Studying Li Evolution in Mixed Ionic-Electronic Conducting Interlayer of Sulfide Based All-Solid-State Batteries Through Operando Raman and Neutron Images

Lithium-metal (Li⁰) anode potentially enables all-solid-state batteries with high energy density. However it shows incompatibility with sulfide solid-state electrolytes (SEs). One strategy is introducing an interlayer, generally made of a mixed ionic-electronic conductor (MIEC). Yet, how Li behaves within MIEC remains unknown. Herein, we investigated the Li dynamics in a graphite interlayer, a typical MIEC, using <i>operando</i> neutron imaging and Raman spectroscopy. This study revealed intercalation–extrusion-dominated mechano-chemical reactions during cell assembly transform the graphite into a Li-graphite interlayer consisting of SE, Li⁰, and graphite-intercalation compounds. During charging, Li⁰ preferentially platted at the Li-graphite|SE interface and then transferred into the Li-graphite interlayer without intercalation. Upon further plating, Li⁰-dendrites formed, inducing short circuits and reverse immigration of Li⁰. Continuum modeling was conducted to explain the Li dynamics. We concluded that the lowest nucleation barrier at the Li⁰ side is necessary to drive the Li⁺ across the MIEC and preferentially deposit onto the Li⁰.