Origin of High Rate Capability of LiFePO4 Investigated By Time-Resolved X-Ray Diffraction at Elevated Temperatures
Authors: Mori Takuya , Otani Kazufumi , Munesada Toshiyuki , Yoshinari Takahiro , Orikasa Yuki , Koyama Yukinori , Ohara Koji , Fukuda Katsutoshi , Nohira Toshiyuki , Hagiwara Rika , Uchimoto Yoshiharu 
Published: February 2020

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 Li1-αFePO4 (LFP) and Li-poor LiβFePO4 (FP).[
1
] Under high rate cycling, we have clarified the formation of a metastable phase of intermediate phase in Li
x
FePO4 (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 LxFP 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 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
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, leads to shift of 020 peak of L
x
FP which finally merged with the 211 peak of FP. In coclusion, phase transition from LxFP to FP entails not only a two-phase reaction, but also a solid-solution mechanism of the L
x
FP phase and L
x
FP 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.
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