Origin of High Rate Capability of LiFePO 4 Investigated By Time-Resolved X-Ray Diffraction at Elevated Temperatures

LiFePO 4 is one of the promising cathode material for lithium-ion batteries as it exhibits high rate capability and safety performance. The origin of the high rate performance exemplified in LiFePO 4 should provide design principles for further development of high rate cathode materials. The (dis)charge reaction of LiFePO 4 proceeds through a two phase behavior between Li-rich Li 1-α FePO 4 (LFP) and Li-poor Li β FePO 4 (FP). [ 1 ] Under high rate cycling, we have clarified the formation of a metastable phase of intermediate phase in Li x FePO 4 ( x = 0.6–0.75) (L x FP) which acts as a buffer layer between LFP and FP. [ 2 ] However, the detailed role of the intermediate phase L x FP during high rate cycling has not fully been understood due to short lifetime of the metastable L x FP phase. To investigate the phase transition mechanism, cycling was conducted at elevated temperatures, since L x FP is thermodynamically stable above 200ºC. [ 3 ] Phase transition mechanism is analyzed by operando time-resolved X-ray diffraction (XRD) measurements at intermediate temperature regimes (100 ~ 300ºC). Charge and discharge proeprties were measured by using three-electrode cells at 170ºC, and 230ºC, in which molten LiTFSA - CsTFSA (molar ration 20:80) was used as the electrolyte. The worling electrode comprised a composite mixture of LiFePO 4 /C, acetylene balck, and polyimide binder (90:5:5(wt%)) coated onto Al foil current collector. operando time-resolved XRD measurements were performed in reflection mode at the beam line BL28XU at SPring-8 (Japan). Elechtrochemical cells were cycled in a temperature-controlled unit set in Ar atmosphere. The cycling temperature was maintained at 230ºC. From the charge curves of LiFePO 4 electrodes at 170ºC and 230ºC, conventional single plateau curve is observed at 170ºC, while the charge curve at 230ºC shows two plateau regions. Based on the reported phase diagram, [ 3 ] the first low potential plateaux is the phase transition of LFP to L x FP, and the second high potential plateaux indicates the phase transition of L x FP to FP. The phase transition behavior was investigated by operando time-resolved XRD at 230ºC. operando time-resolved XRD patterns at 230ºC indicated upon delithiation, 211 and 020 diffraction peaks of LFP vanished and a new peak indexed to 020 diffraction plane of L x FP emerged. These results suggested that LFP transforms to an intermediate L x FP phase via a two-phase mechanism. Further delithiation, however,