title: Thermodynamic Stability of Low- and High-Index Spinel LiMn2O4 Surface Terminations.
abstract: Density functional theory calculations are performed within the generalized gradient approximation (GGA+U) to determine stable terminations of both low- and high-index spinel LiMn2O4 (LMO) surfaces. A grand canonical thermodynamic approach is employed, permitting a direct comparison of off-stoichiometric surfaces with previously reported stoichiometric surface terminations at various environmental conditions. Within this formalism, we have identified trends in the structure of the low-index surfaces as a function of the Li and O chemical potentials. The results suggest that, under a range of chemical potentials for which bulk LMO is stable, Li/O and Li-rich (111) surface terminations are favored, neither of which adopts an inverse spinel structure in the subsurface region. This thermodynamic analysis is extended to identify stable structures for certain high-index surfaces, including (311), (331), (511), and (531), which constitute simple models for steps or defects that may be present on real LMO particles. The low- and high-index results are combined to determine the relative stability of each surface facet under a range of environmental conditions. The relative surface energies are further employed to predict LMO particle shapes through a Wulff construction approach, which suggests that LMO particles will adopt either an octahedron or a truncated octahedron shape at conditions in which LMO is thermodynamically stable. These results are in agreement with the experimental observations of LMO particle shapes.
authors.affiliation: School of Chemical Engineering, Purdue University , West Lafayette, Indiana 47907, United States.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Warburton
authors.affiliation: Materials Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Hakim
authors.initials: H
authors.lastname: Iddir
authors.affiliation: Materials Science Division, Argonne National Laboratory , Argonne, Illinois 60439, United States.
authors.forename: Larry A
authors.initials: LA
authors.lastname: Curtiss
authors.affiliation: School of Chemical Engineering, Purdue University , West Lafayette, Indiana 47907, United States.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Greeley
keywords: DFT; LMO; Li-ion batteries; LiMn2O4; reducible oxides; surface thermodynamics