Editors’ Choice—Alleviating the Kinetic Limitations of the Li-In Alloy Anode in All-Solid-State Batteries

Li-In electrodes are widely applied as counter electrodes in fundamental research on Li-metal all-solid-state batteries. It is commonly assumed that the Li-In anode is not rate limiting, i.e. the measurement results are expected to be representative of the investigated electrode of interest. However, this assumption is rarely verified, and some counterexamples were recently demonstrated in literature. Herein, we fabricate Li-In anodes in three different ways and systematically evaluate the electrochemical properties in two- and three-electrode half-cells. The most common method of pressing Li and In metal sheets together during cell assembly resulted in poor homogeneity and low rate performance, which may result in data misinterpretation when applied for investigations on cathodic phenomena. The formation of a Li-poor region on the separator side of the anode is identified as a major kinetic bottleneck. An alternative fabrication of a Li-In powder anode resulted in no kinetic benefits. In contrast, preparing a composite from Li-In powder and sulfide electrolyte powder alleviated the kinetic limitation, resulted in superior rate performance, and minimized the impedance. The results emphasize the need to fabricate optimized Li-In anodes to ensure suitability as a counter electrode in solid-state cells. The fabrication of Li-In anodes needs to be optimized to ensure suitability as a counter electrode in sulfide all-solid-state batteries. The Li-In counter electrode may often be the limiting factor of sulfide all-solid-state halfcells. Pressing Li and In foil together results in a kinetically limited anode. Composites from Li-In and sulfide electrolyte result in stable reference potential, superior rate performance and low impedance of the counter electrode.