Structural Evolution during Lithiation Cycles of the Li-Au System Observed By in-Situ High-Energy x-Ray Diffraction

Recent new Li-Ion Battery (LIB) developments include alloy anodes such as Si-Li, and a general trend towards nanostructured components. With technological development LIB were proposed to store energy for stationary applications as well as electromobility. Batteries need to become more robust achieving longer cycle  lifetime in use to achieve this future goal. Therefore fundamental insight into the acting atomic-scale processes and structural evolution within the active materials during the charging, i.e. lithiation and delithiation cycles is required. Crystalline phases are often formed during cycling as well as amorphous phases which prominently form in alloy anodes based on Si or Ge. Due to its inert nature Au has been often used as a model material in electrochemical research including Li ion storage (1,2). Au was as well employed in composite LIB anodes for example as seed layers to further grow Si nanowires or as coatings. For gold model electrodes as well as gold contained in a LIB anode several studies have pointed to many intermediate metastable crystalline Li-Au phases.

X-ray diffraction is a classical non-destructive method to characterize crystalline phases and synchrotron light has more recently facilitated the analysis of surfaces and thin films as well as in-situ or in-operando characterization. Using high-energy x-ray sources a complete reciprocal plain can be imaged on short time scales necessary to follow also quick structural changes on the time-scale of a few seconds. The huge benefit of high-energy x-rays has been demonstrated in-operando for detailed studies from catalytic surface reactions and molecular interface structures and layering to battery processes.

We describe an in-situ high-energy x-ray diffraction study (3) following the alloying of Li (charging) and dealloying (de-charging) with a Au thin film and discuss the observations with the result of a theoretical screening of potential metastable phases. During the entire charging and discharging cycles we observed in total 6 different crystalline phases which are mainly so far unknown metastable intermediate phases leading in the here chosen end state to the known stable Li₃Au phase. The detailed evolution of the different phases have been followed during cycling. Using a thin Au film grown on single crystalline Cu substrates we also observe the final crystalline cubic Li₃Au to emerge in oriented relation to the below Cu substrate thus indicating this phase nucleates at the interface. 

1. P. Bach, M. Stratmann, I. Valencia-Jaime, A. H. Romero, F. U. Renner, Lithiation and Delithiation Mechanisms of Gold Thin Film Model Anodes for Lithium Ion Batteries: Electrochemical Characterization, Electrochimica Acta (2015), 164, 81–89.

2. F.U. Renner, H. Kageyama, Z. Siroma, M. Shikano, S. Schöder, Y. Gründer, O. Sakata, Gold model anodes for Li-ion batteries: Single crystalline systems studied by in situ X-ray diffraction, Electrochim. Acta 53 (2008), 6064-6069.

3. P. Bach, I. Valencia-Jaime, U. Rütt, O. Gutowski, I. Valencia-Jaime, A. H. Romero, F. U. Renner, to be published.