A Micro-Reference Electrode for Electrode-Resolved Impedance and Potential Measurements in All-Solid-State Battery Pouch Cells and Its Application to the Study of Indium-Lithium Anodes

Impedance measurements are a powerful tool to investigate interfaces in lithium-ion batteries (LIBs). In order to deconvolute the anode and cathode contributions to the cell impedance, a reference electrode (RE) is required. However, there are only very few reports on the use of a three-electrode setup with an RE for all-solid-state batteries (ASSBs), which is due to the complexity of integrating an RE with a suitable geometry into the typical ASSB test cells that are based on a compressed electrolyte pellet. In this study, we present a straightforward approach to implement a micro-reference electrode ( μ -RE) for electrode-resolved impedance and potential measurements into ASSB pouch cells. The μ -RE consists of an insulated ∼64 μ m diameter gold wire that is sandwiched between two Li 6 PS 5 Cl/polymer separator sheets and activated by in situ electrochemical lithiation. Using this μ -RE, we investigate the electrode potential and the accessibility of cyclable lithium at the separator interface of indium-lithium anodes, which are prepared by stacking lithium and indium foils with a molar excess of indium. We compare two different cell assembly configurations, with the separator faced by either ( i ) the formerly In-side or ( ii ) the formerly Li-side, showing that only the latter case provides a reservoir of cyclable lithium.