Since the discovery of intercalation chemistry in the early 70s, lithium battery technology has been rapidly developed through the efforts in expanding cathodes chemistry for higher energy density, exploring advanced electrolytes for nonflammability and wider voltage window, and enabling high theoretical capacity anode materials. Nevertheless, the ideal anode candidate, Li metal, remains as the holy grail for researchers in the fields of both liquid-electrolyte battery and its solid-state analogues. Future advancement of safe Li metal battery calls for new strategies to enable uniform Li metal plating/stripping at lower stacking pressure and to stabilize Li metal interfaces by engineering. In this dissertation, by leveraging the well-defined interface platforms in all-solid-state thin film battery and utilizing advanced electron microscopy, we demonstrate the fresh understanding at electrode/electrolyte interfaces in a battery system that employs lithium phosphorus oxynitride (LiPON) as the solid-state electrolyte. Firstly, cryogenic electron microscopy unveils a 76-nm-thick Li/LiPON interface that consists of Li2O, Li3N and Li3PO4 as the interphase components, which are embedded in an amorphous matrix and exhibit chemical gradients across the interface. Next, the interface between LiPON and a high-voltage LiNi0.5Mn1.5O1.5 (LNMO) cathode shows overlithiation on the surface of pristine LNMO near the interface. The LNMO/LiPON interface contact remains intact after over 500 cycles, suggesting the essence of both atomic interface contact and removing conductive agents on achieving interfacial stability at high voltage. The efforts on producing a freestanding LiPON film offered invaluable quantitative insights on interface formation between Li metal and LiPON by solid-state NMR, which serves as supportive evidence to the electron microscopy observation. DSC measurement yields a well-defined glass transition temperature of LiPON. Nanoindentation shows a Young’s modulus of ~33GPa of LiPON, which could be related to the residual stress release process in the freestanding form. Moreover, freestanding LiPON is demonstrated to enable Li metal plating in a uniform and fully dense manner without external pressure. Such observations not only provide new insight on interface engineering strategy in bulk batteries, but also shed light on reducing the external pressure on Li metal all-solid-state batteries that is required for stable cycling.

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