Understanding Lithium-Gold Electrochemical Reactions Using Operando X-Ray Diffraction

Gold electrodes are used in lithium-based battery research as catalysts and micro / quasi-reference electrodes despite their high cost and unclear reactivity with lithium. Here we present an investigation of the electrochemical reactions of gold-lithium (Au-Li) mixtures using variable-temperature electrochemistry and operando X-ray diffraction. Multiple solid solutions (alpha, beta, delta) and intermetallic phases (AuLi 3 , Au 4 Li 15 ) are expected based on the equilibrium phase diagram. 1 Previous electrochemical investigations conclusively identified only AuLi 3 ; 2,3 alpha and beta may be bypassed during lithium insertion, and the structure of delta is unclear. 4 We are unaware of any previous reports of the electrochemical formation of Au 4 Li 15 . Metastable crystallographic solutions have been proposed. 5 X-ray diffraction patterns collected during the galvanostatic lithiation of gold leaf (at 60 °C and a rate of ca. C/50) are provided in Figure 1. Reference diffraction patterns and the associated Powder Diffraction File card number are indicated, along with proposed phases. 4,5 X-ray and electrochemical data align with earlier results suggesting that the equilibrium alpha and beta phases are seemingly bypassed during the first insertion. Delta is the first phase to form at a potential of 0.25 V vs. Li/Li + , followed by the formation of AuLi 3 at 0.15 V vs. Li/Li + at all conditions and Au 4 Li 15 at 0.05 V vs. Li/Li + in select conditions. Electrochemical data suggests AuLi 3 may also have a small range of solubility. Peaks associated with AuLi 3 and Au 4 Li 15 align with existing literature but peaks associated with delta do not. Reactions are reversible to delta, followed by the formation of another phase near 0.3 V vs. Li/Li + and alpha at potentials above 0.4 V vs. Li/Li + during lithium removal. Significant hysteresis is present in cell potential and reaction pathways in the low-lithium content region. The second cycle (not shown) is reversible between alpha (not pure gold) and the terminal phase without a significant loss in capacity. Alternative crystallographic unit cell(s) for delta and other gold-lithium phases will be described. While the delta / AuLi 3 reaction exhibited a consistent potential during lithium insertion and removal, the potential otherwise varied strongly with temperature, rate, and composition, implying that gold quasi-reference electrodes may not be suitable for lithium-ion battery research. References: A. D. Pelton, Bull. Alloy Phase Diagrams, 7, 228–231 (1986). A. N. Dey, J. Electrochem. Soc., 118, 1547 (1971). P. Bach, M. Stratmann, I. Valencia-Jaime, A. H. Romero, and F. U. Renner, Electrochim. Acta, 164, 81–89 (2015). A. J. Leenheer, K. L. Jungjohann, K. R. Zavadil, and C. T. Harris, ACS Nano, 10, 5670–5678 (2016). P. Bach, I. Valencia-Jaime, U. Rütt, O. Gutowski, A.H. Romero, and F.U. Renner, Chem. Mater., 28, 2941–2948 (2016). Figure 1. Galvanostatic and X-ray diffraction data collected during the first lithium insertion (alloying) and removal (de-alloying) cycle of a gold foil at 60 °C and a rate of C/50. X-ray intensity is shown on a logarithmic scale (provided). Peaks from X-ray powder diffraction cards and the associated card numbers are indicated. Figure 1