Phase Transition Dynamics of LiFePO4 Via Intermediate Phase By Using Molten Salt Electrolyte
Authors: Munesada Toshiyuki , Otani Kazufumi , Mori Takuya , Orikasa Yuki , Koyama Yukinori , Ohara Koji , Fukuda Katsutoshi , Nohira Toshiyuki , Hagiwara Rika , Uchimoto Yoshiharu 
Published: February 2020

Introduction
LiFePO4 is a promising cathode material for lithium-ion batteries as it exhibits high rate 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 revealed 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 reaction between LFP and FP during high rate cycling has not fully been understood due to short lifetime of metastable L
x
FP phase. To investigate the phase transition mechanism, with respect to L
x
FP phase, electrochemical measurements were conducted at elevated temperatures, since L
x
FP is thermodynamically stable above 200ºC.[
3
] Also, the phase transition mechanism is analyzed by operando time-resolved X-ray diffraction (TR-XRD) measurements at intermediate temperature regimes (100 ~ 300ºC).
Experimental
Charge-discharge measurements were performed by using three-electrode cells at 170ºC, and 230ºC. Molten LiTFSA - CsTFSA (molar ratio 20:80) was used as the electrolyte. The working electrode comprised a composite mixture of LiFePO4/C, acetylene black, and polyimide binder (90:5:5(wt %)) coated onto Al foil current collector. Li-Al alloy wires prepared by electrochemical lithiation of Al wire at 160ºC were used as the counter and reference electrodes. Operando TR-XRD measurements were performed in reflection mode at the beam line BL28XU at SPring-8 (Japan). Electrochemical cells were cycled in a temperature-controlled unit set in Ar atmosphere. The cycling temperature was maintained at 230ºC.
Results and Discusssions
At the charge-discharge tests, typical single plateau curve was observed at 170ºC, while two plateau regions appeared at 230ºC. Based on the reported phase diagram,[3] the first low potential plateau is the phase transition of LFP to L
x
FP, and the second high potential plateau indicates the phase transition of L
x
FP to FP. The phase transition behavior was investigated by operando TR-XRD at 230ºC. Upon delithiation, the 211 and 020 diffraction peaks of LFP vanished and a new peak indexed to 020 diffraction plane of L
x
FP emerged. LFP transformed to an intermediate L
x
FP phase via a two-phase mechanism. Further delithiation, however, led to shift of 020 peak of L
x
FP which finally merged with the 211 peak of FP. The reaction of LxFP to FP proceeded not only through a two-phase reaction, but also through a solid-solution mechanism of the L
x
FP phase. 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.
Reference
[1] A. Yamada, H. Koizumi, S. Nishimura, N. Sonoyama, R. Kanno, M. Yonemura, T. Nakamura, Y. Kobayashi, Nat. Mater.,
5, 357-360 (2006).
[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.,
135, 5497-5500 (2013).
[3] J.L. Dodd, R. Yazami, B. Fultz, Electrochem. Solid-State Lett.,
9, A151 (2006).
Acknowlegment
This study was partially supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery (RISING) Project under the auspices of New Energy and Industrial Technology Development Organization (NEDO), Japan.