Electrochemical Lithiation Cycles of Gold Anodes Observed by In Situ High-Energy X‑ray Diffraction

Significant developments of Li-ion batteries will be necessary to cope with the growing demands in electromobility or home storage of (sustainable) electrical energy. A detailed knowledge on the microscopic processes during battery cycling will be increasingly crucial for improvements. Involved phase changes at ambient temperature often involve metastable intermediate states, making both experimental observation and theoretical prediction of process pathways difficult. Here we describe an in situ high energy X-ray diffraction study following the initial alloying and dealloying of Li with an Au thin-film model anode using ionic liquid electrolyte. Six different crystalline alloy phases were observed to be involved in the cyclic phase transitions. Apart from the highest lithiated phase determined in this study, Li₃Au, none of the observed phases could be related to known, thermodynamically stable Li–Au phases. Structural search calculations following the minima hopping method (MHM) allowed the assignment of these phases to distinct metastable Au–Li alloy unit cells