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 In 4 Li 5 or In 2 Li 3 . 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 Li 3 PS 4 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.