Direct observation of reversible ionic polarization at a buried solid-state battery interface

Abstract

Electric fields control ionic distributions at solid-solid interfaces. However, whether buried interfaces possess an intrinsic ionic reversibility has remained experimentally unresolved. Here, we directly observe reversible ionic polarization at the buried amorphous LiCoO2/Li1.5Al0.5Ge1.5P3O12 electrolyte interface. By combining the Elastic Recoil Detection Analysis (ERDA) method, utilizing time-of-flight (ToF) measurements and nanometer-scale resolution, with the Neutron Depth Profiling (NDP) method for the absolute quantification of lithium, we show that an applied bias of +-1 V results in a reversible redistribution of approximately 9.3% of lithium in the interface region, while conserving its total content. Upon polarity reversal, the lithium redistribution is completely reversed, indicating reversible interfacial ionic polarization rather than irreversible lithium loss or permanent interphase formation. These results demonstrate that buried interfaces in solidelectrolyte systems can behave as electrically reconfigurable ionic regions with an intrinsic reversible freedom for ion motion, thereby providing direct microscopic insight into the physics of space-charge formation and low-voltage interfacial response in solid-state electrochemical systems.