Beta-iron oxy-hydroxide, which exhibits a tunnel-type structure similar to that of #alpha#-MnO_2 was found to intercalate reversibly lithium in the tunnels. This material exhibit three voltage plateaus at 2.3, 1.5 and 0.7 V and has an over all discharge capacity of 1100 mAh/g. When cycling in the 2-V region, the material exhibits high capacity of 275 mAh/g and very good cyclic reversibility. X-ray photoelectron spectroscopy of the discharged material showed that iron is reduced to the divalent state, and the lithium incorporated in the tunnels is purely ionic. This result explains the good reversibility of this electrode material. When discharged to 0.5 V, however, the structure of the material collapsed, and metallic iron was detected in the X-ray diffraction pattern.
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