  LiNi0.5Mn1.5O4, which has a spinel framework structure, is a promising
candidate for the cathode material of next-generation lithium-ion batteries
with high energy density. We investigate the structural transition in
LixNi0.5Mn1.5O4 (0 <= x <= 1) through first-principles calculations using the
projector augmented wave method with the generalized gradient approximation. We
calculate all the unique Li-site occupation configurations in a unit cell to
obtain the total energies and the most stable structures for various
compositions. Thermodynamic analysis shows that Li0.5Ni0.5Mn1.5O4 with x = 0.5
is the only stable phase for 0 < x < 1. The decomposition energy is lower than
0.1 eV for 0 < x < 0.5, but is distinctly higher for 0.5 < x < 1. The
decomposition energy reaches 0.39 eV at x = 0.75. The ratios of the structures
at room temperature are calculated from Boltzmann factors by using the energy
differences between structures. The crystal structure of the unit cell changes
gradually from x = 0 to 0.5, but changes markedly from x = 0.5 to 1. This
first-principles study provides a general evaluation of the variation in the
crystal structure with the composition of the bulk material, which affects the
cyclability of the electrode.
