Spatially Resolved Phase Analysis in Sodium Metal Halide Batteries: Neutron Diffraction and Tomography

Abstract

Neutron methods are powerful tools for non-destructive investigation of batteries. Taking as an example the case study of a Fe/NaCl based battery, we investigate a discharged and a half-charged battery using spatially resolved neutron tomography and powder diffraction. Both batteries, the discharged and the half-charged, were studied by neutron tomography to identify the inner structure of the cells. The analysis of the neutron diffraction data allowed identification and quantification of the reaction zones as well as the phase distribution across the cross section of the half-charged battery.Usually, a battery must be cut open to be characterized and to understand what actually takes place, both chemically and morphologically, inside the battery during operation. This is not only destructive and time-consuming, but the materials and their morphology can also be altered during cutting and exposure to air and moisture. Insight into the detailed microstructure and chemistry within a battery is a key to understanding processes in the battery. Consequently, power and energy densities could be improved and degradation mitigated. It is therefore highly desirable to study the phase distributions and electrochemical processes directly inside a battery without opening it. Neutron and high energy X-ray synchrotron radiation, with their unique penetrating power are ideal probes to study electrochemical processes non-destructively inside a battery. Neutron radiation is particularly suited to study battery materials because of its good scattering contrast for lighter elements when compared to X-rays.Neutron tomography 1 and in particular neutron diffraction allow one to identify and study the cell constituents and their distribution within a battery with high resolution, eventually even under operational conditions. For instance, it was shown that with neutron diffraction it is possible to investigate the cell chemistry in-situ during charge and discharge in conventional Li-batteries 2,3 and in alkaline batteries. 4,5 However, in all these studies no spatial-resolved information was obtained as the complete battery was always immersed in the neutron beam. Additional information is gained when one probes the spatial variation of electrochemical processes within batteries nondestructively with neutron diffraction. Here the complex chemistry within sodium metal halide batteries was chosen as a case study to show the power of spatially resolved neutron scattering for battery research.Sodium metal halide batteries e.g. 6-8 use sodium chloride and metal (Ni or Fe) as the cathode material. The electrolyte and the separator is β -alumina 9 which conducts Na + ions but is an electronic insulator. The operating temperature of this type of batteries is between 270-350 • C. These types of batteries were first introduced in the late 1980's as they combine several advantages for battery applications, such as a high energy density, long cycle life, insensitivity to external temperature, and tolerance to short circuits. This comes in additio...

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].…”

Materials science at the diffractometer STRESS-SPEC at FRM II

“…From the tomography data initial information about the inner structure, porosity and the charge state of the respective batteries could be obtained (for more details see [10]). In the diffraction experiment we took advantage of the setup at the diffractometer STRESS-SPEC to investigate small volumes within bulky samples usually used for strain measurements.…”

“…The reaction front was identifi ed as the border between the FeCl 2 and the NaCl/Na 6 FeCl 8 phase, respectively, with its position being consistent with the tomography results. Add-on studies were recently successfully performed to follow the electrochemical process in-situ during charge and discharge, again taking advantage of the high fl ux of The measurement position is shown schematically as line in the horizontal image from the tomography data of the battery (see [10] for more details). STRESS-SPEC and the possibility to resolve and determine locally within the battery different crystallographic phases.…”

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

“…Our previous experiments showed that spatially resolved neutron diffraction can be used to map the phase distribution within a Na/FeCl 2 cell. 7 Because of their high penetration depth, neutrons are very well suited for non-destructive in situ measurements on the thick sodium metal halide cells and even allow to obtain crystallographic information on the phases by standard structure refinement techniques like the Rietveld method. 7 In the current study we have collected neutron diffraction data at several positions in the cathode of a sodium metal halide cell with mixed Fe/Ni chemistry during charge and discharge.…”

“…7 Because of their high penetration depth, neutrons are very well suited for non-destructive in situ measurements on the thick sodium metal halide cells and even allow to obtain crystallographic information on the phases by standard structure refinement techniques like the Rietveld method. 7 In the current study we have collected neutron diffraction data at several positions in the cathode of a sodium metal halide cell with mixed Fe/Ni chemistry during charge and discharge. Since the reaction travels through the cathode of the cell during charge, looking at one fixed position in the cell only gives an incomplete picture of the processes in the cell, and collecting data at several positions with different distance to the ceramic separator is necessary to see how the reaction front propagates through the cell with time during charge.…”