Lithium-Gold Reference Electrode for Potential Stability During In Situ Electron Microscopy Studies of Lithium-Ion Batteries

Electrochemical liquid-phase transmission electron microscopy (TEM) is showing excellent promise in fundamental studies of energy-related processes including lithium-ion battery (LIB) cycling. A key requirement to accurately interpret the measurements and acquire quantitative information is the implementation of a reliable reference electrode. Quasi-reference electrodes (QRE) remain commonly used due to microfabrication constraints of the electrochemical cell, however, they typically yield dramatic potential drifts making the electrochemical results inconclusive. Here, we present a method of producing a stable and readily interpretable lithium-gold alloy micro-reference electrode, which exhibits a reference potential of 0.1 V vs Li/Li+. We first examine the feasibility of electrochemically alloying a pristine gold electrode, patterned on a chip for in situ TEM, using a benchtop setup, and investigate various sources to support the lithiation. We confirm the presence of the Li-Au alloy using chronopotentiometry (CP) and open circuit voltage (OCV) measurements, and by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS) and high-resolution (HR) TEM. Finally, we apply this methodology in situ and use LiFePO4 as a model cathode material to demonstrate the merit of the Li-Au alloy reference electrode for obtaining reproducible cyclic voltammetry (CV) measurements on a liquid cell microelectrode system.