Local structure and chemical bonding of protonated LixMn2O4 spinels from first principles

Local symmetry and structure of protonated LixMn2O4 (0 <= x <= 1) spinels were systematically investigated using a first-principles method. Introduction of protons into LixMn2O4 (0 <= x <= 1) spinels does not change the basic Mn-O framework, whereas the local symmetry is broken. Every proton, which prefers to reside in one tetrahedral 8a cavity, moves to one of its four neighboring oxygen ions, forming one OH cluster, while the Li ions are still in the 8a sites. The corresponding O(H) atom relaxes about 0.1 angstrom away from the ideal position in the original Mn-O network, toward the proton, which enables the H-O bonds to keep a length of 0.99 angstrom. The calculated results not only are in good agreement with the experiments in the literature but also explain the apparent long H-O bonds obtained from the recent structural refinements.