Yamada Atsuo, Koizumi Hiroshi, Nishimura Shin-Ichi, Sonoyama Noriyuki, Kanno Ryoji, Yonemura Masao, Nakamura Tatsuya, Kobayashi Yo
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8502, Japan.
Nat Mater. 2006 May;5(5):357-60. doi: 10.1038/nmat1634. Epub 2006 Apr 16.
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, LiFePO(4), solves these problems, but has a kinetic problem involving strong electron/hole localization. One reason for this is believed to be the limited carrier density in the fixed monovalent Fe(3+)PO(4)/LiFe(2+)PO(4) two-phase electrode reaction in LixFePO4. Here, we provide experimental evidence that LixFePO4, at room temperature, can be described as a mixture of the Fe(3+)/Fe(2+) mixed-valent intermediate LialphaFePO4 and Li1-betaFePO4 phases. Using powder neutron diffraction, the site occupancy numbers for lithium in each phase were refined to be alpha=0.05 and 1-beta=0.89. The corresponding solid solution ranges outside the miscibility gap (0<x<alpha,1-beta<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.
可充电锂离子电池是一种先进的储能系统。然而，高成本、安全隐患和化学不稳定性阻碍了其在大规模应用中的使用。一种替代阴极材料LiFePO₄解决了这些问题，但存在涉及强电子/空穴局域化的动力学问题。据信其中一个原因是在LiₓFePO₄中固定的一价Fe³⁺PO₄/LiFe²⁺PO₄两相电极反应中载流子密度有限。在此，我们提供实验证据表明，在室温下，LiₓFePO₄可描述为Fe³⁺/Fe²⁺混合价中间体LiαFePO₄和Li₁-βFePO₄相的混合物。使用粉末中子衍射，各相中锂的占位数被精修为α = 0.05和1-β = 0.89。通过构型熵的异常以及开路电压与恒定平衡电位的偏差，检测到了混溶间隙（0 < x < α，1-β < x < 1）之外的相应固溶体范围。这些发现促使人们在环境温度下进一步改进这类重要的化合物。
