Standard Li+/Li redox potential at the Li/amorphous-Li3PO4 interface probed by photoelectron spectroscopy for all-solid-state (photo)electrochemical systems

(Photo)electrochemistry in all-solid-state systems has advanced significantly, driven by the improvement of superionic conductors used as solid electrolytes. However, no general reference electrodes exist for all-solid-state systems that allow the discussion of (photo)electrochemical reactions in terms of potentials rather than voltages. Therefore, we focus on the standard redox potential of Li+/Li [Eº (Li+/Li)], because the charge–discharge behavior of all-solid-state batteries is nearly identical to that of Li-ion batteries. In this study, we have demonstrated that the Eº(Li+/Li) at the Li/amorphous-Li3PO4 interface can serve as a reliable standard potential, comparable to the Eº(Li+/Li) at Li/liquid-electrolyte interfaces, by using ultraviolet and low-energy inverse photoelectron spectroscopy. Our finding enables the discussion of (photo)electrochemical reactions in all-solid-state systems using potentials instead of voltages.