Stability of lithium aluminum manganese oxide cathodes for rechargeable lithium batteries

Abstract

Lithium manganese oxides have attracted wide attention as low-cost, nontoxic intercalation cathode materials for rechargeable lithium batteries. In this work, the stability of these compounds during synthesis and in use has been studied in several respects. (1) Phase stability of LiMnO2 polymorphs has been determined under the high temperature synthesis conditions. Effects of temperature, oxygen partial pressure, and dopant (Al) content on the phase stability have been discussed based on a possible stability mechanism. (2) The mechanism of improved cycling stability of electrochemically transformed spinel compared to conventional spinel has been identified. Atomic rearrangement from the ordered rocksalt to spinel type cation ordering results in an antiphase nanodomain structure, which becomes a ferroelastic domain structure during the cubic ---> tetragonal Jahn-Teller transformation, and thereby accommodates the transformation strains. (3) Al-doped spinels exhibit much improved capacity stability at elevated temperatures compared to undoped spinels. This effect has been discussed with respect to proposed mechanisms of Mn dissolution and capacity loss. (4) Magnetic properties are critically influenced by phase stability, cation ordering, and Mn valence in lithium manganese oxides. In the paramagnetic temperature regime, it has been observed that antiferromagnetic interactions between the Mn ions are strongest in the orthorhombic phase among LiMnO2 polymorphs having the average Mn valence of 3+, while decreasing Mn valence strengthens the antiferromagnetic interactions in LixMn2O4 spinel. At temperatures below the paramagnetic temperature regime, spin-glass behavior is observed in both LixMn2O4 and monoclinic LiMnO2 compounds, which is attributed to geometrical frustration due to structure ( cation ordering) and magnetic disorder due to a disordered distribution of Mn valence. As spin-glass behavior is commonly observed in both well-crystallized, conventional spinel and highly disordered, transformed spinel, magnetic characterization cannot easily be used to distinguish the two different spinels.