Stabilization of LiMnO2 in the alpha-NaFeO2 structure type by LiAlO2 addition

LiMnO2 of the alpha-NaFeO2 structure type has previously been obtained only by the ion-exchange of lithium salts with alpha-NaMnO2. In this paper, we show that LiAlxMn1-xO2 solid solutions can be crystallized in this structur e under conditions where neither pure end-member does. The compounds were s ynthesized by firing homogeneous hydroxide precursors in a reducing atmosph ere to control the manganese valence state. A composition LiAl0.25Mn0.75O2 shows a first-charging voltage of similar to 4 V against a lithium electrod e, but develops 4 and 3 V plateaus upon cycling, indicating transformation to spinel-like cation ordering within the oxide. Unlike previously reported lithium manganese spinels, the compound of this study shows excellent reve rsible capacity (148 mAh/g at C/5 rate; 182 mAh/g at C/15 rate) when cycled over both the 4 and 3 V plateaus, and an energy density (545 Wh/kg) surpas sing that of the spinels. (C) 1998 The Electrochemical Society, Inc. S1099- 0062(97)11-069-0.