Why Neutrons for Li Ion Battery Research?

In the research field of Li ion batteries is an increasing request to characterize complete battery cells and study Li ion batteries in operando. At the Heinz Maier-Leibnitz Zentrum in Garching near Munich (Germany), neutrons are provided as a probe to carry out a variety of diverse and complementary in situ experiments with Li ion batteries. Neutrons have the unique property to penetrate deeply into materials (including metals) because the scattering power is unrelated to the Z number. In addition neutrons are quite sensitive to light elements and even neighboring elements can have excellent contrast to each other (important for example to discern Mn, Fe, Co and Ni). As neutrons are electrically neutral with typical low energy transfers in the meV range no ionization or local heating occurs. Therefore the neutron methods with their possible large beam cross sections (up to cm2 is feasible) are non-destructive.

In the last years neutron scattering in particular with neutron diffraction started to be established as a method to follow in situ phase transformations in batteries as function of temperature and/or charge state. Many experiments characterize the intercalation process of lithium in graphite including the different phases of LiCx formed during the charging and discharging process. Here, neutrons are an ideal probe to monitor the intercalation process due to the fact that they are sensitive to LiCx compounds. Due to the high penetration depth even spatially resolved measurements at various positions within a battery are possible. Measurements on Li ion batteries (NMC/graphite) will be presented to show how neutrons can monitor the intercalation during charging and discharging under different temperatures in detail (Fig. 1). In addition the process of Li plating can be observed and has been described in dependence to relaxation processes of LiCx phases.

Recently further neutron techniques have been employed to extract additional information on the mechanism in Li ion batteries. For example small-angle neutron scattering was applied to identify nanoscaled structures (1 – 300 nm) in Li ion batteries. As the method works in transmission mode thin Li ion batteries of pouch bag format were used in these experiments. A measurement of a single NMC-cathode was fitted to a mass fractal model yielding a spherical particle radius of ~ 85 nm. Further we applied the prompt gamma activation analysis technique (PGAA) which uses the neutron capture in nuclei of the sample material and the subsequent detection of prompt gamma rays emitted during de-excitation of the compound nuclei. The method determines elemental composition and concentration of samples down to the ppm range. Thus PGAA can detect even trace amounts of elements on electrodes. This method was used to study cation dissolution (Mn, Co, Ni) and transport to the anode quantitatively, giving an insight into cell ageing. Finally, to study lateral structures on the nanometer scale another method called time-of-flight grazing incidence small-angle neutron scattering (GISANS) has been used. In this method, the neutron beam impinges on the sample surface at grazing incidence and the scattered neutron pattern enables the description of objects laterally organized in a layered structure near the surface. An example of self-organized anodic titanium oxide nanorods will be presented.

In this presentation a comprehensive overview on the potential of the available neutron scattering methods to study Li ion batteries will be given.

Achnowledgement: The authors gratefully acknowledge the funding by the German Federal Ministry of Education and Research (BMBF) under the auspices of the project ExZellTUM with grant number 03X4633A and the funding by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology under the auspices of the EEBatt project. J. Kunze-Liebhäuser thanks the DFG (project KU 2397/3-1) for financial support.