Revealing Li Dynamics in Mixed Ionic-Electronic Conducting Interlayer of All-Solid-State Batteries

Lithium-metal (Li⁰) anode is considered the holy grail of all-solid-state batteries owing to their exceedingly high energy density; in practice, their stability remains unsatisfactory because of the incompatibility between Li⁰ and solid-state electrolytes (SEs). One strategy is to introduce an interlayer, which often consists of a mixed ionic-electronic conductor (MIEC), to stabilize the Li⁰. However, how Li ions (Li⁺) transport within MIEC remains unknown. Herein, we investigate the Li, including Li⁰ and Li⁺, dynamics in a graphite interlayer, a typical MIEC, using <i>operando</i> neutron imaging and Raman spectroscopy. Our study reveals the Li evolution during mechano-chemistry and mechano-electrochemistry reactions. During cell assembly, intercalation–extrusion-dominated mechano-chemical reactions transform the graphite into a Li-graphite interlayer consisting of SE, Li⁰, and diluted graphite-intercalation compounds. During battery operation, dictated by the lowest nucleation energy, Li⁰ plating preferentially occurred 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 from the anode to the cathode during charging. Continuum modeling was conducted to explain the Li dynamics. We concluded that with the MIEC interlayer, a lowest nucleation barrier at the Li⁰ side is necessary to drive the Li⁺ to transport across the MIEC and preferentially deposit onto the Li⁰.