Fabrication and High-temperature Electrochemical Stability of LiFePO₄ Cathode/Li₃PO₄ Electrolyte Interface (Supporting Information)

<div>A thin-film battery composed of a LiFePO₄ cathode/Li₃PO₄ electrolyte/Li anode was fabricated on a Pt/Ti/Si (PTS) substrate via RF magnetron sputtering. The amorphous Li₃PO₄ film was densely stacked on a 60 nm-thick LiFePO₄ film, which provided a suitable reaction field for understanding the electrochemical properties of LiFePO₄ at the interface with the solid electrolyte. The LiFePO₄ cathode film exhibited highly reversible lithium desertion/insertion at the interface at room temperature and 60 °C, without any side reactions. An irreversible oxidation reaction occurred during the initial charging process at 100 °C, leading to an increase in the charge-transfer resistance of the LiFePO₄/Li₃PO₄ interface with no significant decrease in the lithium desertion/insertion capacity of LiFePO₄. This result suggests the formation of a resistive interphase via the decomposition of Li₃PO₄ at 100 °C. A severe decrease in capacity is observed at 125 °C, which indicates the LiFePO₄-side interface contributed to the side reactions. The film battery exhibits a severe decrease in capacity at 125 °C.</div>