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(1). 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 Li(x)FePO(4). 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 Li(alpha)FePO(4) and Li(1-beta)FePO(4) 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 themiscibility 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.

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opencircuit voltage constant equilibrium rechargeable lithiumion cell the configurational entropy anomaly fixed monovalent fe3po4life2po4 twophase electrode reaction electronhole neutron solution fe3fe2 mixedvalent intermediate lialphafepo4 1beta lt cathode material lifepo4 site occupancy

Sonoyama, N ,Nishimura, SI ,Nakamura, T ,Yonemura, M ,Kobayashi, Y,Yamada, A ,Kanno, R ,Koizumi, H ,.Room-temperature miscibility gap in Li(x)FePO(4). 5 (),4.

Sonoyama, N , et al."Room-temperature miscibility gap in Li(x)FePO(4)" 5

Sonoyama, N ,Nishimura, SI ,Nakamura, T ,Yonemura, M ,Kobayashi, Y,Yamada, A ,Kanno, R ,Koizumi, H ,"Room-temperature miscibility gap in Li(x)FePO(4)"