The data and method described in this application note resulted from a common work of Dr. Stefan Seidlmayer (TU München) and Armin Kriele (Helmholtz Zentrum Geesthacht) from the Heinz Maier-Leibnitz Zentrum, MLZ in Garching (Germany). The MLZ is a neutron research facility open for user experiments in science and industrial application.
Introduction
Lithium-ion batteries have become increasingly important in key technologies of our day-to-day lives. Especially the long-term performance of batteries is crucial for their applications in new markets such as energy storage in electric vehicles or even large-scale electric storage devices combined with photovoltaic energy sources.
For these large-scale applications a key requirement is very good long-term and cycling performance. Improvement of this performance requires studying ongoing aging processes inside batteries continuously and without the need to destroy and disassemble them prior to the analysis (in operando analysis). This is especially important as many cell components deteriorate due to the disassembly process or when exposed to air or moisture.
Often X-ray synchrotron or neutron radiation based methods are used in this context [1-4]. The strengths of these methods are their high penetration capabilities and their ability to use specially encapsulated samples or even industrially manufactured samples directly. Nevertheless, a drawback of these methods is the restricted access as they are only available at large-scale research facilities and obtaining the required beam time is typically a longer process.
While neutron scattering is very sensitive to light elements such as Li, X-ray powder diffraction provides a much higher resolution of the cell parameters and therefore both are complementary techniques. A large variety of experimental cells for XRD studies has been developed such as specially adapted coin cell type batteries for reflection geometry. Typically XRD setups of lab diffractometers are based on using copper radiation and rely on using so-called ‘half-cells’, containing only one active electrode while the other electrode is then only a thin foil of lithium metal. Although this is adequate to study the principal transformation mechanism and the corresponding structural changes of the electrode active material via diffraction methods, it is insufficient for studying complex aging processes. These depend heavily on even the smallest changes in cell chemistry and material composition and require monitoring the structural evolutions of both electrodes at the same time. For such aging studies one cannot rely on half-cell studies but has to observe the cells under real operating conditions in their native environment.
The pouch bag cell design offers this possibility. It is easy to manufacture reproducibly. It is versatile enough for experimental changes and offers the possibility to use all standard battery grade materials and components. An industrial prototype can be assembled for the continuous monitoring of cells subjected to different aging procedures. In synchrotron beam lines such pouch cells have already been used successfully [5]. The battery research group headed by Dr. habil. Ralph Gilles (TU München) at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching (Germany) has adapted and further developed this method in order to use standardized pouch cells for diffraction experiments on a lab XRD instrument like the Empyrean using hard X-ray radiation. This allows investigation of the structural dynamics of fully closed pouch bag type batteries as used as industrial prototype samples during typical aging procedures directly and without any prior dismantling.
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