Smart Neutrons for in-Situ and Operando Characterization of Battery Components and Cells
Authors: Gilles Ralph , Trunk Markus , Wetjen Morten , Werner Lukas , Gernhäuser Roman , Märkisch Bastian , Gasteiger Hubert A. , Révay Zsolt 
Published: February 2020

In the last years, a huge demand emerged to follow electrochemical processes in situ or operando to understand in detail the mechanisms of energy storage systems. Neutrons were identified as a highly suitable probe thanks to their special properties, such as large penetration depths in materials, the non-destructive interaction due to meV energy transfer, the high sensitivity for light elements as especially Li and H in the presence of heavy elements and the easy distinction of neighbor elements or various isotopes of a single element.
Besides the well-established method of neutron diffraction using the LiCx peaks to study the intercalation or de-intercalation of Li in the graphite anode [1], other techniques became available to study other phenomena in battery research. Using the imaging techniques, the visualization of electrolyte filling [2], gassing [3] or the Na liquid level of a ZEBRA cell in operation [4] could be examined. Small-angle neutron scattering with the method of scattering-contrast variation (protonated and deuterated labelled samples to vary the sensitivity for example of the electrolyte) open another kind of application to receive further information on shell structures of electrode materials [5]. Further techniques as neutron reflectivity [6], grazing incidence small-angle neutron scattering [7], positron annihilation spectroscopy using neutrons converted into positrons [8] or prompt gamma activation analysis [9] have been successful used.
This contribution focuses on the new neutron depth profiling facility [10] of Research Neutron Source Heinz Maier-Leibnitz (FRM II) at Technical University of Munich. The method can be applied for surface studies of the upper 50 μm of a sample. The method is suitable e.g. for Li concentration profiles, as the 6Li nucleus and the incoming neutron form an α particle and a triton leaving the reaction location with 180° direction to each other. Both particles are charged and therefore are easily detectable. The precise reaction depth can be determined due to the fact that the α particle and the triton lose energy passing through the sample. A good depth resolution of about ten nanometers is possible with α particles emitted from <15 μm. With a somewhat worse resolution, a depth down to 50 μm (depending on the sample composition) can be reached with triton particles. Examples of first test measurements including reference samples [11] and the distribution of electrolyte decomposition products in silicon-graphite electrodes [12] will be presented.
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