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 introducing an interlayer, which often consists of the 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 operando 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 MIEC and preferentially deposit onto the Li0.