Active reaction sites for reduction in electrode have been characterized by addressing the origin of the cathodic polarization effects on this electrode material. Cathodic polarization (up to −1.2 V vs. Pt reference electrode) had several effects on reduction kinetics. First, the reduction rate was favorably increased when the perovskite electrode was cathodically polarized. Second, in situ x‐ray photoelectron spectroscopy results indicated that the Mn ions are electrochemically reduced and concomitantly the oxygen stoichiometry decreases. Reduction of Mn ions was further demonstrated in the cyclic voltammogram traced under nitrogen atmosphere. Third, hysteresis in cathodic currents was observed in the cyclic voltammograms of the perovskite/YSZ/Pt system, and the hysteresis phenomena were more prominent at higher pressure. We interpreted these findings to mean that the internal and/or external surface oxide vacancies participate in the reduction reaction. However, it has been explained from the hysteresis phenomena that, even though those surface sites are active in the reduction, their activity is less than that of the three‐phase boundary sites since additional diffusional processes are required for the former sites. Consequently, the three‐phase boundary sites are the major reaction sites at lower pressure, which leads to a small hysteresis. However, at higher pressure, the surface sites also participate in the reaction, resulting in a larger hysteresis.

Direct integration of a metallic lithium anode with the ceramic Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) electrolyte into an all-solid-state battery is highly challenging due to their chemical and electrochemical incompatibility. Herein, a layered hybrid solid electrolyte is designed by coating the ceramic LATP electrolyte with a protective polymer electrolyte, polyphosphazene/PVDF-HFP/LiBOB. This polymer electrolyte comprises highly Li⁺ conductive polyphosphazene and mechanically stable PVDF-HFP as the polymer matrix, and the mobile lithium ions in the polymer layer are supplied by LiBOB. Equipped with both polymer and ceramic components, the hybrid electrolyte possesses favorable features, such as a flexible surface, high ionic conductivity, high chemical stability against lithium and wide electrochemical stability window (4.7 V), which all to help realize its application in all-solid-state lithium batteries. The prepared all-solid-state battery with a metallic lithium anode and high-voltage Li₃V₂(PO₄)₃/CNT cathode shows high capacity and excellent cycling performance with negligible capacity loss over 500 cycles at 50 °C. Furthermore, the analysis of the hybrid solid electrolyte after long-term cycling demonstrates outstanding electrode/electrolyte interfacial stability. This study suggests that use of solid organic–inorganic hybrid electrolyte is a promising approach to circumvent the mechanical, chemical and electrochemical limitations at the interface of electrodes and ceramic electrolyte for all-solid-state batteries.