 are used today both for stationary and mobile applications. 2 Advantages of such molten salt Na/MCl 2 (M = Ni, Fe) batteries are a high specific energy density of ∼140 Wh/kg-four times higher than lead acid batteriesin combination with a high cyclability (more than 3000 cycles at 80% depth of discharge (DoD)) without need of maintenance. These features make sodium metal halide batteries well suited for stationary telecom applications, for example in areas with poor grid connection and frequent power outages, where also the fact that the batteries can be operated under elevated or extreme temperature conditions proves to be useful. Additionally, sodium metal halide cells can be charged and discharged rapidly and are safe in operation, which makes them especially suitable for mobile applications such as cars, vans and busses. Current applications also comprise use as energy backup system for railed vehicles. Finally, sodium metal halide batteries have the benefit of being 100% recyclable: The metal is used for alloys while salt and ceramic go into road beds. Figure 1 shows a simplified scheme of sodium metal halide cell operation. In the discharged state, the cathode of a sodium metal halide cell consists of a porous Ni or Fe metal electrode, sodium chloride and NaAlCl 4 , which is liquid above 154 • C and serves as electrolyte, ensuring good ion conductivity in the cathode. The cathode is separated from the anode compartment by a tube of ceramic β -alumina (BASE), that offers a high sodium conductivity at the optimal cell operation temperature of 270-350 • C. During charging, NaCl reacts with the metal (M = Ni or Fe) to form MCl 2 and Na + ions. The sodium ions travel through the β -alumina separator and a...

Na-β″-alumina ceramics are archetypical ion conductors with excellent sodium-ion conductivity. Their processing is, however, challenging and results in large variations in reported conductivity measurements. We systematically reexamine the impact of sintering conditions on microstructure and sodium-ion conductivity of Na-β″-alumina ceramics. Depending on sintering temperature and sintering time, we measure conductivities between 0.04 and 0.37 S/cm at 300 °C on ceramics prepared from identical starting powders. During sintering, formation of a liquid phase is observed above 1500 °C, which promotes densification but leads to abnormal grain growth for extended sintering times. While such conditions result in the highest conductivities measured for our sample series (0.37 S/cm at 300 °C), the corresponding microstructures are mechanically fragile. For mechanically robust, densely sintered samples, we identify the average grain size as the dominating factor controlling ion conductivity. For average grain sizes between 1 and 6 μm, we obtain conductivities between 0.17 and 0.27 S/cm at 300 °C. The influence of porosity in undersintered, highly porous samples is well accounted for by Archie’s law and results in low ion conductivities down to 0.04 S/cm at 68% density. Our insights into microstructural factors controlling ionic conductivity such as grain size and density are instrumental for the successful integration of Na-β″-alumina ceramic electrolytes into next-generation batteries.

No abstract

Here we present a neutron tomography and in-situ radiography study on a sodium metal halide cell with mixed Fe/Ni chemistry during charge and discharge at 280°C. The neutron tomography data visualizes the cathode containing porous wick, Ni current collector and partly filled macro pores. Upon charging, the appearance of the cathode changes drastically. More macro pores appear due to material transport, especially in the porous wick above 12 cm cell height. Qualitative estimation shows almost no changes in macro pore volume during Fe oxidation and an increase during Ni oxidation in accordance with trends expected from calculations. Spatially resolved neutron diffraction data collected along the height of the cell shows that despite different appearance of upper and lower part at 100% SOC, the cell is fully charged and electrolyte is present at all heights. Thus, the differences in tomography are attributed to morphology differences of the Ni-Fe conductive network and redistribution of electrolyte in the macro pores, the rest of the cathode being well immersed in electrolyte. Difference images of neutron tomography data at different SOCs picture the reaction front of metal chloride formation and show its progress is faster along the cell diagonal than at the concave areas of the β’’-alumina tube.