Challenges toward higher temperature operation of LiFePO4

Large-scale lithium-ion batteries operating at higher temperature may provide additional advantageous aspects such as higher power and use of new materials which are inactive at ambient temperatures. As a first step for this direction, we applied LiFePO4 cathode to middle-temperature region of 60 °C–115 °C and investigated temperature-dependent charge–discharge properties. By selecting suitable electrolyte and battery components tolerant to the elevated temperature, stable operation was attained for no less than 50 cycles below 115 °C. High-rate performance was significantly improved as operating temperature increased up to 100 °C, but suffered from abrupt increase in polarization above 100 °C, where the corresponding impedance signal emerged, which might be assigned to some side reactions occurring at the surface of LiFePO4 particles. At 100 °C, the discharge capacity over 100 mAh g−1 was achieved at 200C rate, even with 10wt% of carbon in the electrode composite of LiFePO4. Shrinkage of miscibility gap was not confirmed for samples larger than 40 nm at < 115 °C. ► Charge–discharge properties of LiFePO4 at 60 °C–115 °C were investigated. ► Elevating temperature up to 100 °C significantly enhanced reaction speed of LiFePO4. ► Side reaction to form resistive surface layer impede high-rate performance above 100 °C. ► Trade-off between fast kinetics and side reaction is to be considered at higher temperature. ► Miscibility gap do not shrink for samples with >40 nm at 115 °C.