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, 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...

In-situ Studies of High Temperature Thermal Batteries: A Perspective

“…We should point out that thermal batteries are not the only high temperature battery technology. Sodium-metal halide batteries, which are also often referred to as "ZEBRA" (Zero-Emission Battery Research Activities) cells are another high temperature battery technology which have been studied using in-situ techniques (Zinth et al, 2015(Zinth et al, , 2016. These cells typically operate at around 300 • C and have also been studied by in-situ neutron diffraction (Zinth et al, 2015).…”

“…Sodium-metal halide batteries, which are also often referred to as "ZEBRA" (Zero-Emission Battery Research Activities) cells are another high temperature battery technology which have been studied using in-situ techniques (Zinth et al, 2015(Zinth et al, , 2016. These cells typically operate at around 300 • C and have also been studied by in-situ neutron diffraction (Zinth et al, 2015). Spatially resolved neutron diffraction data were collected at different points within the cathode of such a cell, enabling reaction fronts to be monitored as they move through the battery (Zinth et al, 2015).…”

“…These cells typically operate at around 300 • C and have also been studied by in-situ neutron diffraction (Zinth et al, 2015). Spatially resolved neutron diffraction data were collected at different points within the cathode of such a cell, enabling reaction fronts to be monitored as they move through the battery (Zinth et al, 2015). In-situ neutron tomography and radiography have been used to probe the changes in morphology of the cell components during battery cycling (Zinth et al, 2016).…”

Neutrons for industry

“…Thanks to the selectivity of neutron diffraction data it is possible to study every single reaction that takes place inside the cell at the same time and the relationship between them, in time and space. The study is a basis for future improvements in the composition of the cathode through the optimization of the quantity of main components (NaCl, Fe, Ni) in order to increase the energy density of a sodium metal halide cell [53,54].…”

Battery research as progress pacemaker

“…A typical Zebra cell has a diameter of about 5 cm and a steel casing, which prevents gathering information about the processes within the battery by conventional methods. During charging the iron oxidation and the formation of an intermediate sodium and chloride containing phase is followed by depletion of that phase during nickel oxidation and fi nally consumption of all remaining sodium chloride [2]. Figure 1 shows the experimental setup for experiments on a Zebra battery at the instrument STRESS-SPEC.…”