Solid-state lithium-ion batteries (SLIBs) hold great promise for obtaining high energy densities but their power density is currently limited by the inferior charge transfer properties and chemical instability of the electrode-electrolyte interfaces. The goal of this research is to advance the fundamental understanding of the solid electrolyte interfaces with Li metal anode and with high-voltage cathodes. The results obtained from this project will provide scientific insights into how solid-solid interfaces in SLIBs evolve chemically and structurally when driven by temperature and electrochemical potential, and what the electron and Li+ transfer properties of the interfaces are. Such understanding is necessary for providing physically-based breakthroughs in the development of SLIB technology. The work proposes to perform in operando interface sensitive X-ray spectroscopy and specialized scanning-probe measurements on thin-film electrode-electrolyte systems as a function of temperature and applied voltage. Ac and dc electrochemical measurements concurrent with the X-ray measurements will correlate electrochemical performance to interface chemistry. This work will constitute the first time that solid electrolyte-electrode interfaces are studied in operando systematically and in a non-destructive manner. These in operando experiments provide a novel and unique enabler to uncover the chemistry, stability and charge transfer properties of these critical interfaces. Results will guide the engineering of stable interface compositions, and determine electrode-electrolyte combinations that could improve the power density and durability of SLIBs significantly, which then can enable viable energy storage technologies for the Army equipment and improve survivability of Soldiers on the field. Furthermore, the new experimental methods developed in this work can also contribute broadly to other advances in physical sciences studies supported by the Army.