Fabrication and High-temperature Electrochemical Stability of LiFePO4 Cathode/Li3PO4 Electrolyte Interface

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 degrees C, without any side reactions. An irreversible oxidation reaction occurred during the initial charging process at 100 degrees 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 degrees C. A severe decrease in capacity is observed at 125 degrees C, which indicates the LiFePO4-side interface contributed to the side reactions. The film battery exhibits a severe decrease in capacity at 125 degrees C.