Synthesis of the Fe/Li2O

The Fe/Li₂O mixed electronic/ionic conductor material was prepared using a high-temperature lithium thermal reduction method in a glove box (Mikrouna super 1220/750, Shanghai, China) filled with Ar gas to isolate it from air. Firstly, 0.18 g lithium metal foil (thickness 50 μm, 99%, Suzhou Duoduo Chemical Technology Co.) was weighed and placed in a tantalum crucible and heated on a heating stage set to 185 °C, and the lithium metal was slowly mechanically stirred until it melted. Then, 0.6 g Fe₃O₄ powder (AR, Macklin, Shanghai, China) was weighed and added into the molten lithium gradually and batch-wise, while the mechanical stirring was maintained during the addition process. After the addition was completed, the mixture was continuously stirred at 185 °C for an additional 20 min until the molten lithium was fully mixed with the Fe₃O₄ powder. Subsequently, the heating temperature was raised to 200 °C and the molten mixture underwent a solid-state calcination process, typically for a duration of 2 h. Mechanical stirring was maintained throughout the calcination process to ensure a complete reaction. After the reaction was terminated, the mixture was allowed to cool to room temperature, resulting in the formation of a black Fe/Li₂O solid powder. It is important to note that the final product should be avoided contacting the air or moisture and it has to be stored in the glove box.