TEM studies: The key for understanding the origin of the 3.3 V and 4.5 V steps observed in LiMn2O4-based spinels

Transmission electron microscopy (TEM) measurements were performed on elect rochemically partially delithiated prepared spinel Li1-xMn2O4 samples. The potential-composition profile of LiMn2O4 exhibits (besides the two plateaus at 4.05 and 4.1 V) two additional redox steps of identical capacity at 4.5 and 3.3/3.95 V, We found by TEM studies that these extra steps are the sig nature of a reversible phase transition between LiMn2O4 spinel type structu re and a new Li1-xMn2O4 double hexagonal (DH) type structure (a approximate to 5.8 Angstrom, c approximate to 8.9 Angstrom, P6(3)mc), The latter is is otypic with DH LiFeSnO4. Selected area electron diffraction (SAED) and high -resolution transmission electron microscopy (HRTEM) made it possible to id entify the mechanism by which this cubic-DH phase transition occurs within a particle. Based on the structural findings the origin as well as the simi lar electrochemical capacity of the 3.3/3.95 and 4.5 V anomalies are explai ned. (C) 2000 Academic Press.