THE EFFECT OF AMMONIA REDUCTION ON THE SPINEL ELECTRODE MATERIALS, LIMN2O4 AND LI(LI1/3MN5/3)O4

Treating the lithium manganese oxide spinels, LiMn2O4 and Li(Li1/3Mn5/3)O4, with ammonia at elevated temperatures causes oxygen loss. The reaction between these materials and ammonia as a function of temperature has been studied using X-ray diffraction and Thermal Gravimetric Analysis (TGA). Electrochemical studies of the subsequent deintercalation of Li from these materials have been made using Li/non-aqueous electrolyte/spinel cells. Very little oxygen can be removed from LiMn2O4 (synthesized at 750-degrees-C) by NH3 reduction before disproportionation to Li2Mn2O4 and Mn3O4 occurs. By contrast, treatment of Li(Li1/3Mn5/3)O4 and LiMn2O4 (both synthesized at 400-degrees-C) with ammonia at temperatures near 200-degrees-C removes oxygen without inducing a phase change. For Li(Li1/3Mn5/3)O4, the product of the reduction is best characterized as Li(Li1/3Mns/3)O4-delta with delta as large as 0.25. Unlike LiMn2O4 (prepared at 750-degrees-C), where half the Mn is in the 3+ oxidation state and almost all the Li can be removed electrochemically, very little Li can be removed from Li(Lil/3Mns/3)04 (all Mn4+) because it is apparently difficult to oxidize the Mn atoms in it beyond the 4+ state. When delta increases, the average oxidation state of Mn decreases and then substantial Li can be deintercalated. Similarly, reducing LiMn2O4 (prepared at 400-degrees-C) with ammonia increases the amount of lithium which can be extracted from the material. These results demonstrate the striking effect of the Mn: 0 ratio, or equivalently the Mn oxidation state, on the electrochemical behavior of Li/spinel cells.