For understanding and improving functional materials it is very important to know their crystal structure and structural changes in time or after application of a large number duty cycles.
Structural effects due to fatigue and aging can be determined and later on used to improve the behavior of the material. Laboratory diffraction is routinely used at IAM-ESS to investigate materials concerning crystallographic phases ex-situ. For non-destructive investigation of materials and devices in-situ or in-operando diffraction techniques can also be used. Since these methods are difficult to implement at laboratory instruments, synchrotron and neutron sources are used. With their increased penetration and resolution much more information can be gathered. IAM-ESS employs scientists at large scale facilities in Germany as PETRA III in Hamburg and MLZ/FRM II in Munich, who, in cooperation with local scientists, operate and develop experimental stations and characterization methods. Moreover, the institute uses numerous other facilities all over Europe for their experiments. The methodical development comprises mostly optimized electrochemical cells for in-situ synchrotron diffraction [1] and configurations for in-operando neutron radiography and tomography. For periodical excitations stroboscopic techniques are used with time resolution down to microseconds [3].
Not only the average structure (long range order) but also short range order/disorder or nano sized materials can be measured with diffraction methods. The total scattering or PDF method (Pair distribution function) considers all contributions from scattering centers in the sample and results in information about the real distances between atoms in disordered or nanostructured materials [4].
Effects as clustering of defects or structural distortion can quantitatively be determined.A typical representative of a high voltage cathode material is the Ni-Mn Spinel LiNi0.5Mn1.5O4 (LNMO). Figure 2 shows a detail of a synchrotron diffraction pattern of a Fe doped LNMO spinel.
Doping with Fe improves the cyclability of the cathode material. The de-/intercalation of Li into the host matrix cause changes in atomic distances that are identified by a shift of reflections in the diffraction pattern. In contrast to undoped LNMO the Li intercalation in LNFMO is mostly single phase [5].For fast and periodic processes stroboscopic techniques are used to investigate microstructural changes. Time resolution down to microseconds can be reached. Figure 3 shows this exemplarily with a ferroelectric actuator.
Ferroelectric materials are usually used as actuators, sensors or as data storages. Phase transitions and domain switching lead to mechanical strain under applied electric field and vice versa. The influence of doping or microstructural changes due to fatigue and aging are investigated at synchrotron and neutron sources by application of sinusoidal or rectangular periodic electric fields that are properly synchronized with the detector read out during a diffraction experiment.
Neutron Scattering Experiments
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.
Experimental equipement:
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.
Contact:
Dr. Martin Mühlbauer
E-Mail: martin muehlbauer ∂does-not-exist.kit edu
Phone: +49 (0)89 289 14373
Dr. Anatoliy Senyshyn
E-Mail:anatoliy.senyshyn∂frm2.tum.de
Phone: +49 (0)89 289 14316
References:
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O. Dolotko, A. Senyshyn, M.J. Mühlbauer, K. Nikolowski, F. Scheiba, H. Ehrenberg, J. Electrochem. Soc. 2012, Volume 159, Issue 12, Pages A2082-A2088.
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A. Senyshyn, M.J. Mühlbauer, K. Nikolowski, T. Pirling, H. Ehrenberg, J. Power Sources 203 (2012) 126-129.
