Letter abstract
Nature Materials 5, 357 - 360 (2006)
doi:10.1038/nmat1634
Subject Categories: Materials for energy | Ceramics
Room-temperature miscibility gap in LixFePO4
Atsuo Yamada1, Hiroshi Koizumi1, Shin-ichi Nishimura1, Noriyuki Sonoyama1, Ryoji Kanno1, Masao Yonemura2, Tatsuya Nakamura3 and Yo Kobayashi4
The rechargeable lithium-ion cell is an advanced energy-storage system. However, high cost, safety hazards, and chemical instability prohibit its use in large-scale applications. An alternative cathode material, LiFePO4, solves these problems, but has a kinetic problem involving strong electron/hole localization1. One reason for this is believed to be the limited carrier density in the fixed monovalent Fe3+PO4/LiFe2+PO4 two-phase electrode reaction in LixFePO4. Here, we provide experimental evidence that LixFePO4, at room temperature, can be described as a mixture of the Fe3+/Fe2+ mixed-valent intermediate Li
FePO4 and Li1-
FePO4 phases. Using powder neutron diffraction, the site occupancy numbers for lithium in each phase were refined to be 
=0.05 and 1-
=0.89. The corresponding solid solution ranges outside the miscibility gap (0<x<,1-<x<1) were detected by the anomaly in the configurational entropy, and also by the deviation of the open-circuit voltage from the constant equilibrium potential. These findings encourage further improvement of this important class of compounds at ambient temperatures.
- Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8502, Japan
- Institute of Applied Beam Science, Graduate School of Science and Engineering, Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan
- Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201, Japan
- Central Research Institute of Electric Power Industry, 2-11-1 Iwadokita, Komae, Tokyo 201-8511, Japan
Correspondence to: Atsuo Yamada1 e-mail: yamada@echem.titech.ac.jp
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