The Indium-Lithium Electrode in Solid-State Lithium-Ion Batteries: Phase Formation, Redox Potentials, and Interface Stability

Lithium solid-state batteries (Li-SSBs) require electrodes that provide a sufficiently stable interface with the solid electrolyte. Due to the often limited stability window of solid electrolytes, researchers frequently favor an In-Li alloy instead of lithium metal as counter electrode for two-electrode measurements. Maintaining a stable potential at the counter electrode is especially important because three-electrode measurements are hard to realize in solid-state cells. Although a constant potential of about 0.6 V vs. Li+/Li is commonly accepted for the In-Li electrode, only little is known about the behavior of this electrode. Moreover, the In-Li phase diagram is complex containing several intermetallic phases/compounds such as the InLi phase, or line compounds such as In4Li5 or In2Li3. This means that the redox potential of the In-Li electrode depends on the alloy composition, i. e. the In/Li ratio. Here, we study the behavior of In-Li electrodes in cells with liquid electrolyte to determine their phase evolution and several equilibrium potentials vs. Li+/Li. The room temperature equilibrium redox potential of the In-Li electrode with the favored composition (or more precisely the Li+/(In-InLi) electrode) is 0.62 V vs. Li+/Li. We then discuss the use of In-Li electrodes in solid state cells using Li3PS4 as solid electrolyte and give examples on the importance of the right In/Li ratio of the electrode. While the right In/Li ratio enables stable lithium insertion/deinsertion in symmetrical cells for at least 100 cycles, too much lithium in the electrode leads to a drop in redox potential combined with a rapid build-up of interface resistance.