The effect of ammonia reduction on the spinel electrode materials, LiMn 2O 4 and Li(Li [formula omitted]Mn [formula omitted])O 4

Abstract

Treating the lithium manganese oxide spinels, LiMn 2O 4 and Li( Li 1 3 Mn 5 3 ) O 4 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 LiMn 2O 4 (synthesized at 750°C) by NH 3 reduction before disproportionation to Li 2Mn 2O 4 and Mn 3O 4 occurs. By contrast, treatment of Li( Li 1 3 Mn 5 3 ) O 4 and LiMn 2O 4 (both synthesized at 400°C) with ammonia at temperatures near 200°C removes oxygen without inducing a phase change. For Li( Li 1 3 Mn 5 3 ) O 4 , the product of the reduction is best characterized as Li( Li 1 3 Mn 5 3 ) O 4−δ with δ as large as 0.25. Unlike LiMn 2O 4 (prepared at 750°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( Li 1 3 Mn 5 3 ) O 4 (all Mn 4+) because it is apparently difficult to oxidize the Mn atoms in it beyond the 4 + state. When δ increases, the average oxidation state of Mn decreases and then substantial Li can be deintercalated. Similarly, reducing LiMn 2O 4 (prepared at 400°C) with ammonia increases the amount of lithium which can be extracted from the material. These results demonstrate the striking effect of the Mn:O ratio, or equivalently the Mn oxidation state, on the electrochemical behavior of Li/spinel cells.