(LLZO) as the lithium cation solid electrolyte. For the molten salt, we chose to use a eutectic mixture of KCl-LiCl due to its relatively low eutectic melting point of ~350°C. Interestingly, molten lithium is slightly soluble in molten LiCl-KCl , and causes the molten salt to become electronically conductive, which might lead to reduction of the oxide solid electrolyte. This highlights the importance of the combination of molten salt and cation solid electrolyte to enable the use of molten lithium electrodes. On the positive electrode, YSZ was chosen for the oxygen cation conductor due to its good conductivity and commercial availability. Porous platinum was used for the oxygen catalyst because of the ease of coating onto the curved surface of the YSZ.