  Spinel lithium manganese oxide (LixMn2O4) is used as an active material in
battery cathodes. It is a relatively inexpensive and environmentally friendly
material, but suffers from capacity fade during use. The capacity losses are
generally attributed to the formation of the tetragonal phase (x > 1) due to
overpotentials at the surfaces of the micron-sized particles that are used in
commercial electrodes. In this study, we investigate the mechanisms of
tetragonal phase formation by performing electrochemical lithiation
(discharging) in-situ in the transmission electron microscope (TEM) utilizing
diffraction and high resolution as well as spectroscopy. We observe a sharp
interface between the cubic spinel (x = 1) and the tetragonal phase (x = 2),
that moves under lithium diffusion-control. The tetragonal phase forms as a
complex nanotwinned microstructure, presumably to relieve the stresses due to
expansion during lithiation. We propose that the twinned microstructure
stabilizes the tetragonal phase, adding to capacity loss upon deep discharge.
