In situ, multi-modal X-ray Characterization of Energy Materials
Dr Johanna Nelson Weker, Stanford Synchrotron Lightsource
Synchrotron radiation provides an intense, energy tunable source of x-rays ideal for an array of in situ characterization experiments. By varying the sample conditions such as potential, current, temperature, pressure, or environmental atmosphere the morphological, structural, and chemical changes occurring in the sample can be tracked to understand how these energy materials evolve during synthesis or under operating conditions. In situ X-ray diffraction (XRD) follows the structural evolution of crystals and can capture metastable or transient phases. In situ X-ray absorption spectroscopy (XAS) tracks the chemical and structural changes even in poorly crystalline or amorphous materials. In situ XAS can be used to identify intermediate species which are relatively short lived. In situ transmission X-ray microscopy (TXM) is a full-field, high resolution imaging technique which can follow the morphological and local chemical changes in 2D and 3D.
Using the three in situ characterization techniques (XRD, XAS, and TXM), Li-Ge batteries were studied under typical galvanostatic cycling. Germanium is a promising anode material that has a theoretical capacity more than five times larger than the currently used graphite anodes. However, large volume changes during lithiation and delithiation are believed to cause facturing leading to particle pulverization and capacity fading in Ge anodes. With in situ 2D TXM we observe particle size dependencies during charge/discharge. Particles with diameters below ~2 µm do not fracture during lithiation and only the largest particles remained active in the second cycle. From in situ tomography we show unambiguously for the first time particles fracturing into completely isolated pieces. Finally, with in situ XRD and XAS the sequence of intermediate species as well as the end state species of the discharged/lithiated germanium anode are identified.