A Stable and Reliable Li–In Alloy Counter Electrode for Sulfide‐Based All‐Solid‐State Batteries

ABSTRACT Stable and reliable counter electrodes are essential for the accurate performance evaluation of individual electrodes in sulfide‐based all‐solid‐state batteries. Lithium–indium (Li–In) alloys are widely used as counter electrodes due to their stable voltage plateau (∼0.62 V vs. Li/Li ⁺ ) and good compatibility with sulfide electrolytes. However, conventional cold‐pressed Li–In (CP‐LiIn/In) alloys suffer from sluggish lithium‐extraction kinetics and limited capacity due to incomplete alloying and heterogeneous phase distribution, rendering them unsuitable for characterizing high‐capacity, initially lithium‐free electrode systems. To address this limitation, we developed a chemical pre‐lithiation strategy to fabricate Li–In alloy powder to construct reliable counter electrodes for the first time. By precisely controlling the stoichiometric ratio between the pre‐lithiation agent (lithium naphthalene) and indium foil, a homogeneous LiIn/In biphasic alloy powder and the corresponding electrode (PL‐LiIn/In) were successfully prepared. The resulting PL‐LiIn/In electrode exhibits fast reaction kinetics and high lithium‐extraction capacity. Even at a high mass loading of 70.73 mg cm ⁻² , the electrode delivers a lithium‐extraction capacity of 11.75 mA h cm ⁻² , corresponding to 90.6% of its theoretical value. It enables sufficient lithium replenishment for high‐loading silicon electrodes (4.75 mA h cm ⁻² ), ensuring accurate and reliable performance assessment. This work presents a robust technical solution for constructing high‐performance Li–In alloy counter electrodes, thereby facilitating the advancement of sulfide‐based all‐solid‐state batteries.