| Summary: | 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.
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