LiFePO₄ 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 LiFePO₄ should provide design principles for further development of high rate cathode materials. The (dis)charge reaction of LiFePO₄ proceeds through a two phase behavior between Li-rich Li_1-αFePO₄ (LFP) and Li-poor Li_βFePO₄ (FP).^[1] Under high rate cycling, we have clarified the formation of a metastable phase of intermediate phase in Li_xFePO₄ (x = 0.6–0.75) (L_xFP) which acts as a buffer layer between LFP and FP.^[2] However, the detailed role of the intermediate phase L_xFP during high rate cycling has not fully been understood due to short lifetime of the metastable L_xFP phase. To investigate the phase transition mechanism, cycling was conducted at elevated temperatures, since L_xFP 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 LiFePO₄/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 LiFePO₄ 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_xFP, and the second high potential plateaux indicates the phase transition of L_xFP 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_xFP emerged. These results suggested that LFP transforms to an intermediate L_xFP phase via a two-phase mechanism. Further delithiation, however, leads to shift of 020 peak of L_xFP which finally merged with the 211 peak of FP. In coclusion, phase transition from L_xFP to FP entails not only a two-phase reaction, but also a solid-solution mechanism of the L_xFP phase and L_xFP phase facilitates a solid-solution reaction route during delithiation of LFP. This solid-solution mechanism confers high diffusivity of lithium within the host structure, accounting for the fast charge reaction process of LiFePO₄.

References :

[1] A. Yamada, H. Koizumi, S. Nishimura, N. Sonoyama, R. Kanno, M. Yonemura, T. Nakamura, Y. Kobayashi, Nat. Mater.,2006, 5, 357.

[2]Y. Orikasa, T. Maeda, Y. Koyama, H. Murayama, K. Fukuda, H. Tanida, H. Arai, E. Matsubara, Y. Uchimoto, Z. Ogumi, J. Am. Chem. Soc.,2013, 135, 5497

[3]J.L.Dodd, R. Yazami, B. Fultz, Elechtrochem. Solid State Lett., 2006, 9, A151