The IAM-ESS has an outstation at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) in Garching near Munich. There our colleagues apply non-destructive testing techniques based on neutron radiation to investigate the processes inside of energy storage systems. In the first instance neutron powder diffraction and neutron imaging methods, namely neutron radiography and neutron tomography, are used. The interaction cross sections of neutrons show a considerable difference to the ones of X-ray of synchrotron radiation. Therefore, neutron methods yield complementary results. In addition, neutrons show large scattering and absorption cross sections for low-Z materials like hydrogen or lithium. Thus, neutrons offer outstanding capabilities for the investigation of state of the art energy storage systems like hydrogen storage materials or lithium-ion-batteries.
Besides monitoring structural changes during charge or discharge, effects caused by aging or fatigue of the storage materials are tracked. Hereby, changes on an atomic length scale are observed by neutron diffraction.
Neutron radiography and neutron tomography on the other hand is based on the analysis of the transmitted neutron intensity after the neutron beam has been attenuated while penetrating the sample. Two-dimensional projections or three-dimensional volume data sets allow for a reconstruction of the macroscopic composition/organisation of the sample on a sub millimeter scale. As most metals show high transmission values for neutrons, the experiments can be carried out in operando under real operating conditions.
The aim of the experiments is to gain more knowledge about the internal processes occurring during the operation of energy storage systems in order to improve their performance. For example the maximum number of cycles, meaning the number of charge and discharge processes, should be increased, which is equivalent to a prolonged life time. It should be pointed out, that well-directed investigations at critical operating conditions may reveal the reasons for failure/breakdown of such storage systems and help to improve their reliability and safety.
Figure 1 shows the results of Rietveld refinement of neutron diffraction data acquired for a 18650 type Li-ion cell, the same type as it is used in many notebook battery packs. Besides the accurate structural information (material symmetry, cell metrics, fractional atomic coordinates) the Rietveld analysis enables to follow the evolution of the electrode materials. Therefore, it is possible to monitor the structural and phase changes at different operating states of the cell. Intercalation and deintercalation of lithium causes, for instance, a volume change which in turn is a reason for micro cracks inside the electrode material and may also result in its delamination. This limits the life time of a battery.
By means of neutron radiography and tomography the exchange of lithium between anode and cathode can be tracked. A decrease or increase of the lithium concentration inside the electrode material alters its absorption behaviour (Fig. 2, stripe like structure on the left), which is reflected by a change of the recorded transmission values. Besides the movement of the lithium one can observe the expansion of the electrodes which, again, comes along with the intercalation of lithium ions.
In addition to the large scale facilities used for the neutron scattering investigations (high-resolution powder diffractometer SPODI, engineering diffractometer STRESS-SPEC, neutron radiography and neutron tomography facilities ANTARES and NECTAR) two potentiostats from Bio-Logic are available for process monitoring and sample preparation, a multi potentiostat VMP3 (4 channels with impedance option, 400mA) and a SP-240 (1 channel with impedance option and internal booster, 4A). Dedicated furnaces or cryostats may be used on the instruments to simulate different operating conditions for the energy storage systems.
An X-ray inspection system v|tome|x s 240 for X-ray radiography and tomography with energies up to 240keV is available at the institute of biomedical physics.
Dr. Martin Mühlbauer
E-Mail: martin muehlbauer ∂does-not-exist.kit edu
Phone: +49 (0)89 289 14373
Dr. Anatoliy Senyshyn
Phone: +49 (0)89 289 14316
O. Dolotko, A. Senyshyn, M.J. Mühlbauer, K. Nikolowski, F. Scheiba, H. Ehrenberg, J. Electrochem. Soc. 2012, Volume 159, Issue 12, Pages A2082-A2088.
A. Senyshyn, M.J. Mühlbauer, K. Nikolowski, T. Pirling, H. Ehrenberg, J. Power Sources 203 (2012) 126-129.